mod.rs

//! Defines the [`World`] and APIs for accessing it directly.

pub(crate) mod command_queue;
mod deferred_world;
mod entity_fetch;
mod entity_ref;
mod filtered_resource;
mod identifier;
mod spawn_batch;

pub mod error;
#[cfg(feature = "bevy_reflect")]
pub mod reflect;
pub mod unsafe_world_cell;

pub use crate::{
    change_detection::{Mut, Ref, CHECK_TICK_THRESHOLD},
    world::command_queue::CommandQueue,
};
pub use bevy_ecs_macros::FromWorld;
pub use deferred_world::DeferredWorld;
pub use entity_fetch::{EntityFetcher, WorldEntityFetch};
pub use entity_ref::{
    ComponentEntry, DynamicComponentFetch, EntityMut, EntityMutExcept, EntityRef, EntityRefExcept,
    EntityWorldMut, FilteredEntityMut, FilteredEntityRef, OccupiedComponentEntry,
    TryFromFilteredError, VacantComponentEntry,
};
pub use filtered_resource::*;
pub use identifier::WorldId;
pub use spawn_batch::*;

use crate::{
    archetype::{ArchetypeId, Archetypes},
    bundle::{
        Bundle, BundleEffect, BundleId, BundleInfo, BundleInserter, BundleSpawner, Bundles,
        InsertMode, NoBundleEffect,
    },
    change_detection::{MaybeLocation, MutUntyped, TicksMut},
    component::{
        CheckChangeTicks, Component, ComponentDescriptor, ComponentId, ComponentIds, ComponentInfo,
        ComponentTicks, Components, ComponentsQueuedRegistrator, ComponentsRegistrator, Mutable,
        RequiredComponents, RequiredComponentsError, Tick,
    },
    entity::{Entities, Entity, EntityDoesNotExistError},
    entity_disabling::DefaultQueryFilters,
    error::{DefaultErrorHandler, ErrorHandler},
    event::{BufferedEvent, EventId, Events, WriteBatchIds},
    lifecycle::{ComponentHooks, RemovedComponentEvents, ADD, DESPAWN, INSERT, REMOVE, REPLACE},
    observer::Observers,
    prelude::{Add, Despawn, Insert, Remove, Replace},
    query::{DebugCheckedUnwrap, QueryData, QueryFilter, QueryState},
    relationship::RelationshipHookMode,
    resource::Resource,
    schedule::{Schedule, ScheduleLabel, Schedules},
    storage::{ResourceData, Storages},
    system::Commands,
    world::{
        command_queue::RawCommandQueue,
        error::{
            EntityDespawnError, EntityMutableFetchError, TryInsertBatchError, TryRunScheduleError,
        },
    },
};
use alloc::{boxed::Box, vec::Vec};
use bevy_platform::sync::atomic::{AtomicU32, Ordering};
use bevy_ptr::{OwningPtr, Ptr, UnsafeCellDeref};
use bevy_utils::prelude::DebugName;
use core::{any::TypeId, fmt};
use log::warn;
use unsafe_world_cell::{UnsafeEntityCell, UnsafeWorldCell};

/// Stores and exposes operations on [entities](Entity), [components](Component), resources,
/// and their associated metadata.
///
/// Each [`Entity`] has a set of unique components, based on their type.
/// Entity components can be created, updated, removed, and queried using a given
///
/// For complex access patterns involving [`SystemParam`](crate::system::SystemParam),
/// consider using [`SystemState`](crate::system::SystemState).
///
/// To mutate different parts of the world simultaneously,
/// use [`World::resource_scope`] or [`SystemState`](crate::system::SystemState).
///
/// ## Resources
///
/// Worlds can also store [`Resource`]s,
/// which are unique instances of a given type that don't belong to a specific Entity.
/// There are also *non send resources*, which can only be accessed on the main thread.
/// See [`Resource`] for usage.
pub struct World {
    id: WorldId,
    pub(crate) entities: Entities,
    pub(crate) components: Components,
    pub(crate) component_ids: ComponentIds,
    pub(crate) archetypes: Archetypes,
    pub(crate) storages: Storages,
    pub(crate) bundles: Bundles,
    pub(crate) observers: Observers,
    pub(crate) removed_components: RemovedComponentEvents,
    pub(crate) change_tick: AtomicU32,
    pub(crate) last_change_tick: Tick,
    pub(crate) last_check_tick: Tick,
    pub(crate) last_trigger_id: u32,
    pub(crate) command_queue: RawCommandQueue,
}

impl Default for World {
    fn default() -> Self {
        let mut world = Self {
            id: WorldId::new().expect("More `bevy` `World`s have been created than is supported"),
            entities: Entities::new(),
            components: Default::default(),
            archetypes: Archetypes::new(),
            storages: Default::default(),
            bundles: Default::default(),
            observers: Observers::default(),
            removed_components: Default::default(),
            // Default value is `1`, and `last_change_tick`s default to `0`, such that changes
            // are detected on first system runs and for direct world queries.
            change_tick: AtomicU32::new(1),
            last_change_tick: Tick::new(0),
            last_check_tick: Tick::new(0),
            last_trigger_id: 0,
            command_queue: RawCommandQueue::new(),
            component_ids: ComponentIds::default(),
        };
        world.bootstrap();
        world
    }
}

impl Drop for World {
    fn drop(&mut self) {
        // SAFETY: Not passing a pointer so the argument is always valid
        unsafe { self.command_queue.apply_or_drop_queued(None) };
        // SAFETY: Pointers in internal command queue are only invalidated here
        drop(unsafe { Box::from_raw(self.command_queue.bytes.as_ptr()) });
        // SAFETY: Pointers in internal command queue are only invalidated here
        drop(unsafe { Box::from_raw(self.command_queue.cursor.as_ptr()) });
        // SAFETY: Pointers in internal command queue are only invalidated here
        drop(unsafe { Box::from_raw(self.command_queue.panic_recovery.as_ptr()) });
    }
}

impl World {
    /// This performs initialization that _must_ happen for every [`World`] immediately upon creation (such as claiming specific component ids).
    /// This _must_ be run as part of constructing a [`World`], before it is returned to the caller.
    #[inline]
    fn bootstrap(&mut self) {
        // The order that we register these events is vital to ensure that the constants are correct!
        let on_add = self.register_event_key::<Add>();
        assert_eq!(ADD, on_add);

        let on_insert = self.register_event_key::<Insert>();
        assert_eq!(INSERT, on_insert);

        let on_replace = self.register_event_key::<Replace>();
        assert_eq!(REPLACE, on_replace);

        let on_remove = self.register_event_key::<Remove>();
        assert_eq!(REMOVE, on_remove);

        let on_despawn = self.register_event_key::<Despawn>();
        assert_eq!(DESPAWN, on_despawn);

        // This sets up `Disabled` as a disabling component, via the FromWorld impl
        self.init_resource::<DefaultQueryFilters>();
    }
    /// Creates a new empty [`World`].
    ///
    /// # Panics
    ///
    /// If [`usize::MAX`] [`World`]s have been created.
    /// This guarantee allows System Parameters to safely uniquely identify a [`World`],
    /// since its [`WorldId`] is unique
    #[inline]
    pub fn new() -> World {
        World::default()
    }

    /// Retrieves this [`World`]'s unique ID
    #[inline]
    pub fn id(&self) -> WorldId {
        self.id
    }

    /// Creates a new [`UnsafeWorldCell`] view with complete read+write access.
    #[inline]
    pub fn as_unsafe_world_cell(&mut self) -> UnsafeWorldCell<'_> {
        UnsafeWorldCell::new_mutable(self)
    }

    /// Creates a new [`UnsafeWorldCell`] view with only read access to everything.
    #[inline]
    pub fn as_unsafe_world_cell_readonly(&self) -> UnsafeWorldCell<'_> {
        UnsafeWorldCell::new_readonly(self)
    }

    /// Retrieves this world's [`Entities`] collection.
    #[inline]
    pub fn entities(&self) -> &Entities {
        &self.entities
    }

    /// Retrieves this world's [`Entities`] collection mutably.
    ///
    /// # Safety
    /// Mutable reference must not be used to put the [`Entities`] data
    /// in an invalid state for this [`World`]
    #[inline]
    pub unsafe fn entities_mut(&mut self) -> &mut Entities {
        &mut self.entities
    }

    /// Retrieves this world's [`Archetypes`] collection.
    #[inline]
    pub fn archetypes(&self) -> &Archetypes {
        &self.archetypes
    }

    /// Retrieves this world's [`Components`] collection.
    #[inline]
    pub fn components(&self) -> &Components {
        &self.components
    }

    /// Prepares a [`ComponentsQueuedRegistrator`] for the world.
    /// **NOTE:** [`ComponentsQueuedRegistrator`] is easily misused.
    /// See its docs for important notes on when and how it should be used.
    #[inline]
    pub fn components_queue(&self) -> ComponentsQueuedRegistrator<'_> {
        // SAFETY: These are from the same world.
        unsafe { ComponentsQueuedRegistrator::new(&self.components, &self.component_ids) }
    }

    /// Prepares a [`ComponentsRegistrator`] for the world.
    #[inline]
    pub fn components_registrator(&mut self) -> ComponentsRegistrator<'_> {
        // SAFETY: These are from the same world.
        unsafe { ComponentsRegistrator::new(&mut self.components, &mut self.component_ids) }
    }

    /// Retrieves this world's [`Storages`] collection.
    #[inline]
    pub fn storages(&self) -> &Storages {
        &self.storages
    }

    /// Retrieves this world's [`Bundles`] collection.
    #[inline]
    pub fn bundles(&self) -> &Bundles {
        &self.bundles
    }

    /// Retrieves this world's [`RemovedComponentEvents`] collection
    #[inline]
    pub fn removed_components(&self) -> &RemovedComponentEvents {
        &self.removed_components
    }

    /// Retrieves this world's [`Observers`] list
    #[inline]
    pub fn observers(&self) -> &Observers {
        &self.observers
    }

    /// Creates a new [`Commands`] instance that writes to the world's command queue
    /// Use [`World::flush`] to apply all queued commands
    #[inline]
    pub fn commands(&mut self) -> Commands<'_, '_> {
        // SAFETY: command_queue is stored on world and always valid while the world exists
        unsafe { Commands::new_raw_from_entities(self.command_queue.clone(), &self.entities) }
    }

    /// Registers a new [`Component`] type and returns the [`ComponentId`] created for it.
    ///
    /// # Usage Notes
    /// In most cases, you don't need to call this method directly since component registration
    /// happens automatically during system initialization.
    pub fn register_component<T: Component>(&mut self) -> ComponentId {
        self.components_registrator().register_component::<T>()
    }

    /// Registers a component type as "disabling",
    /// using [default query filters](DefaultQueryFilters) to exclude entities with the component from queries.
    pub fn register_disabling_component<C: Component>(&mut self) {
        let component_id = self.register_component::<C>();
        let mut dqf = self.resource_mut::<DefaultQueryFilters>();
        dqf.register_disabling_component(component_id);
    }

    /// Returns a mutable reference to the [`ComponentHooks`] for a [`Component`] type.
    ///
    /// Will panic if `T` exists in any archetypes.
    #[must_use]
    pub fn register_component_hooks<T: Component>(&mut self) -> &mut ComponentHooks {
        let index = self.register_component::<T>();
        assert!(!self.archetypes.archetypes.iter().any(|a| a.contains(index)), "Components hooks cannot be modified if the component already exists in an archetype, use register_component if {} may already be in use", core::any::type_name::<T>());
        // SAFETY: We just created this component
        unsafe { self.components.get_hooks_mut(index).debug_checked_unwrap() }
    }

    /// Returns a mutable reference to the [`ComponentHooks`] for a [`Component`] with the given id if it exists.
    ///
    /// Will panic if `id` exists in any archetypes.
    pub fn register_component_hooks_by_id(
        &mut self,
        id: ComponentId,
    ) -> Option<&mut ComponentHooks> {
        assert!(!self.archetypes.archetypes.iter().any(|a| a.contains(id)), "Components hooks cannot be modified if the component already exists in an archetype, use register_component if the component with id {id:?} may already be in use");
        self.components.get_hooks_mut(id)
    }

    /// Registers the given component `R` as a [required component] for `T`.
    ///
    /// When `T` is added to an entity, `R` and its own required components will also be added
    /// if `R` was not already provided. The [`Default`] `constructor` will be used for the creation of `R`.
    /// If a custom constructor is desired, use [`World::register_required_components_with`] instead.
    ///
    /// For the non-panicking version, see [`World::try_register_required_components`].
    ///
    /// Note that requirements must currently be registered before `T` is inserted into the world
    /// for the first time. This limitation may be fixed in the future.
    ///
    /// [required component]: Component#required-components
    ///
    /// # Panics
    ///
    /// Panics if `R` is already a directly required component for `T`, or if `T` has ever been added
    /// on an entity before the registration.
    ///
    /// Indirect requirements through other components are allowed. In those cases, any existing requirements
    /// will only be overwritten if the new requirement is more specific.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct A;
    ///
    /// #[derive(Component, Default, PartialEq, Eq, Debug)]
    /// struct B(usize);
    ///
    /// #[derive(Component, Default, PartialEq, Eq, Debug)]
    /// struct C(u32);
    ///
    /// # let mut world = World::default();
    /// // Register B as required by A and C as required by B.
    /// world.register_required_components::<A, B>();
    /// world.register_required_components::<B, C>();
    ///
    /// // This will implicitly also insert B and C with their Default constructors.
    /// let id = world.spawn(A).id();
    /// assert_eq!(&B(0), world.entity(id).get::<B>().unwrap());
    /// assert_eq!(&C(0), world.entity(id).get::<C>().unwrap());
    /// ```
    pub fn register_required_components<T: Component, R: Component + Default>(&mut self) {
        self.try_register_required_components::<T, R>().unwrap();
    }

    /// Registers the given component `R` as a [required component] for `T`.
    ///
    /// When `T` is added to an entity, `R` and its own required components will also be added
    /// if `R` was not already provided. The given `constructor` will be used for the creation of `R`.
    /// If a [`Default`] constructor is desired, use [`World::register_required_components`] instead.
    ///
    /// For the non-panicking version, see [`World::try_register_required_components_with`].
    ///
    /// Note that requirements must currently be registered before `T` is inserted into the world
    /// for the first time. This limitation may be fixed in the future.
    ///
    /// [required component]: Component#required-components
    ///
    /// # Panics
    ///
    /// Panics if `R` is already a directly required component for `T`, or if `T` has ever been added
    /// on an entity before the registration.
    ///
    /// Indirect requirements through other components are allowed. In those cases, any existing requirements
    /// will only be overwritten if the new requirement is more specific.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct A;
    ///
    /// #[derive(Component, Default, PartialEq, Eq, Debug)]
    /// struct B(usize);
    ///
    /// #[derive(Component, PartialEq, Eq, Debug)]
    /// struct C(u32);
    ///
    /// # let mut world = World::default();
    /// // Register B and C as required by A and C as required by B.
    /// // A requiring C directly will overwrite the indirect requirement through B.
    /// world.register_required_components::<A, B>();
    /// world.register_required_components_with::<B, C>(|| C(1));
    /// world.register_required_components_with::<A, C>(|| C(2));
    ///
    /// // This will implicitly also insert B with its Default constructor and C
    /// // with the custom constructor defined by A.
    /// let id = world.spawn(A).id();
    /// assert_eq!(&B(0), world.entity(id).get::<B>().unwrap());
    /// assert_eq!(&C(2), world.entity(id).get::<C>().unwrap());
    /// ```
    pub fn register_required_components_with<T: Component, R: Component>(
        &mut self,
        constructor: fn() -> R,
    ) {
        self.try_register_required_components_with::<T, R>(constructor)
            .unwrap();
    }

    /// Tries to register the given component `R` as a [required component] for `T`.
    ///
    /// When `T` is added to an entity, `R` and its own required components will also be added
    /// if `R` was not already provided. The [`Default`] `constructor` will be used for the creation of `R`.
    /// If a custom constructor is desired, use [`World::register_required_components_with`] instead.
    ///
    /// For the panicking version, see [`World::register_required_components`].
    ///
    /// Note that requirements must currently be registered before `T` is inserted into the world
    /// for the first time. This limitation may be fixed in the future.
    ///
    /// [required component]: Component#required-components
    ///
    /// # Errors
    ///
    /// Returns a [`RequiredComponentsError`] if `R` is already a directly required component for `T`, or if `T` has ever been added
    /// on an entity before the registration.
    ///
    /// Indirect requirements through other components are allowed. In those cases, any existing requirements
    /// will only be overwritten if the new requirement is more specific.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct A;
    ///
    /// #[derive(Component, Default, PartialEq, Eq, Debug)]
    /// struct B(usize);
    ///
    /// #[derive(Component, Default, PartialEq, Eq, Debug)]
    /// struct C(u32);
    ///
    /// # let mut world = World::default();
    /// // Register B as required by A and C as required by B.
    /// world.register_required_components::<A, B>();
    /// world.register_required_components::<B, C>();
    ///
    /// // Duplicate registration! This will fail.
    /// assert!(world.try_register_required_components::<A, B>().is_err());
    ///
    /// // This will implicitly also insert B and C with their Default constructors.
    /// let id = world.spawn(A).id();
    /// assert_eq!(&B(0), world.entity(id).get::<B>().unwrap());
    /// assert_eq!(&C(0), world.entity(id).get::<C>().unwrap());
    /// ```
    pub fn try_register_required_components<T: Component, R: Component + Default>(
        &mut self,
    ) -> Result<(), RequiredComponentsError> {
        self.try_register_required_components_with::<T, R>(R::default)
    }

    /// Tries to register the given component `R` as a [required component] for `T`.
    ///
    /// When `T` is added to an entity, `R` and its own required components will also be added
    /// if `R` was not already provided. The given `constructor` will be used for the creation of `R`.
    /// If a [`Default`] constructor is desired, use [`World::register_required_components`] instead.
    ///
    /// For the panicking version, see [`World::register_required_components_with`].
    ///
    /// Note that requirements must currently be registered before `T` is inserted into the world
    /// for the first time. This limitation may be fixed in the future.
    ///
    /// [required component]: Component#required-components
    ///
    /// # Errors
    ///
    /// Returns a [`RequiredComponentsError`] if `R` is already a directly required component for `T`, or if `T` has ever been added
    /// on an entity before the registration.
    ///
    /// Indirect requirements through other components are allowed. In those cases, any existing requirements
    /// will only be overwritten if the new requirement is more specific.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct A;
    ///
    /// #[derive(Component, Default, PartialEq, Eq, Debug)]
    /// struct B(usize);
    ///
    /// #[derive(Component, PartialEq, Eq, Debug)]
    /// struct C(u32);
    ///
    /// # let mut world = World::default();
    /// // Register B and C as required by A and C as required by B.
    /// // A requiring C directly will overwrite the indirect requirement through B.
    /// world.register_required_components::<A, B>();
    /// world.register_required_components_with::<B, C>(|| C(1));
    /// world.register_required_components_with::<A, C>(|| C(2));
    ///
    /// // Duplicate registration! Even if the constructors were different, this would fail.
    /// assert!(world.try_register_required_components_with::<B, C>(|| C(1)).is_err());
    ///
    /// // This will implicitly also insert B with its Default constructor and C
    /// // with the custom constructor defined by A.
    /// let id = world.spawn(A).id();
    /// assert_eq!(&B(0), world.entity(id).get::<B>().unwrap());
    /// assert_eq!(&C(2), world.entity(id).get::<C>().unwrap());
    /// ```
    pub fn try_register_required_components_with<T: Component, R: Component>(
        &mut self,
        constructor: fn() -> R,
    ) -> Result<(), RequiredComponentsError> {
        let requiree = self.register_component::<T>();

        // TODO: Remove this panic and update archetype edges accordingly when required components are added
        if self.archetypes().component_index().contains_key(&requiree) {
            return Err(RequiredComponentsError::ArchetypeExists(requiree));
        }

        let required = self.register_component::<R>();

        // SAFETY: We just created the `required` and `requiree` components.
        unsafe {
            self.components
                .register_required_components::<R>(requiree, required, constructor)
        }
    }

    /// Retrieves the [required components](RequiredComponents) for the given component type, if it exists.
    pub fn get_required_components<C: Component>(&self) -> Option<&RequiredComponents> {
        let id = self.components().valid_component_id::<C>()?;
        let component_info = self.components().get_info(id)?;
        Some(component_info.required_components())
    }

    /// Retrieves the [required components](RequiredComponents) for the component of the given [`ComponentId`], if it exists.
    pub fn get_required_components_by_id(&self, id: ComponentId) -> Option<&RequiredComponents> {
        let component_info = self.components().get_info(id)?;
        Some(component_info.required_components())
    }

    /// Registers a new [`Component`] type and returns the [`ComponentId`] created for it.
    ///
    /// This method differs from [`World::register_component`] in that it uses a [`ComponentDescriptor`]
    /// to register the new component type instead of statically available type information. This
    /// enables the dynamic registration of new component definitions at runtime for advanced use cases.
    ///
    /// While the option to register a component from a descriptor is useful in type-erased
    /// contexts, the standard [`World::register_component`] function should always be used instead
    /// when type information is available at compile time.
    pub fn register_component_with_descriptor(
        &mut self,
        descriptor: ComponentDescriptor,
    ) -> ComponentId {
        self.components_registrator()
            .register_component_with_descriptor(descriptor)
    }

    /// Returns the [`ComponentId`] of the given [`Component`] type `T`.
    ///
    /// The returned `ComponentId` is specific to the `World` instance
    /// it was retrieved from and should not be used with another `World` instance.
    ///
    /// Returns [`None`] if the `Component` type has not yet been initialized within
    /// the `World` using [`World::register_component`].
    ///
    /// ```
    /// use bevy_ecs::prelude::*;
    ///
    /// let mut world = World::new();
    ///
    /// #[derive(Component)]
    /// struct ComponentA;
    ///
    /// let component_a_id = world.register_component::<ComponentA>();
    ///
    /// assert_eq!(component_a_id, world.component_id::<ComponentA>().unwrap())
    /// ```
    ///
    /// # See also
    ///
    /// * [`ComponentIdFor`](crate::component::ComponentIdFor)
    /// * [`Components::component_id()`]
    /// * [`Components::get_id()`]
    #[inline]
    pub fn component_id<T: Component>(&self) -> Option<ComponentId> {
        self.components.component_id::<T>()
    }

    /// Registers a new [`Resource`] type and returns the [`ComponentId`] created for it.
    ///
    /// The [`Resource`] doesn't have a value in the [`World`], it's only registered. If you want
    /// to insert the [`Resource`] in the [`World`], use [`World::init_resource`] or
    /// [`World::insert_resource`] instead.
    pub fn register_resource<R: Resource>(&mut self) -> ComponentId {
        self.components_registrator().register_resource::<R>()
    }

    /// Returns the [`ComponentId`] of the given [`Resource`] type `T`.
    ///
    /// The returned [`ComponentId`] is specific to the [`World`] instance it was retrieved from
    /// and should not be used with another [`World`] instance.
    ///
    /// Returns [`None`] if the [`Resource`] type has not yet been initialized within the
    /// [`World`] using [`World::register_resource`], [`World::init_resource`] or [`World::insert_resource`].
    pub fn resource_id<T: Resource>(&self) -> Option<ComponentId> {
        self.components.get_resource_id(TypeId::of::<T>())
    }

    /// Returns [`EntityRef`]s that expose read-only operations for the given
    /// `entities`. This will panic if any of the given entities do not exist. Use
    /// [`World::get_entity`] if you want to check for entity existence instead
    /// of implicitly panicking.
    ///
    /// This function supports fetching a single entity or multiple entities:
    /// - Pass an [`Entity`] to receive a single [`EntityRef`].
    /// - Pass a slice of [`Entity`]s to receive a [`Vec<EntityRef>`].
    /// - Pass an array of [`Entity`]s to receive an equally-sized array of [`EntityRef`]s.
    ///
    /// # Panics
    ///
    /// If any of the given `entities` do not exist in the world.
    ///
    /// # Examples
    ///
    /// ## Single [`Entity`]
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let entity = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    ///
    /// let position = world.entity(entity).get::<Position>().unwrap();
    /// assert_eq!(position.x, 0.0);
    /// ```
    ///
    /// ## Array of [`Entity`]s
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    /// let e2 = world.spawn(Position { x: 1.0, y: 1.0 }).id();
    ///
    /// let [e1_ref, e2_ref] = world.entity([e1, e2]);
    /// let e1_position = e1_ref.get::<Position>().unwrap();
    /// assert_eq!(e1_position.x, 0.0);
    /// let e2_position = e2_ref.get::<Position>().unwrap();
    /// assert_eq!(e2_position.x, 1.0);
    /// ```
    ///
    /// ## Slice of [`Entity`]s
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e2 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e3 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    ///
    /// let ids = vec![e1, e2, e3];
    /// for eref in world.entity(&ids[..]) {
    ///     assert_eq!(eref.get::<Position>().unwrap().y, 1.0);
    /// }
    /// ```
    ///
    /// ## [`EntityHashSet`](crate::entity::EntityHashSet)
    ///
    /// ```
    /// # use bevy_ecs::{prelude::*, entity::EntityHashSet};
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e2 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e3 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    ///
    /// let ids = EntityHashSet::from_iter([e1, e2, e3]);
    /// for (_id, eref) in world.entity(&ids) {
    ///     assert_eq!(eref.get::<Position>().unwrap().y, 1.0);
    /// }
    /// ```
    ///
    /// [`EntityHashSet`]: crate::entity::EntityHashSet
    #[inline]
    #[track_caller]
    pub fn entity<F: WorldEntityFetch>(&self, entities: F) -> F::Ref<'_> {
        #[inline(never)]
        #[cold]
        #[track_caller]
        fn panic_no_entity(world: &World, entity: Entity) -> ! {
            panic!(
                "Entity {entity} {}",
                world.entities.entity_does_not_exist_error_details(entity)
            );
        }

        match self.get_entity(entities) {
            Ok(fetched) => fetched,
            Err(error) => panic_no_entity(self, error.entity),
        }
    }

    /// Returns [`EntityMut`]s that expose read and write operations for the
    /// given `entities`. This will panic if any of the given entities do not
    /// exist. Use [`World::get_entity_mut`] if you want to check for entity
    /// existence instead of implicitly panicking.
    ///
    /// This function supports fetching a single entity or multiple entities:
    /// - Pass an [`Entity`] to receive a single [`EntityWorldMut`].
    ///    - This reference type allows for structural changes to the entity,
    ///      such as adding or removing components, or despawning the entity.
    /// - Pass a slice of [`Entity`]s to receive a [`Vec<EntityMut>`].
    /// - Pass an array of [`Entity`]s to receive an equally-sized array of [`EntityMut`]s.
    /// - Pass a reference to a [`EntityHashSet`](crate::entity::EntityHashMap) to receive an
    ///   [`EntityHashMap<EntityMut>`](crate::entity::EntityHashMap).
    ///
    /// In order to perform structural changes on the returned entity reference,
    /// such as adding or removing components, or despawning the entity, only a
    /// single [`Entity`] can be passed to this function. Allowing multiple
    /// entities at the same time with structural access would lead to undefined
    /// behavior, so [`EntityMut`] is returned when requesting multiple entities.
    ///
    /// # Panics
    ///
    /// If any of the given `entities` do not exist in the world.
    ///
    /// # Examples
    ///
    /// ## Single [`Entity`]
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let entity = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    ///
    /// let mut entity_mut = world.entity_mut(entity);
    /// let mut position = entity_mut.get_mut::<Position>().unwrap();
    /// position.y = 1.0;
    /// assert_eq!(position.x, 0.0);
    /// entity_mut.despawn();
    /// # assert!(world.get_entity_mut(entity).is_err());
    /// ```
    ///
    /// ## Array of [`Entity`]s
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    /// let e2 = world.spawn(Position { x: 1.0, y: 1.0 }).id();
    ///
    /// let [mut e1_ref, mut e2_ref] = world.entity_mut([e1, e2]);
    /// let mut e1_position = e1_ref.get_mut::<Position>().unwrap();
    /// e1_position.x = 1.0;
    /// assert_eq!(e1_position.x, 1.0);
    /// let mut e2_position = e2_ref.get_mut::<Position>().unwrap();
    /// e2_position.x = 2.0;
    /// assert_eq!(e2_position.x, 2.0);
    /// ```
    ///
    /// ## Slice of [`Entity`]s
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e2 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e3 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    ///
    /// let ids = vec![e1, e2, e3];
    /// for mut eref in world.entity_mut(&ids[..]) {
    ///     let mut pos = eref.get_mut::<Position>().unwrap();
    ///     pos.y = 2.0;
    ///     assert_eq!(pos.y, 2.0);
    /// }
    /// ```
    ///
    /// ## [`EntityHashSet`](crate::entity::EntityHashSet)
    ///
    /// ```
    /// # use bevy_ecs::{prelude::*, entity::EntityHashSet};
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e2 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e3 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    ///
    /// let ids = EntityHashSet::from_iter([e1, e2, e3]);
    /// for (_id, mut eref) in world.entity_mut(&ids) {
    ///     let mut pos = eref.get_mut::<Position>().unwrap();
    ///     pos.y = 2.0;
    ///     assert_eq!(pos.y, 2.0);
    /// }
    /// ```
    ///
    /// [`EntityHashSet`]: crate::entity::EntityHashSet
    #[inline]
    #[track_caller]
    pub fn entity_mut<F: WorldEntityFetch>(&mut self, entities: F) -> F::Mut<'_> {
        #[inline(never)]
        #[cold]
        #[track_caller]
        fn panic_on_err(e: EntityMutableFetchError) -> ! {
            panic!("{e}");
        }

        match self.get_entity_mut(entities) {
            Ok(fetched) => fetched,
            Err(e) => panic_on_err(e),
        }
    }

    /// Returns the components of an [`Entity`] through [`ComponentInfo`].
    #[inline]
    pub fn inspect_entity(
        &self,
        entity: Entity,
    ) -> Result<impl Iterator<Item = &ComponentInfo>, EntityDoesNotExistError> {
        let entity_location = self
            .entities()
            .get(entity)
            .ok_or(EntityDoesNotExistError::new(entity, self.entities()))?;

        let archetype = self
            .archetypes()
            .get(entity_location.archetype_id)
            .expect("ArchetypeId was retrieved from an EntityLocation and should correspond to an Archetype");

        Ok(archetype
            .iter_components()
            .filter_map(|id| self.components().get_info(id)))
    }

    /// Returns [`EntityRef`]s that expose read-only operations for the given
    /// `entities`, returning [`Err`] if any of the given entities do not exist.
    /// Instead of immediately unwrapping the value returned from this function,
    /// prefer [`World::entity`].
    ///
    /// This function supports fetching a single entity or multiple entities:
    /// - Pass an [`Entity`] to receive a single [`EntityRef`].
    /// - Pass a slice of [`Entity`]s to receive a [`Vec<EntityRef>`].
    /// - Pass an array of [`Entity`]s to receive an equally-sized array of [`EntityRef`]s.
    /// - Pass a reference to a [`EntityHashSet`](crate::entity::EntityHashMap) to receive an
    ///   [`EntityHashMap<EntityRef>`](crate::entity::EntityHashMap).
    ///
    /// # Errors
    ///
    /// If any of the given `entities` do not exist in the world, the first
    /// [`Entity`] found to be missing will return an [`EntityDoesNotExistError`].
    ///
    /// # Examples
    ///
    /// For examples, see [`World::entity`].
    ///
    /// [`EntityHashSet`]: crate::entity::EntityHashSet
    #[inline]
    pub fn get_entity<F: WorldEntityFetch>(
        &self,
        entities: F,
    ) -> Result<F::Ref<'_>, EntityDoesNotExistError> {
        let cell = self.as_unsafe_world_cell_readonly();
        // SAFETY: `&self` gives read access to the entire world, and prevents mutable access.
        unsafe { entities.fetch_ref(cell) }
    }

    /// Returns [`EntityMut`]s that expose read and write operations for the
    /// given `entities`, returning [`Err`] if any of the given entities do not
    /// exist. Instead of immediately unwrapping the value returned from this
    /// function, prefer [`World::entity_mut`].
    ///
    /// This function supports fetching a single entity or multiple entities:
    /// - Pass an [`Entity`] to receive a single [`EntityWorldMut`].
    ///    - This reference type allows for structural changes to the entity,
    ///      such as adding or removing components, or despawning the entity.
    /// - Pass a slice of [`Entity`]s to receive a [`Vec<EntityMut>`].
    /// - Pass an array of [`Entity`]s to receive an equally-sized array of [`EntityMut`]s.
    /// - Pass a reference to a [`EntityHashSet`](crate::entity::EntityHashMap) to receive an
    ///   [`EntityHashMap<EntityMut>`](crate::entity::EntityHashMap).
    ///
    /// In order to perform structural changes on the returned entity reference,
    /// such as adding or removing components, or despawning the entity, only a
    /// single [`Entity`] can be passed to this function. Allowing multiple
    /// entities at the same time with structural access would lead to undefined
    /// behavior, so [`EntityMut`] is returned when requesting multiple entities.
    ///
    /// # Errors
    ///
    /// - Returns [`EntityMutableFetchError::EntityDoesNotExist`] if any of the given `entities` do not exist in the world.
    ///     - Only the first entity found to be missing will be returned.
    /// - Returns [`EntityMutableFetchError::AliasedMutability`] if the same entity is requested multiple times.
    ///
    /// # Examples
    ///
    /// For examples, see [`World::entity_mut`].
    ///
    /// [`EntityHashSet`]: crate::entity::EntityHashSet
    #[inline]
    pub fn get_entity_mut<F: WorldEntityFetch>(
        &mut self,
        entities: F,
    ) -> Result<F::Mut<'_>, EntityMutableFetchError> {
        let cell = self.as_unsafe_world_cell();
        // SAFETY: `&mut self` gives mutable access to the entire world,
        // and prevents any other access to the world.
        unsafe { entities.fetch_mut(cell) }
    }

    /// Returns an [`Entity`] iterator of current entities.
    ///
    /// This is useful in contexts where you only have read-only access to the [`World`].
    #[deprecated(since = "0.17.0", note = "use world.query::<EntityRef>()` instead")]
    #[inline]
    pub fn iter_entities(&self) -> impl Iterator<Item = EntityRef<'_>> + '_ {
        self.archetypes.iter().flat_map(|archetype| {
            archetype
                .entities_with_location()
                .map(|(entity, location)| {
                    // SAFETY: entity exists and location accurately specifies the archetype where the entity is stored.
                    let cell = UnsafeEntityCell::new(
                        self.as_unsafe_world_cell_readonly(),
                        entity,
                        location,
                        self.last_change_tick,
                        self.read_change_tick(),
                    );
                    // SAFETY: `&self` gives read access to the entire world.
                    unsafe { EntityRef::new(cell) }
                })
        })
    }

    /// Returns a mutable iterator over all entities in the `World`.
    #[deprecated(since = "0.17.0", note = "use world.query::<EntityMut>()` instead")]
    pub fn iter_entities_mut(&mut self) -> impl Iterator<Item = EntityMut<'_>> + '_ {
        let last_change_tick = self.last_change_tick;
        let change_tick = self.change_tick();
        let world_cell = self.as_unsafe_world_cell();
        world_cell.archetypes().iter().flat_map(move |archetype| {
            archetype
                .entities_with_location()
                .map(move |(entity, location)| {
                    // SAFETY: entity exists and location accurately specifies the archetype where the entity is stored.
                    let cell = UnsafeEntityCell::new(
                        world_cell,
                        entity,
                        location,
                        last_change_tick,
                        change_tick,
                    );
                    // SAFETY: We have exclusive access to the entire world. We only create one borrow for each entity,
                    // so none will conflict with one another.
                    unsafe { EntityMut::new(cell) }
                })
        })
    }

    /// Simultaneously provides access to entity data and a command queue, which
    /// will be applied when the world is next flushed.
    ///
    /// This allows using borrowed entity data to construct commands where the
    /// borrow checker would otherwise prevent it.
    ///
    /// See [`DeferredWorld::entities_and_commands`] for the deferred version.
    ///
    /// # Example
    ///
    /// ```rust
    /// # use bevy_ecs::{prelude::*, world::DeferredWorld};
    /// #[derive(Component)]
    /// struct Targets(Vec<Entity>);
    /// #[derive(Component)]
    /// struct TargetedBy(Entity);
    ///
    /// let mut world: World = // ...
    /// #    World::new();
    /// # let e1 = world.spawn_empty().id();
    /// # let e2 = world.spawn_empty().id();
    /// # let eid = world.spawn(Targets(vec![e1, e2])).id();
    /// let (entities, mut commands) = world.entities_and_commands();
    ///
    /// let entity = entities.get(eid).unwrap();
    /// for &target in entity.get::<Targets>().unwrap().0.iter() {
    ///     commands.entity(target).insert(TargetedBy(eid));
    /// }
    /// # world.flush();
    /// # assert_eq!(world.get::<TargetedBy>(e1).unwrap().0, eid);
    /// # assert_eq!(world.get::<TargetedBy>(e2).unwrap().0, eid);
    /// ```
    pub fn entities_and_commands(&mut self) -> (EntityFetcher<'_>, Commands<'_, '_>) {
        let cell = self.as_unsafe_world_cell();
        // SAFETY: `&mut self` gives mutable access to the entire world, and prevents simultaneous access.
        let fetcher = unsafe { EntityFetcher::new(cell) };
        // SAFETY:
        // - `&mut self` gives mutable access to the entire world, and prevents simultaneous access.
        // - Command queue access does not conflict with entity access.
        let raw_queue = unsafe { cell.get_raw_command_queue() };
        // SAFETY: `&mut self` ensures the commands does not outlive the world.
        let commands = unsafe { Commands::new_raw_from_entities(raw_queue, cell.entities()) };

        (fetcher, commands)
    }

    /// Spawns a new [`Entity`] and returns a corresponding [`EntityWorldMut`], which can be used
    /// to add components to the entity or retrieve its id.
    ///
    /// ```
    /// use bevy_ecs::{component::Component, world::World};
    ///
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    /// #[derive(Component)]
    /// struct Label(&'static str);
    /// #[derive(Component)]
    /// struct Num(u32);
    ///
    /// let mut world = World::new();
    /// let entity = world.spawn_empty()
    ///     .insert(Position { x: 0.0, y: 0.0 }) // add a single component
    ///     .insert((Num(1), Label("hello"))) // add a bundle of components
    ///     .id();
    ///
    /// let position = world.entity(entity).get::<Position>().unwrap();
    /// assert_eq!(position.x, 0.0);
    /// ```
    #[track_caller]
    pub fn spawn_empty(&mut self) -> EntityWorldMut<'_> {
        self.flush();
        let entity = self.entities.alloc();
        // SAFETY: entity was just allocated
        unsafe { self.spawn_at_empty_internal(entity, MaybeLocation::caller()) }
    }

    /// Spawns a new [`Entity`] with a given [`Bundle`] of [components](`Component`) and returns
    /// a corresponding [`EntityWorldMut`], which can be used to add components to the entity or
    /// retrieve its id. In case large batches of entities need to be spawned, consider using
    /// [`World::spawn_batch`] instead.
    ///
    /// ```
    /// use bevy_ecs::{bundle::Bundle, component::Component, world::World};
    ///
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// #[derive(Component)]
    /// struct Velocity {
    ///     x: f32,
    ///     y: f32,
    /// };
    ///
    /// #[derive(Component)]
    /// struct Name(&'static str);
    ///
    /// #[derive(Bundle)]
    /// struct PhysicsBundle {
    ///     position: Position,
    ///     velocity: Velocity,
    /// }
    ///
    /// let mut world = World::new();
    ///
    /// // `spawn` can accept a single component:
    /// world.spawn(Position { x: 0.0, y: 0.0 });
    ///
    /// // It can also accept a tuple of components:
    /// world.spawn((
    ///     Position { x: 0.0, y: 0.0 },
    ///     Velocity { x: 1.0, y: 1.0 },
    /// ));
    ///
    /// // Or it can accept a pre-defined Bundle of components:
    /// world.spawn(PhysicsBundle {
    ///     position: Position { x: 2.0, y: 2.0 },
    ///     velocity: Velocity { x: 0.0, y: 4.0 },
    /// });
    ///
    /// let entity = world
    ///     // Tuples can also mix Bundles and Components
    ///     .spawn((
    ///         PhysicsBundle {
    ///             position: Position { x: 2.0, y: 2.0 },
    ///             velocity: Velocity { x: 0.0, y: 4.0 },
    ///         },
    ///         Name("Elaina Proctor"),
    ///     ))
    ///     // Calling id() will return the unique identifier for the spawned entity
    ///     .id();
    /// let position = world.entity(entity).get::<Position>().unwrap();
    /// assert_eq!(position.x, 2.0);
    /// ```
    #[track_caller]
    pub fn spawn<B: Bundle>(&mut self, bundle: B) -> EntityWorldMut<'_> {
        self.spawn_with_caller(bundle, MaybeLocation::caller())
    }

    pub(crate) fn spawn_with_caller<B: Bundle>(
        &mut self,
        bundle: B,
        caller: MaybeLocation,
    ) -> EntityWorldMut<'_> {
        self.flush();
        let change_tick = self.change_tick();
        let entity = self.entities.alloc();
        let mut bundle_spawner = BundleSpawner::new::<B>(self, change_tick);
        // SAFETY: bundle's type matches `bundle_info`, entity is allocated but non-existent
        let (entity_location, after_effect) =
            unsafe { bundle_spawner.spawn_non_existent(entity, bundle, caller) };

        let mut entity_location = Some(entity_location);

        // SAFETY: command_queue is not referenced anywhere else
        if !unsafe { self.command_queue.is_empty() } {
            self.flush();
            entity_location = self.entities().get(entity);
        }

        // SAFETY: entity and location are valid, as they were just created above
        let mut entity = unsafe { EntityWorldMut::new(self, entity, entity_location) };
        after_effect.apply(&mut entity);
        entity
    }

    /// # Safety
    /// must be called on an entity that was just allocated
    unsafe fn spawn_at_empty_internal(
        &mut self,
        entity: Entity,
        caller: MaybeLocation,
    ) -> EntityWorldMut<'_> {
        let archetype = self.archetypes.empty_mut();
        // PERF: consider avoiding allocating entities in the empty archetype unless needed
        let table_row = self.storages.tables[archetype.table_id()].allocate(entity);
        // SAFETY: no components are allocated by archetype.allocate() because the archetype is
        // empty
        let location = unsafe { archetype.allocate(entity, table_row) };
        let change_tick = self.change_tick();
        self.entities.set(entity.index(), Some(location));
        self.entities
            .mark_spawn_despawn(entity.index(), caller, change_tick);

        EntityWorldMut::new(self, entity, Some(location))
    }

    /// Spawns a batch of entities with the same component [`Bundle`] type. Takes a given
    /// [`Bundle`] iterator and returns a corresponding [`Entity`] iterator.
    /// This is more efficient than spawning entities and adding components to them individually
    /// using [`World::spawn`], but it is limited to spawning entities with the same [`Bundle`]
    /// type, whereas spawning individually is more flexible.
    ///
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World};
    ///
    /// #[derive(Component)]
    /// struct Str(&'static str);
    /// #[derive(Component)]
    /// struct Num(u32);
    ///
    /// let mut world = World::new();
    /// let entities = world.spawn_batch(vec![
    ///   (Str("a"), Num(0)), // the first entity
    ///   (Str("b"), Num(1)), // the second entity
    /// ]).collect::<Vec<Entity>>();
    ///
    /// assert_eq!(entities.len(), 2);
    /// ```
    #[track_caller]
    pub fn spawn_batch<I>(&mut self, iter: I) -> SpawnBatchIter<'_, I::IntoIter>
    where
        I: IntoIterator,
        I::Item: Bundle<Effect: NoBundleEffect>,
    {
        SpawnBatchIter::new(self, iter.into_iter(), MaybeLocation::caller())
    }

    /// Retrieves a reference to the given `entity`'s [`Component`] of the given type.
    /// Returns `None` if the `entity` does not have a [`Component`] of the given type.
    /// ```
    /// use bevy_ecs::{component::Component, world::World};
    ///
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let entity = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    /// let position = world.get::<Position>(entity).unwrap();
    /// assert_eq!(position.x, 0.0);
    /// ```
    #[inline]
    pub fn get<T: Component>(&self, entity: Entity) -> Option<&T> {
        self.get_entity(entity).ok()?.get()
    }

    /// Retrieves a mutable reference to the given `entity`'s [`Component`] of the given type.
    /// Returns `None` if the `entity` does not have a [`Component`] of the given type.
    /// ```
    /// use bevy_ecs::{component::Component, world::World};
    ///
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let entity = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    /// let mut position = world.get_mut::<Position>(entity).unwrap();
    /// position.x = 1.0;
    /// ```
    #[inline]
    pub fn get_mut<T: Component<Mutability = Mutable>>(
        &mut self,
        entity: Entity,
    ) -> Option<Mut<'_, T>> {
        self.get_entity_mut(entity).ok()?.into_mut()
    }

    /// Temporarily removes a [`Component`] `T` from the provided [`Entity`] and
    /// runs the provided closure on it, returning the result if `T` was available.
    /// This will trigger the `Remove` and `Replace` component hooks without
    /// causing an archetype move.
    ///
    /// This is most useful with immutable components, where removal and reinsertion
    /// is the only way to modify a value.
    ///
    /// If you do not need to ensure the above hooks are triggered, and your component
    /// is mutable, prefer using [`get_mut`](World::get_mut).
    ///
    /// # Examples
    ///
    /// ```rust
    /// # use bevy_ecs::prelude::*;
    /// #
    /// #[derive(Component, PartialEq, Eq, Debug)]
    /// #[component(immutable)]
    /// struct Foo(bool);
    ///
    /// # let mut world = World::default();
    /// # world.register_component::<Foo>();
    /// #
    /// # let entity = world.spawn(Foo(false)).id();
    /// #
    /// world.modify_component(entity, |foo: &mut Foo| {
    ///     foo.0 = true;
    /// });
    /// #
    /// # assert_eq!(world.get::<Foo>(entity), Some(&Foo(true)));
    /// ```
    #[inline]
    #[track_caller]
    pub fn modify_component<T: Component, R>(
        &mut self,
        entity: Entity,
        f: impl FnOnce(&mut T) -> R,
    ) -> Result<Option<R>, EntityMutableFetchError> {
        let mut world = DeferredWorld::from(&mut *self);

        let result = world.modify_component_with_relationship_hook_mode(
            entity,
            RelationshipHookMode::Run,
            f,
        )?;

        self.flush();
        Ok(result)
    }

    /// Temporarily removes a [`Component`] identified by the provided
    /// [`ComponentId`] from the provided [`Entity`] and runs the provided
    /// closure on it, returning the result if the component was available.
    /// This will trigger the `Remove` and `Replace` component hooks without
    /// causing an archetype move.
    ///
    /// This is most useful with immutable components, where removal and reinsertion
    /// is the only way to modify a value.
    ///
    /// If you do not need to ensure the above hooks are triggered, and your component
    /// is mutable, prefer using [`get_mut_by_id`](World::get_mut_by_id).
    ///
    /// You should prefer the typed [`modify_component`](World::modify_component)
    /// whenever possible.
    #[inline]
    #[track_caller]
    pub fn modify_component_by_id<R>(
        &mut self,
        entity: Entity,
        component_id: ComponentId,
        f: impl for<'a> FnOnce(MutUntyped<'a>) -> R,
    ) -> Result<Option<R>, EntityMutableFetchError> {
        let mut world = DeferredWorld::from(&mut *self);

        let result = world.modify_component_by_id_with_relationship_hook_mode(
            entity,
            component_id,
            RelationshipHookMode::Run,
            f,
        )?;

        self.flush();
        Ok(result)
    }

    /// Despawns the given [`Entity`], if it exists. This will also remove all of the entity's
    /// [`Components`](Component).
    ///
    /// Returns `true` if the entity is successfully despawned and `false` if
    /// the entity does not exist.
    ///
    /// # Note
    ///
    /// This will also despawn the entities in any [`RelationshipTarget`](crate::relationship::RelationshipTarget) that is configured
    /// to despawn descendants. For example, this will recursively despawn [`Children`](crate::hierarchy::Children).
    ///
    /// ```
    /// use bevy_ecs::{component::Component, world::World};
    ///
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let entity = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    /// assert!(world.despawn(entity));
    /// assert!(world.get_entity(entity).is_err());
    /// assert!(world.get::<Position>(entity).is_none());
    /// ```
    #[track_caller]
    #[inline]
    pub fn despawn(&mut self, entity: Entity) -> bool {
        if let Err(error) = self.despawn_with_caller(entity, MaybeLocation::caller()) {
            warn!("{error}");
            false
        } else {
            true
        }
    }

    /// Despawns the given `entity`, if it exists. This will also remove all of the entity's
    /// [`Components`](Component).
    ///
    /// Returns an [`EntityDespawnError`] if the entity does not exist.
    ///
    /// # Note
    ///
    /// This will also despawn the entities in any [`RelationshipTarget`](crate::relationship::RelationshipTarget) that is configured
    /// to despawn descendants. For example, this will recursively despawn [`Children`](crate::hierarchy::Children).
    #[track_caller]
    #[inline]
    pub fn try_despawn(&mut self, entity: Entity) -> Result<(), EntityDespawnError> {
        self.despawn_with_caller(entity, MaybeLocation::caller())
    }

    #[inline]
    pub(crate) fn despawn_with_caller(
        &mut self,
        entity: Entity,
        caller: MaybeLocation,
    ) -> Result<(), EntityDespawnError> {
        self.flush();
        let entity = self.get_entity_mut(entity)?;
        entity.despawn_with_caller(caller);
        Ok(())
    }

    /// Clears the internal component tracker state.
    ///
    /// The world maintains some internal state about changed and removed components. This state
    /// is used by [`RemovedComponents`] to provide access to the entities that had a specific type
    /// of component removed since last tick.
    ///
    /// The state is also used for change detection when accessing components and resources outside
    /// of a system, for example via [`World::get_mut()`] or [`World::get_resource_mut()`].
    ///
    /// By clearing this internal state, the world "forgets" about those changes, allowing a new round
    /// of detection to be recorded.
    ///
    /// When using `bevy_ecs` as part of the full Bevy engine, this method is called automatically
    /// by `bevy_app::App::update` and `bevy_app::SubApp::update`, so you don't need to call it manually.
    /// When using `bevy_ecs` as a separate standalone crate however, you do need to call this manually.
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// # #[derive(Component, Default)]
    /// # struct Transform;
    /// // a whole new world
    /// let mut world = World::new();
    ///
    /// // you changed it
    /// let entity = world.spawn(Transform::default()).id();
    ///
    /// // change is detected
    /// let transform = world.get_mut::<Transform>(entity).unwrap();
    /// assert!(transform.is_changed());
    ///
    /// // update the last change tick
    /// world.clear_trackers();
    ///
    /// // change is no longer detected
    /// let transform = world.get_mut::<Transform>(entity).unwrap();
    /// assert!(!transform.is_changed());
    /// ```
    ///
    /// [`RemovedComponents`]: crate::lifecycle::RemovedComponents
    pub fn clear_trackers(&mut self) {
        self.removed_components.update();
        self.last_change_tick = self.increment_change_tick();
    }

    /// Returns [`QueryState`] for the given [`QueryData`], which is used to efficiently
    /// run queries on the [`World`] by storing and reusing the [`QueryState`].
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World};
    ///
    /// #[derive(Component, Debug, PartialEq)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// #[derive(Component)]
    /// struct Velocity {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let entities = world.spawn_batch(vec![
    ///     (Position { x: 0.0, y: 0.0}, Velocity { x: 1.0, y: 0.0 }),
    ///     (Position { x: 0.0, y: 0.0}, Velocity { x: 0.0, y: 1.0 }),
    /// ]).collect::<Vec<Entity>>();
    ///
    /// let mut query = world.query::<(&mut Position, &Velocity)>();
    /// for (mut position, velocity) in query.iter_mut(&mut world) {
    ///    position.x += velocity.x;
    ///    position.y += velocity.y;
    /// }
    ///
    /// assert_eq!(world.get::<Position>(entities[0]).unwrap(), &Position { x: 1.0, y: 0.0 });
    /// assert_eq!(world.get::<Position>(entities[1]).unwrap(), &Position { x: 0.0, y: 1.0 });
    /// ```
    ///
    /// To iterate over entities in a deterministic order,
    /// sort the results of the query using the desired component as a key.
    /// Note that this requires fetching the whole result set from the query
    /// and allocation of a [`Vec`] to store it.
    ///
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World};
    ///
    /// #[derive(Component, PartialEq, Eq, PartialOrd, Ord, Debug)]
    /// struct Order(i32);
    /// #[derive(Component, PartialEq, Debug)]
    /// struct Label(&'static str);
    ///
    /// let mut world = World::new();
    /// let a = world.spawn((Order(2), Label("second"))).id();
    /// let b = world.spawn((Order(3), Label("third"))).id();
    /// let c = world.spawn((Order(1), Label("first"))).id();
    /// let mut entities = world.query::<(Entity, &Order, &Label)>()
    ///     .iter(&world)
    ///     .collect::<Vec<_>>();
    /// // Sort the query results by their `Order` component before comparing
    /// // to expected results. Query iteration order should not be relied on.
    /// entities.sort_by_key(|e| e.1);
    /// assert_eq!(entities, vec![
    ///     (c, &Order(1), &Label("first")),
    ///     (a, &Order(2), &Label("second")),
    ///     (b, &Order(3), &Label("third")),
    /// ]);
    /// ```
    #[inline]
    pub fn query<D: QueryData>(&mut self) -> QueryState<D, ()> {
        self.query_filtered::<D, ()>()
    }

    /// Returns [`QueryState`] for the given filtered [`QueryData`], which is used to efficiently
    /// run queries on the [`World`] by storing and reusing the [`QueryState`].
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World, query::With};
    ///
    /// #[derive(Component)]
    /// struct A;
    /// #[derive(Component)]
    /// struct B;
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(A).id();
    /// let e2 = world.spawn((A, B)).id();
    ///
    /// let mut query = world.query_filtered::<Entity, With<B>>();
    /// let matching_entities = query.iter(&world).collect::<Vec<Entity>>();
    ///
    /// assert_eq!(matching_entities, vec![e2]);
    /// ```
    #[inline]
    pub fn query_filtered<D: QueryData, F: QueryFilter>(&mut self) -> QueryState<D, F> {
        QueryState::new(self)
    }

    /// Returns [`QueryState`] for the given [`QueryData`], which is used to efficiently
    /// run queries on the [`World`] by storing and reusing the [`QueryState`].
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World};
    ///
    /// #[derive(Component, Debug, PartialEq)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// world.spawn_batch(vec![
    ///     Position { x: 0.0, y: 0.0 },
    ///     Position { x: 1.0, y: 1.0 },
    /// ]);
    ///
    /// fn get_positions(world: &World) -> Vec<(Entity, &Position)> {
    ///     let mut query = world.try_query::<(Entity, &Position)>().unwrap();
    ///     query.iter(world).collect()
    /// }
    ///
    /// let positions = get_positions(&world);
    ///
    /// assert_eq!(world.get::<Position>(positions[0].0).unwrap(), positions[0].1);
    /// assert_eq!(world.get::<Position>(positions[1].0).unwrap(), positions[1].1);
    /// ```
    ///
    /// Requires only an immutable world reference, but may fail if, for example,
    /// the components that make up this query have not been registered into the world.
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World};
    ///
    /// #[derive(Component)]
    /// struct A;
    ///
    /// let mut world = World::new();
    ///
    /// let none_query = world.try_query::<&A>();
    /// assert!(none_query.is_none());
    ///
    /// world.register_component::<A>();
    ///
    /// let some_query = world.try_query::<&A>();
    /// assert!(some_query.is_some());
    /// ```
    #[inline]
    pub fn try_query<D: QueryData>(&self) -> Option<QueryState<D, ()>> {
        self.try_query_filtered::<D, ()>()
    }

    /// Returns [`QueryState`] for the given filtered [`QueryData`], which is used to efficiently
    /// run queries on the [`World`] by storing and reusing the [`QueryState`].
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World, query::With};
    ///
    /// #[derive(Component)]
    /// struct A;
    /// #[derive(Component)]
    /// struct B;
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(A).id();
    /// let e2 = world.spawn((A, B)).id();
    ///
    /// let mut query = world.try_query_filtered::<Entity, With<B>>().unwrap();
    /// let matching_entities = query.iter(&world).collect::<Vec<Entity>>();
    ///
    /// assert_eq!(matching_entities, vec![e2]);
    /// ```
    ///
    /// Requires only an immutable world reference, but may fail if, for example,
    /// the components that make up this query have not been registered into the world.
    #[inline]
    pub fn try_query_filtered<D: QueryData, F: QueryFilter>(&self) -> Option<QueryState<D, F>> {
        QueryState::try_new(self)
    }

    /// Returns an iterator of entities that had components of type `T` removed
    /// since the last call to [`World::clear_trackers`].
    pub fn removed<T: Component>(&self) -> impl Iterator<Item = Entity> + '_ {
        self.components
            .get_valid_id(TypeId::of::<T>())
            .map(|component_id| self.removed_with_id(component_id))
            .into_iter()
            .flatten()
    }

    /// Returns an iterator of entities that had components with the given `component_id` removed
    /// since the last call to [`World::clear_trackers`].
    pub fn removed_with_id(&self, component_id: ComponentId) -> impl Iterator<Item = Entity> + '_ {
        self.removed_components
            .get(component_id)
            .map(|removed| removed.iter_current_update_events().cloned())
            .into_iter()
            .flatten()
            .map(Into::into)
    }

    /// Registers a new [`Resource`] type and returns the [`ComponentId`] created for it.
    ///
    /// This enables the dynamic registration of new [`Resource`] definitions at runtime for
    /// advanced use cases.
    ///
    /// # Note
    ///
    /// Registering a [`Resource`] does not insert it into [`World`]. For insertion, you could use
    /// [`World::insert_resource_by_id`].
    pub fn register_resource_with_descriptor(
        &mut self,
        descriptor: ComponentDescriptor,
    ) -> ComponentId {
        self.components_registrator()
            .register_resource_with_descriptor(descriptor)
    }

    /// Initializes a new resource and returns the [`ComponentId`] created for it.
    ///
    /// If the resource already exists, nothing happens.
    ///
    /// The value given by the [`FromWorld::from_world`] method will be used.
    /// Note that any resource with the [`Default`] trait automatically implements [`FromWorld`],
    /// and those default values will be here instead.
    #[inline]
    #[track_caller]
    pub fn init_resource<R: Resource + FromWorld>(&mut self) -> ComponentId {
        let caller = MaybeLocation::caller();
        let component_id = self.components_registrator().register_resource::<R>();
        if self
            .storages
            .resources
            .get(component_id)
            .is_none_or(|data| !data.is_present())
        {
            let value = R::from_world(self);
            OwningPtr::make(value, |ptr| {
                // SAFETY: component_id was just initialized and corresponds to resource of type R.
                unsafe {
                    self.insert_resource_by_id(component_id, ptr, caller);
                }
            });
        }
        component_id
    }

    /// Inserts a new resource with the given `value`.
    ///
    /// Resources are "unique" data of a given type.
    /// If you insert a resource of a type that already exists,
    /// you will overwrite any existing data.
    #[inline]
    #[track_caller]
    pub fn insert_resource<R: Resource>(&mut self, value: R) {
        self.insert_resource_with_caller(value, MaybeLocation::caller());
    }

    /// Split into a new function so we can pass the calling location into the function when using
    /// as a command.
    #[inline]
    pub(crate) fn insert_resource_with_caller<R: Resource>(
        &mut self,
        value: R,
        caller: MaybeLocation,
    ) {
        let component_id = self.components_registrator().register_resource::<R>();
        OwningPtr::make(value, |ptr| {
            // SAFETY: component_id was just initialized and corresponds to resource of type R.
            unsafe {
                self.insert_resource_by_id(component_id, ptr, caller);
            }
        });
    }

    /// Initializes a new non-send resource and returns the [`ComponentId`] created for it.
    ///
    /// If the resource already exists, nothing happens.
    ///
    /// The value given by the [`FromWorld::from_world`] method will be used.
    /// Note that any resource with the `Default` trait automatically implements `FromWorld`,
    /// and those default values will be here instead.
    ///
    /// # Panics
    ///
    /// Panics if called from a thread other than the main thread.
    #[inline]
    #[track_caller]
    pub fn init_non_send_resource<R: 'static + FromWorld>(&mut self) -> ComponentId {
        let caller = MaybeLocation::caller();
        let component_id = self.components_registrator().register_non_send::<R>();
        if self
            .storages
            .non_send_resources
            .get(component_id)
            .is_none_or(|data| !data.is_present())
        {
            let value = R::from_world(self);
            OwningPtr::make(value, |ptr| {
                // SAFETY: component_id was just initialized and corresponds to resource of type R.
                unsafe {
                    self.insert_non_send_by_id(component_id, ptr, caller);
                }
            });
        }
        component_id
    }

    /// Inserts a new non-send resource with the given `value`.
    ///
    /// `NonSend` resources cannot be sent across threads,
    /// and do not need the `Send + Sync` bounds.
    /// Systems with `NonSend` resources are always scheduled on the main thread.
    ///
    /// # Panics
    /// If a value is already present, this function will panic if called
    /// from a different thread than where the original value was inserted from.
    #[inline]
    #[track_caller]
    pub fn insert_non_send_resource<R: 'static>(&mut self, value: R) {
        let caller = MaybeLocation::caller();
        let component_id = self.components_registrator().register_non_send::<R>();
        OwningPtr::make(value, |ptr| {
            // SAFETY: component_id was just initialized and corresponds to resource of type R.
            unsafe {
                self.insert_non_send_by_id(component_id, ptr, caller);
            }
        });
    }

    /// Removes the resource of a given type and returns it, if it exists. Otherwise returns `None`.
    #[inline]
    pub fn remove_resource<R: Resource>(&mut self) -> Option<R> {
        let component_id = self.components.get_valid_resource_id(TypeId::of::<R>())?;
        let (ptr, _, _) = self.storages.resources.get_mut(component_id)?.remove()?;
        // SAFETY: `component_id` was gotten via looking up the `R` type
        unsafe { Some(ptr.read::<R>()) }
    }

    /// Removes a `!Send` resource from the world and returns it, if present.
    ///
    /// `NonSend` resources cannot be sent across threads,
    /// and do not need the `Send + Sync` bounds.
    /// Systems with `NonSend` resources are always scheduled on the main thread.
    ///
    /// Returns `None` if a value was not previously present.
    ///
    /// # Panics
    /// If a value is present, this function will panic if called from a different
    /// thread than where the value was inserted from.
    #[inline]
    pub fn remove_non_send_resource<R: 'static>(&mut self) -> Option<R> {
        let component_id = self.components.get_valid_resource_id(TypeId::of::<R>())?;
        let (ptr, _, _) = self
            .storages
            .non_send_resources
            .get_mut(component_id)?
            .remove()?;
        // SAFETY: `component_id` was gotten via looking up the `R` type
        unsafe { Some(ptr.read::<R>()) }
    }

    /// Returns `true` if a resource of type `R` exists. Otherwise returns `false`.
    #[inline]
    pub fn contains_resource<R: Resource>(&self) -> bool {
        self.components
            .get_valid_resource_id(TypeId::of::<R>())
            .and_then(|component_id| self.storages.resources.get(component_id))
            .is_some_and(ResourceData::is_present)
    }

    /// Returns `true` if a resource with provided `component_id` exists. Otherwise returns `false`.
    #[inline]
    pub fn contains_resource_by_id(&self, component_id: ComponentId) -> bool {
        self.storages
            .resources
            .get(component_id)
            .is_some_and(ResourceData::is_present)
    }

    /// Returns `true` if a resource of type `R` exists. Otherwise returns `false`.
    #[inline]
    pub fn contains_non_send<R: 'static>(&self) -> bool {
        self.components
            .get_valid_resource_id(TypeId::of::<R>())
            .and_then(|component_id| self.storages.non_send_resources.get(component_id))
            .is_some_and(ResourceData::is_present)
    }

    /// Returns `true` if a resource with provided `component_id` exists. Otherwise returns `false`.
    #[inline]
    pub fn contains_non_send_by_id(&self, component_id: ComponentId) -> bool {
        self.storages
            .non_send_resources
            .get(component_id)
            .is_some_and(ResourceData::is_present)
    }

    /// Returns `true` if a resource of type `R` exists and was added since the world's
    /// [`last_change_tick`](World::last_change_tick()). Otherwise, this returns `false`.
    ///
    /// This means that:
    /// - When called from an exclusive system, this will check for additions since the system last ran.
    /// - When called elsewhere, this will check for additions since the last time that [`World::clear_trackers`]
    ///   was called.
    pub fn is_resource_added<R: Resource>(&self) -> bool {
        self.components
            .get_valid_resource_id(TypeId::of::<R>())
            .is_some_and(|component_id| self.is_resource_added_by_id(component_id))
    }

    /// Returns `true` if a resource with id `component_id` exists and was added since the world's
    /// [`last_change_tick`](World::last_change_tick()). Otherwise, this returns `false`.
    ///
    /// This means that:
    /// - When called from an exclusive system, this will check for additions since the system last ran.
    /// - When called elsewhere, this will check for additions since the last time that [`World::clear_trackers`]
    ///   was called.
    pub fn is_resource_added_by_id(&self, component_id: ComponentId) -> bool {
        self.storages
            .resources
            .get(component_id)
            .is_some_and(|resource| {
                resource.get_ticks().is_some_and(|ticks| {
                    ticks.is_added(self.last_change_tick(), self.read_change_tick())
                })
            })
    }

    /// Returns `true` if a resource of type `R` exists and was modified since the world's
    /// [`last_change_tick`](World::last_change_tick()). Otherwise, this returns `false`.
    ///
    /// This means that:
    /// - When called from an exclusive system, this will check for changes since the system last ran.
    /// - When called elsewhere, this will check for changes since the last time that [`World::clear_trackers`]
    ///   was called.
    pub fn is_resource_changed<R: Resource>(&self) -> bool {
        self.components
            .get_valid_resource_id(TypeId::of::<R>())
            .is_some_and(|component_id| self.is_resource_changed_by_id(component_id))
    }

    /// Returns `true` if a resource with id `component_id` exists and was modified since the world's
    /// [`last_change_tick`](World::last_change_tick()). Otherwise, this returns `false`.
    ///
    /// This means that:
    /// - When called from an exclusive system, this will check for changes since the system last ran.
    /// - When called elsewhere, this will check for changes since the last time that [`World::clear_trackers`]
    ///   was called.
    pub fn is_resource_changed_by_id(&self, component_id: ComponentId) -> bool {
        self.storages
            .resources
            .get(component_id)
            .is_some_and(|resource| {
                resource.get_ticks().is_some_and(|ticks| {
                    ticks.is_changed(self.last_change_tick(), self.read_change_tick())
                })
            })
    }

    /// Retrieves the change ticks for the given resource.
    pub fn get_resource_change_ticks<R: Resource>(&self) -> Option<ComponentTicks> {
        self.components
            .get_valid_resource_id(TypeId::of::<R>())
            .and_then(|component_id| self.get_resource_change_ticks_by_id(component_id))
    }

    /// Retrieves the change ticks for the given [`ComponentId`].
    ///
    /// **You should prefer to use the typed API [`World::get_resource_change_ticks`] where possible.**
    pub fn get_resource_change_ticks_by_id(
        &self,
        component_id: ComponentId,
    ) -> Option<ComponentTicks> {
        self.storages
            .resources
            .get(component_id)
            .and_then(ResourceData::get_ticks)
    }

    /// Gets a reference to the resource of the given type
    ///
    /// # Panics
    ///
    /// Panics if the resource does not exist.
    /// Use [`get_resource`](World::get_resource) instead if you want to handle this case.
    ///
    /// If you want to instead insert a value if the resource does not exist,
    /// use [`get_resource_or_insert_with`](World::get_resource_or_insert_with).
    #[inline]
    #[track_caller]
    pub fn resource<R: Resource>(&self) -> &R {
        match self.get_resource() {
            Some(x) => x,
            None => panic!(
                "Requested resource {} does not exist in the `World`.
                Did you forget to add it using `app.insert_resource` / `app.init_resource`?
                Resources are also implicitly added via `app.add_event`,
                and can be added by plugins.",
                DebugName::type_name::<R>()
            ),
        }
    }

    /// Gets a reference to the resource of the given type
    ///
    /// # Panics
    ///
    /// Panics if the resource does not exist.
    /// Use [`get_resource_ref`](World::get_resource_ref) instead if you want to handle this case.
    ///
    /// If you want to instead insert a value if the resource does not exist,
    /// use [`get_resource_or_insert_with`](World::get_resource_or_insert_with).
    #[inline]
    #[track_caller]
    pub fn resource_ref<R: Resource>(&self) -> Ref<'_, R> {
        match self.get_resource_ref() {
            Some(x) => x,
            None => panic!(
                "Requested resource {} does not exist in the `World`.
                Did you forget to add it using `app.insert_resource` / `app.init_resource`?
                Resources are also implicitly added via `app.add_event`,
                and can be added by plugins.",
                DebugName::type_name::<R>()
            ),
        }
    }

    /// Gets a mutable reference to the resource of the given type
    ///
    /// # Panics
    ///
    /// Panics if the resource does not exist.
    /// Use [`get_resource_mut`](World::get_resource_mut) instead if you want to handle this case.
    ///
    /// If you want to instead insert a value if the resource does not exist,
    /// use [`get_resource_or_insert_with`](World::get_resource_or_insert_with).
    #[inline]
    #[track_caller]
    pub fn resource_mut<R: Resource>(&mut self) -> Mut<'_, R> {
        match self.get_resource_mut() {
            Some(x) => x,
            None => panic!(
                "Requested resource {} does not exist in the `World`.
                Did you forget to add it using `app.insert_resource` / `app.init_resource`?
                Resources are also implicitly added via `app.add_event`,
                and can be added by plugins.",
                DebugName::type_name::<R>()
            ),
        }
    }

    /// Gets a reference to the resource of the given type if it exists
    #[inline]
    pub fn get_resource<R: Resource>(&self) -> Option<&R> {
        // SAFETY:
        // - `as_unsafe_world_cell_readonly` gives permission to access everything immutably
        // - `&self` ensures nothing in world is borrowed mutably
        unsafe { self.as_unsafe_world_cell_readonly().get_resource() }
    }

    /// Gets a reference including change detection to the resource of the given type if it exists.
    #[inline]
    pub fn get_resource_ref<R: Resource>(&self) -> Option<Ref<'_, R>> {
        // SAFETY:
        // - `as_unsafe_world_cell_readonly` gives permission to access everything immutably
        // - `&self` ensures nothing in world is borrowed mutably
        unsafe { self.as_unsafe_world_cell_readonly().get_resource_ref() }
    }

    /// Gets a mutable reference to the resource of the given type if it exists
    #[inline]
    pub fn get_resource_mut<R: Resource>(&mut self) -> Option<Mut<'_, R>> {
        // SAFETY:
        // - `as_unsafe_world_cell` gives permission to access everything mutably
        // - `&mut self` ensures nothing in world is borrowed
        unsafe { self.as_unsafe_world_cell().get_resource_mut() }
    }

    /// Gets a mutable reference to the resource of type `T` if it exists,
    /// otherwise inserts the resource using the result of calling `func`.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #
    /// #[derive(Resource)]
    /// struct MyResource(i32);
    ///
    /// # let mut world = World::new();
    /// let my_res = world.get_resource_or_insert_with(|| MyResource(10));
    /// assert_eq!(my_res.0, 10);
    /// ```
    #[inline]
    #[track_caller]
    pub fn get_resource_or_insert_with<R: Resource>(
        &mut self,
        func: impl FnOnce() -> R,
    ) -> Mut<'_, R> {
        let caller = MaybeLocation::caller();
        let change_tick = self.change_tick();
        let last_change_tick = self.last_change_tick();

        let component_id = self.components_registrator().register_resource::<R>();
        let data = self.initialize_resource_internal(component_id);
        if !data.is_present() {
            OwningPtr::make(func(), |ptr| {
                // SAFETY: component_id was just initialized and corresponds to resource of type R.
                unsafe {
                    data.insert(ptr, change_tick, caller);
                }
            });
        }

        // SAFETY: The resource must be present, as we would have inserted it if it was empty.
        let data = unsafe {
            data.get_mut(last_change_tick, change_tick)
                .debug_checked_unwrap()
        };
        // SAFETY: The underlying type of the resource is `R`.
        unsafe { data.with_type::<R>() }
    }

    /// Gets a mutable reference to the resource of type `T` if it exists,
    /// otherwise initializes the resource by calling its [`FromWorld`]
    /// implementation.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #
    /// #[derive(Resource)]
    /// struct Foo(i32);
    ///
    /// impl Default for Foo {
    ///     fn default() -> Self {
    ///         Self(15)
    ///     }
    /// }
    ///
    /// #[derive(Resource)]
    /// struct MyResource(i32);
    ///
    /// impl FromWorld for MyResource {
    ///     fn from_world(world: &mut World) -> Self {
    ///         let foo = world.get_resource_or_init::<Foo>();
    ///         Self(foo.0 * 2)
    ///     }
    /// }
    ///
    /// # let mut world = World::new();
    /// let my_res = world.get_resource_or_init::<MyResource>();
    /// assert_eq!(my_res.0, 30);
    /// ```
    #[track_caller]
    pub fn get_resource_or_init<R: Resource + FromWorld>(&mut self) -> Mut<'_, R> {
        let caller = MaybeLocation::caller();
        let change_tick = self.change_tick();
        let last_change_tick = self.last_change_tick();

        let component_id = self.components_registrator().register_resource::<R>();
        if self
            .storages
            .resources
            .get(component_id)
            .is_none_or(|data| !data.is_present())
        {
            let value = R::from_world(self);
            OwningPtr::make(value, |ptr| {
                // SAFETY: component_id was just initialized and corresponds to resource of type R.
                unsafe {
                    self.insert_resource_by_id(component_id, ptr, caller);
                }
            });
        }

        // SAFETY: The resource was just initialized if it was empty.
        let data = unsafe {
            self.storages
                .resources
                .get_mut(component_id)
                .debug_checked_unwrap()
        };
        // SAFETY: The resource must be present, as we would have inserted it if it was empty.
        let data = unsafe {
            data.get_mut(last_change_tick, change_tick)
                .debug_checked_unwrap()
        };
        // SAFETY: The underlying type of the resource is `R`.
        unsafe { data.with_type::<R>() }
    }

    /// Gets an immutable reference to the non-send resource of the given type, if it exists.
    ///
    /// # Panics
    ///
    /// Panics if the resource does not exist.
    /// Use [`get_non_send_resource`](World::get_non_send_resource) instead if you want to handle this case.
    ///
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    #[inline]
    #[track_caller]
    pub fn non_send_resource<R: 'static>(&self) -> &R {
        match self.get_non_send_resource() {
            Some(x) => x,
            None => panic!(
                "Requested non-send resource {} does not exist in the `World`.
                Did you forget to add it using `app.insert_non_send_resource` / `app.init_non_send_resource`?
                Non-send resources can also be added by plugins.",
                DebugName::type_name::<R>()
            ),
        }
    }

    /// Gets a mutable reference to the non-send resource of the given type, if it exists.
    ///
    /// # Panics
    ///
    /// Panics if the resource does not exist.
    /// Use [`get_non_send_resource_mut`](World::get_non_send_resource_mut) instead if you want to handle this case.
    ///
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    #[inline]
    #[track_caller]
    pub fn non_send_resource_mut<R: 'static>(&mut self) -> Mut<'_, R> {
        match self.get_non_send_resource_mut() {
            Some(x) => x,
            None => panic!(
                "Requested non-send resource {} does not exist in the `World`.
                Did you forget to add it using `app.insert_non_send_resource` / `app.init_non_send_resource`?
                Non-send resources can also be added by plugins.",
                DebugName::type_name::<R>()
            ),
        }
    }

    /// Gets a reference to the non-send resource of the given type, if it exists.
    /// Otherwise returns `None`.
    ///
    /// # Panics
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    #[inline]
    pub fn get_non_send_resource<R: 'static>(&self) -> Option<&R> {
        // SAFETY:
        // - `as_unsafe_world_cell_readonly` gives permission to access the entire world immutably
        // - `&self` ensures that there are no mutable borrows of world data
        unsafe { self.as_unsafe_world_cell_readonly().get_non_send_resource() }
    }

    /// Gets a mutable reference to the non-send resource of the given type, if it exists.
    /// Otherwise returns `None`.
    ///
    /// # Panics
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    #[inline]
    pub fn get_non_send_resource_mut<R: 'static>(&mut self) -> Option<Mut<'_, R>> {
        // SAFETY:
        // - `as_unsafe_world_cell` gives permission to access the entire world mutably
        // - `&mut self` ensures that there are no borrows of world data
        unsafe { self.as_unsafe_world_cell().get_non_send_resource_mut() }
    }

    /// For a given batch of ([`Entity`], [`Bundle`]) pairs,
    /// adds the `Bundle` of components to each `Entity`.
    /// This is faster than doing equivalent operations one-by-one.
    ///
    /// A batch can be any type that implements [`IntoIterator`] containing `(Entity, Bundle)` tuples,
    /// such as a [`Vec<(Entity, Bundle)>`] or an array `[(Entity, Bundle); N]`.
    ///
    /// This will overwrite any previous values of components shared by the `Bundle`.
    /// See [`World::insert_batch_if_new`] to keep the old values instead.
    ///
    /// # Panics
    ///
    /// This function will panic if any of the associated entities do not exist.
    ///
    /// For the fallible version, see [`World::try_insert_batch`].
    #[track_caller]
    pub fn insert_batch<I, B>(&mut self, batch: I)
    where
        I: IntoIterator,
        I::IntoIter: Iterator<Item = (Entity, B)>,
        B: Bundle<Effect: NoBundleEffect>,
    {
        self.insert_batch_with_caller(batch, InsertMode::Replace, MaybeLocation::caller());
    }

    /// For a given batch of ([`Entity`], [`Bundle`]) pairs,
    /// adds the `Bundle` of components to each `Entity` without overwriting.
    /// This is faster than doing equivalent operations one-by-one.
    ///
    /// A batch can be any type that implements [`IntoIterator`] containing `(Entity, Bundle)` tuples,
    /// such as a [`Vec<(Entity, Bundle)>`] or an array `[(Entity, Bundle); N]`.
    ///
    /// This is the same as [`World::insert_batch`], but in case of duplicate
    /// components it will leave the old values instead of replacing them with new ones.
    ///
    /// # Panics
    ///
    /// This function will panic if any of the associated entities do not exist.
    ///
    /// For the fallible version, see [`World::try_insert_batch_if_new`].
    #[track_caller]
    pub fn insert_batch_if_new<I, B>(&mut self, batch: I)
    where
        I: IntoIterator,
        I::IntoIter: Iterator<Item = (Entity, B)>,
        B: Bundle<Effect: NoBundleEffect>,
    {
        self.insert_batch_with_caller(batch, InsertMode::Keep, MaybeLocation::caller());
    }

    /// Split into a new function so we can differentiate the calling location.
    ///
    /// This can be called by:
    /// - [`World::insert_batch`]
    /// - [`World::insert_batch_if_new`]
    #[inline]
    pub(crate) fn insert_batch_with_caller<I, B>(
        &mut self,
        batch: I,
        insert_mode: InsertMode,
        caller: MaybeLocation,
    ) where
        I: IntoIterator,
        I::IntoIter: Iterator<Item = (Entity, B)>,
        B: Bundle<Effect: NoBundleEffect>,
    {
        struct InserterArchetypeCache<'w> {
            inserter: BundleInserter<'w>,
            archetype_id: ArchetypeId,
        }

        self.flush();
        let change_tick = self.change_tick();
        let bundle_id = self.register_bundle_info::<B>();

        let mut batch_iter = batch.into_iter();

        if let Some((first_entity, first_bundle)) = batch_iter.next() {
            if let Some(first_location) = self.entities().get(first_entity) {
                let mut cache = InserterArchetypeCache {
                    // SAFETY: we initialized this bundle_id in `register_info`
                    inserter: unsafe {
                        BundleInserter::new_with_id(
                            self,
                            first_location.archetype_id,
                            bundle_id,
                            change_tick,
                        )
                    },
                    archetype_id: first_location.archetype_id,
                };
                // SAFETY: `entity` is valid, `location` matches entity, bundle matches inserter
                unsafe {
                    cache.inserter.insert(
                        first_entity,
                        first_location,
                        first_bundle,
                        insert_mode,
                        caller,
                        RelationshipHookMode::Run,
                    )
                };

                for (entity, bundle) in batch_iter {
                    if let Some(location) = cache.inserter.entities().get(entity) {
                        if location.archetype_id != cache.archetype_id {
                            cache = InserterArchetypeCache {
                                // SAFETY: we initialized this bundle_id in `register_info`
                                inserter: unsafe {
                                    BundleInserter::new_with_id(
                                        self,
                                        location.archetype_id,
                                        bundle_id,
                                        change_tick,
                                    )
                                },
                                archetype_id: location.archetype_id,
                            }
                        }
                        // SAFETY: `entity` is valid, `location` matches entity, bundle matches inserter
                        unsafe {
                            cache.inserter.insert(
                                entity,
                                location,
                                bundle,
                                insert_mode,
                                caller,
                                RelationshipHookMode::Run,
                            )
                        };
                    } else {
                        panic!("error[B0003]: Could not insert a bundle (of type `{}`) for entity {entity}, which {}. See: https://bevy.org/learn/errors/b0003", DebugName::type_name::<B>(), self.entities.entity_does_not_exist_error_details(entity));
                    }
                }
            } else {
                panic!("error[B0003]: Could not insert a bundle (of type `{}`) for entity {first_entity}, which {}. See: https://bevy.org/learn/errors/b0003", DebugName::type_name::<B>(), self.entities.entity_does_not_exist_error_details(first_entity));
            }
        }
    }

    /// For a given batch of ([`Entity`], [`Bundle`]) pairs,
    /// adds the `Bundle` of components to each `Entity`.
    /// This is faster than doing equivalent operations one-by-one.
    ///
    /// A batch can be any type that implements [`IntoIterator`] containing `(Entity, Bundle)` tuples,
    /// such as a [`Vec<(Entity, Bundle)>`] or an array `[(Entity, Bundle); N]`.
    ///
    /// This will overwrite any previous values of components shared by the `Bundle`.
    /// See [`World::try_insert_batch_if_new`] to keep the old values instead.
    ///
    /// Returns a [`TryInsertBatchError`] if any of the provided entities do not exist.
    ///
    /// For the panicking version, see [`World::insert_batch`].
    #[track_caller]
    pub fn try_insert_batch<I, B>(&mut self, batch: I) -> Result<(), TryInsertBatchError>
    where
        I: IntoIterator,
        I::IntoIter: Iterator<Item = (Entity, B)>,
        B: Bundle<Effect: NoBundleEffect>,
    {
        self.try_insert_batch_with_caller(batch, InsertMode::Replace, MaybeLocation::caller())
    }
    /// For a given batch of ([`Entity`], [`Bundle`]) pairs,
    /// adds the `Bundle` of components to each `Entity` without overwriting.
    /// This is faster than doing equivalent operations one-by-one.
    ///
    /// A batch can be any type that implements [`IntoIterator`] containing `(Entity, Bundle)` tuples,
    /// such as a [`Vec<(Entity, Bundle)>`] or an array `[(Entity, Bundle); N]`.
    ///
    /// This is the same as [`World::try_insert_batch`], but in case of duplicate
    /// components it will leave the old values instead of replacing them with new ones.
    ///
    /// Returns a [`TryInsertBatchError`] if any of the provided entities do not exist.
    ///
    /// For the panicking version, see [`World::insert_batch_if_new`].
    #[track_caller]
    pub fn try_insert_batch_if_new<I, B>(&mut self, batch: I) -> Result<(), TryInsertBatchError>
    where
        I: IntoIterator,
        I::IntoIter: Iterator<Item = (Entity, B)>,
        B: Bundle<Effect: NoBundleEffect>,
    {
        self.try_insert_batch_with_caller(batch, InsertMode::Keep, MaybeLocation::caller())
    }

    /// Split into a new function so we can differentiate the calling location.
    ///
    /// This can be called by:
    /// - [`World::try_insert_batch`]
    /// - [`World::try_insert_batch_if_new`]
    /// - [`Commands::insert_batch`]
    /// - [`Commands::insert_batch_if_new`]
    /// - [`Commands::try_insert_batch`]
    /// - [`Commands::try_insert_batch_if_new`]
    #[inline]
    pub(crate) fn try_insert_batch_with_caller<I, B>(
        &mut self,
        batch: I,
        insert_mode: InsertMode,
        caller: MaybeLocation,
    ) -> Result<(), TryInsertBatchError>
    where
        I: IntoIterator,
        I::IntoIter: Iterator<Item = (Entity, B)>,
        B: Bundle<Effect: NoBundleEffect>,
    {
        struct InserterArchetypeCache<'w> {
            inserter: BundleInserter<'w>,
            archetype_id: ArchetypeId,
        }

        self.flush();
        let change_tick = self.change_tick();
        let bundle_id = self.register_bundle_info::<B>();

        let mut invalid_entities = Vec::<Entity>::new();
        let mut batch_iter = batch.into_iter();

        // We need to find the first valid entity so we can initialize the bundle inserter.
        // This differs from `insert_batch_with_caller` because that method can just panic
        // if the first entity is invalid, whereas this method needs to keep going.
        let cache = loop {
            if let Some((first_entity, first_bundle)) = batch_iter.next() {
                if let Some(first_location) = self.entities().get(first_entity) {
                    let mut cache = InserterArchetypeCache {
                        // SAFETY: we initialized this bundle_id in `register_bundle_info`
                        inserter: unsafe {
                            BundleInserter::new_with_id(
                                self,
                                first_location.archetype_id,
                                bundle_id,
                                change_tick,
                            )
                        },
                        archetype_id: first_location.archetype_id,
                    };
                    // SAFETY: `entity` is valid, `location` matches entity, bundle matches inserter
                    unsafe {
                        cache.inserter.insert(
                            first_entity,
                            first_location,
                            first_bundle,
                            insert_mode,
                            caller,
                            RelationshipHookMode::Run,
                        )
                    };
                    break Some(cache);
                }
                invalid_entities.push(first_entity);
            } else {
                // We reached the end of the entities the caller provided and none were valid.
                break None;
            }
        };

        if let Some(mut cache) = cache {
            for (entity, bundle) in batch_iter {
                if let Some(location) = cache.inserter.entities().get(entity) {
                    if location.archetype_id != cache.archetype_id {
                        cache = InserterArchetypeCache {
                            // SAFETY: we initialized this bundle_id in `register_info`
                            inserter: unsafe {
                                BundleInserter::new_with_id(
                                    self,
                                    location.archetype_id,
                                    bundle_id,
                                    change_tick,
                                )
                            },
                            archetype_id: location.archetype_id,
                        }
                    }
                    // SAFETY: `entity` is valid, `location` matches entity, bundle matches inserter
                    unsafe {
                        cache.inserter.insert(
                            entity,
                            location,
                            bundle,
                            insert_mode,
                            caller,
                            RelationshipHookMode::Run,
                        )
                    };
                } else {
                    invalid_entities.push(entity);
                }
            }
        }

        if invalid_entities.is_empty() {
            Ok(())
        } else {
            Err(TryInsertBatchError {
                bundle_type: DebugName::type_name::<B>(),
                entities: invalid_entities,
            })
        }
    }

    /// Temporarily removes the requested resource from this [`World`], runs custom user code,
    /// then re-adds the resource before returning.
    ///
    /// This enables safe simultaneous mutable access to both a resource and the rest of the [`World`].
    /// For more complex access patterns, consider using [`SystemState`](crate::system::SystemState).
    ///
    /// # Panics
    ///
    /// Panics if the resource does not exist.
    /// Use [`try_resource_scope`](Self::try_resource_scope) instead if you want to handle this case.
    ///
    /// # Example
    /// ```
    /// use bevy_ecs::prelude::*;
    /// #[derive(Resource)]
    /// struct A(u32);
    /// #[derive(Component)]
    /// struct B(u32);
    /// let mut world = World::new();
    /// world.insert_resource(A(1));
    /// let entity = world.spawn(B(1)).id();
    ///
    /// world.resource_scope(|world, mut a: Mut<A>| {
    ///     let b = world.get_mut::<B>(entity).unwrap();
    ///     a.0 += b.0;
    /// });
    /// assert_eq!(world.get_resource::<A>().unwrap().0, 2);
    /// ```
    #[track_caller]
    pub fn resource_scope<R: Resource, U>(&mut self, f: impl FnOnce(&mut World, Mut<R>) -> U) -> U {
        self.try_resource_scope(f)
            .unwrap_or_else(|| panic!("resource does not exist: {}", DebugName::type_name::<R>()))
    }

    /// Temporarily removes the requested resource from this [`World`] if it exists, runs custom user code,
    /// then re-adds the resource before returning. Returns `None` if the resource does not exist in this [`World`].
    ///
    /// This enables safe simultaneous mutable access to both a resource and the rest of the [`World`].
    /// For more complex access patterns, consider using [`SystemState`](crate::system::SystemState).
    ///
    /// See also [`resource_scope`](Self::resource_scope).
    pub fn try_resource_scope<R: Resource, U>(
        &mut self,
        f: impl FnOnce(&mut World, Mut<R>) -> U,
    ) -> Option<U> {
        let last_change_tick = self.last_change_tick();
        let change_tick = self.change_tick();

        let component_id = self.components.get_valid_resource_id(TypeId::of::<R>())?;
        let (ptr, mut ticks, mut caller) = self
            .storages
            .resources
            .get_mut(component_id)
            .and_then(ResourceData::remove)?;
        // Read the value onto the stack to avoid potential mut aliasing.
        // SAFETY: `ptr` was obtained from the TypeId of `R`.
        let mut value = unsafe { ptr.read::<R>() };
        let value_mut = Mut {
            value: &mut value,
            ticks: TicksMut {
                added: &mut ticks.added,
                changed: &mut ticks.changed,
                last_run: last_change_tick,
                this_run: change_tick,
            },
            changed_by: caller.as_mut(),
        };
        let result = f(self, value_mut);
        assert!(!self.contains_resource::<R>(),
            "Resource `{}` was inserted during a call to World::resource_scope.\n\
            This is not allowed as the original resource is reinserted to the world after the closure is invoked.",
            DebugName::type_name::<R>());

        OwningPtr::make(value, |ptr| {
            // SAFETY: pointer is of type R
            unsafe {
                self.storages.resources.get_mut(component_id).map(|info| {
                    info.insert_with_ticks(ptr, ticks, caller);
                })
            }
        })?;

        Some(result)
    }

    /// Writes a [`BufferedEvent`].
    /// This method returns the [ID](`EventId`) of the written `event`,
    /// or [`None`] if the `event` could not be written.
    #[inline]
    pub fn write_event<E: BufferedEvent>(&mut self, event: E) -> Option<EventId<E>> {
        self.write_event_batch(core::iter::once(event))?.next()
    }

    /// Writes a [`BufferedEvent`].
    /// This method returns the [ID](`EventId`) of the written `event`,
    /// or [`None`] if the `event` could not be written.
    #[inline]
    #[deprecated(since = "0.17.0", note = "Use `World::write_event` instead.")]
    pub fn send_event<E: BufferedEvent>(&mut self, event: E) -> Option<EventId<E>> {
        self.write_event(event)
    }

    /// Writes the default value of the [`BufferedEvent`] of type `E`.
    /// This method returns the [ID](`EventId`) of the written `event`,
    /// or [`None`] if the `event` could not be written.
    #[inline]
    pub fn write_event_default<E: BufferedEvent + Default>(&mut self) -> Option<EventId<E>> {
        self.write_event(E::default())
    }

    /// Writes the default value of the [`BufferedEvent`] of type `E`.
    /// This method returns the [ID](`EventId`) of the written `event`,
    /// or [`None`] if the `event` could not be written.
    #[inline]
    #[deprecated(since = "0.17.0", note = "Use `World::write_event_default` instead.")]
    pub fn send_event_default<E: BufferedEvent + Default>(&mut self) -> Option<EventId<E>> {
        self.write_event_default::<E>()
    }

    /// Writes a batch of [`BufferedEvent`]s from an iterator.
    /// This method returns the [IDs](`EventId`) of the written `events`,
    /// or [`None`] if the `event` could not be written.
    #[inline]
    pub fn write_event_batch<E: BufferedEvent>(
        &mut self,
        events: impl IntoIterator<Item = E>,
    ) -> Option<WriteBatchIds<E>> {
        let Some(mut events_resource) = self.get_resource_mut::<Events<E>>() else {
            log::error!(
                "Unable to send event `{}`\n\tEvent must be added to the app with `add_event()`\n\thttps://docs.rs/bevy/*/bevy/app/struct.App.html#method.add_event ",
                DebugName::type_name::<E>()
            );
            return None;
        };
        Some(events_resource.write_batch(events))
    }

    /// Writes a batch of [`BufferedEvent`]s from an iterator.
    /// This method returns the [IDs](`EventId`) of the written `events`,
    /// or [`None`] if the `event` could not be written.
    #[inline]
    #[deprecated(since = "0.17.0", note = "Use `World::write_event_batch` instead.")]
    pub fn send_event_batch<E: BufferedEvent>(
        &mut self,
        events: impl IntoIterator<Item = E>,
    ) -> Option<WriteBatchIds<E>> {
        self.write_event_batch(events)
    }

    /// Inserts a new resource with the given `value`. Will replace the value if it already existed.
    ///
    /// **You should prefer to use the typed API [`World::insert_resource`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    ///
    /// # Safety
    /// The value referenced by `value` must be valid for the given [`ComponentId`] of this world.
    #[inline]
    #[track_caller]
    pub unsafe fn insert_resource_by_id(
        &mut self,
        component_id: ComponentId,
        value: OwningPtr<'_>,
        caller: MaybeLocation,
    ) {
        let change_tick = self.change_tick();

        let resource = self.initialize_resource_internal(component_id);
        // SAFETY: `value` is valid for `component_id`, ensured by caller
        unsafe {
            resource.insert(value, change_tick, caller);
        }
    }

    /// Inserts a new `!Send` resource with the given `value`. Will replace the value if it already
    /// existed.
    ///
    /// **You should prefer to use the typed API [`World::insert_non_send_resource`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    ///
    /// # Panics
    /// If a value is already present, this function will panic if not called from the same
    /// thread that the original value was inserted from.
    ///
    /// # Safety
    /// The value referenced by `value` must be valid for the given [`ComponentId`] of this world.
    #[inline]
    #[track_caller]
    pub unsafe fn insert_non_send_by_id(
        &mut self,
        component_id: ComponentId,
        value: OwningPtr<'_>,
        caller: MaybeLocation,
    ) {
        let change_tick = self.change_tick();

        let resource = self.initialize_non_send_internal(component_id);
        // SAFETY: `value` is valid for `component_id`, ensured by caller
        unsafe {
            resource.insert(value, change_tick, caller);
        }
    }

    /// # Panics
    /// Panics if `component_id` is not registered as a `Send` component type in this `World`
    #[inline]
    pub(crate) fn initialize_resource_internal(
        &mut self,
        component_id: ComponentId,
    ) -> &mut ResourceData<true> {
        self.flush_components();
        self.storages
            .resources
            .initialize_with(component_id, &self.components)
    }

    /// # Panics
    /// Panics if `component_id` is not registered in this world
    #[inline]
    pub(crate) fn initialize_non_send_internal(
        &mut self,
        component_id: ComponentId,
    ) -> &mut ResourceData<false> {
        self.flush_components();
        self.storages
            .non_send_resources
            .initialize_with(component_id, &self.components)
    }

    /// Empties queued entities and adds them to the empty [`Archetype`](crate::archetype::Archetype).
    /// This should be called before doing operations that might operate on queued entities,
    /// such as inserting a [`Component`].
    #[track_caller]
    pub(crate) fn flush_entities(&mut self) {
        let by = MaybeLocation::caller();
        let at = self.change_tick();
        let empty_archetype = self.archetypes.empty_mut();
        let table = &mut self.storages.tables[empty_archetype.table_id()];
        // PERF: consider pre-allocating space for flushed entities
        // SAFETY: entity is set to a valid location
        unsafe {
            self.entities.flush(
                |entity, location| {
                    // SAFETY: no components are allocated by archetype.allocate() because the archetype
                    // is empty
                    *location = Some(empty_archetype.allocate(entity, table.allocate(entity)));
                },
                by,
                at,
            );
        }
    }

    /// Applies any commands in the world's internal [`CommandQueue`].
    /// This does not apply commands from any systems, only those stored in the world.
    ///
    /// # Panics
    /// This will panic if any of the queued commands are [`spawn`](Commands::spawn).
    /// If this is possible, you should instead use [`flush`](Self::flush).
    pub(crate) fn flush_commands(&mut self) {
        // SAFETY: `self.command_queue` is only de-allocated in `World`'s `Drop`
        if !unsafe { self.command_queue.is_empty() } {
            // SAFETY: `self.command_queue` is only de-allocated in `World`'s `Drop`
            unsafe {
                self.command_queue
                    .clone()
                    .apply_or_drop_queued(Some(self.into()));
            };
        }
    }

    /// Applies any queued component registration.
    /// For spawning vanilla rust component types and resources, this is not strictly necessary.
    /// However, flushing components can make information available more quickly, and can have performance benefits.
    /// Additionally, for components and resources registered dynamically through a raw descriptor or similar,
    /// this is the only way to complete their registration.
    pub(crate) fn flush_components(&mut self) {
        self.components_registrator().apply_queued_registrations();
    }

    /// Flushes queued entities and commands.
    ///
    /// Queued entities will be spawned, and then commands will be applied.
    #[inline]
    #[track_caller]
    pub fn flush(&mut self) {
        self.flush_entities();
        self.flush_components();
        self.flush_commands();
    }

    /// Increments the world's current change tick and returns the old value.
    ///
    /// If you need to call this method, but do not have `&mut` access to the world,
    /// consider using [`as_unsafe_world_cell_readonly`](Self::as_unsafe_world_cell_readonly)
    /// to obtain an [`UnsafeWorldCell`] and calling [`increment_change_tick`](UnsafeWorldCell::increment_change_tick) on that.
    /// Note that this *can* be done in safe code, despite the name of the type.
    #[inline]
    pub fn increment_change_tick(&mut self) -> Tick {
        let change_tick = self.change_tick.get_mut();
        let prev_tick = *change_tick;
        *change_tick = change_tick.wrapping_add(1);
        Tick::new(prev_tick)
    }

    /// Reads the current change tick of this world.
    ///
    /// If you have exclusive (`&mut`) access to the world, consider using [`change_tick()`](Self::change_tick),
    /// which is more efficient since it does not require atomic synchronization.
    #[inline]
    pub fn read_change_tick(&self) -> Tick {
        let tick = self.change_tick.load(Ordering::Acquire);
        Tick::new(tick)
    }

    /// Reads the current change tick of this world.
    ///
    /// This does the same thing as [`read_change_tick()`](Self::read_change_tick), only this method
    /// is more efficient since it does not require atomic synchronization.
    #[inline]
    pub fn change_tick(&mut self) -> Tick {
        let tick = *self.change_tick.get_mut();
        Tick::new(tick)
    }

    /// When called from within an exclusive system (a [`System`] that takes `&mut World` as its first
    /// parameter), this method returns the [`Tick`] indicating the last time the exclusive system was run.
    ///
    /// Otherwise, this returns the `Tick` indicating the last time that [`World::clear_trackers`] was called.
    ///
    /// [`System`]: crate::system::System
    #[inline]
    pub fn last_change_tick(&self) -> Tick {
        self.last_change_tick
    }

    /// Returns the id of the last ECS event that was fired.
    /// Used internally to ensure observers don't trigger multiple times for the same event.
    #[inline]
    pub(crate) fn last_trigger_id(&self) -> u32 {
        self.last_trigger_id
    }

    /// Sets [`World::last_change_tick()`] to the specified value during a scope.
    /// When the scope terminates, it will return to its old value.
    ///
    /// This is useful if you need a region of code to be able to react to earlier changes made in the same system.
    ///
    /// # Examples
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// // This function runs an update loop repeatedly, allowing each iteration of the loop
    /// // to react to changes made in the previous loop iteration.
    /// fn update_loop(
    ///     world: &mut World,
    ///     mut update_fn: impl FnMut(&mut World) -> std::ops::ControlFlow<()>,
    /// ) {
    ///     let mut last_change_tick = world.last_change_tick();
    ///
    ///     // Repeatedly run the update function until it requests a break.
    ///     loop {
    ///         let control_flow = world.last_change_tick_scope(last_change_tick, |world| {
    ///             // Increment the change tick so we can detect changes from the previous update.
    ///             last_change_tick = world.change_tick();
    ///             world.increment_change_tick();
    ///
    ///             // Update once.
    ///             update_fn(world)
    ///         });
    ///
    ///         // End the loop when the closure returns `ControlFlow::Break`.
    ///         if control_flow.is_break() {
    ///             break;
    ///         }
    ///     }
    /// }
    /// #
    /// # #[derive(Resource)] struct Count(u32);
    /// # let mut world = World::new();
    /// # world.insert_resource(Count(0));
    /// # let saved_last_tick = world.last_change_tick();
    /// # let mut num_updates = 0;
    /// # update_loop(&mut world, |world| {
    /// #     let mut c = world.resource_mut::<Count>();
    /// #     match c.0 {
    /// #         0 => {
    /// #             assert_eq!(num_updates, 0);
    /// #             assert!(c.is_added());
    /// #             c.0 = 1;
    /// #         }
    /// #         1 => {
    /// #             assert_eq!(num_updates, 1);
    /// #             assert!(!c.is_added());
    /// #             assert!(c.is_changed());
    /// #             c.0 = 2;
    /// #         }
    /// #         2 if c.is_changed() => {
    /// #             assert_eq!(num_updates, 2);
    /// #             assert!(!c.is_added());
    /// #         }
    /// #         2 => {
    /// #             assert_eq!(num_updates, 3);
    /// #             assert!(!c.is_changed());
    /// #             world.remove_resource::<Count>();
    /// #             world.insert_resource(Count(3));
    /// #         }
    /// #         3 if c.is_changed() => {
    /// #             assert_eq!(num_updates, 4);
    /// #             assert!(c.is_added());
    /// #         }
    /// #         3 => {
    /// #             assert_eq!(num_updates, 5);
    /// #             assert!(!c.is_added());
    /// #             c.0 = 4;
    /// #             return std::ops::ControlFlow::Break(());
    /// #         }
    /// #         _ => unreachable!(),
    /// #     }
    /// #     num_updates += 1;
    /// #     std::ops::ControlFlow::Continue(())
    /// # });
    /// # assert_eq!(num_updates, 5);
    /// # assert_eq!(world.resource::<Count>().0, 4);
    /// # assert_eq!(world.last_change_tick(), saved_last_tick);
    /// ```
    pub fn last_change_tick_scope<T>(
        &mut self,
        last_change_tick: Tick,
        f: impl FnOnce(&mut World) -> T,
    ) -> T {
        struct LastTickGuard<'a> {
            world: &'a mut World,
            last_tick: Tick,
        }

        // By setting the change tick in the drop impl, we ensure that
        // the change tick gets reset even if a panic occurs during the scope.
        impl Drop for LastTickGuard<'_> {
            fn drop(&mut self) {
                self.world.last_change_tick = self.last_tick;
            }
        }

        let guard = LastTickGuard {
            last_tick: self.last_change_tick,
            world: self,
        };

        guard.world.last_change_tick = last_change_tick;

        f(guard.world)
    }

    /// Iterates all component change ticks and clamps any older than [`MAX_CHANGE_AGE`](crate::change_detection::MAX_CHANGE_AGE).
    /// This also triggers [`CheckChangeTicks`] observers and returns the same event here.
    ///
    /// Calling this method prevents [`Tick`]s overflowing and thus prevents false positives when comparing them.
    ///
    /// **Note:** Does nothing and returns `None` if the [`World`] counter has not been incremented at least [`CHECK_TICK_THRESHOLD`]
    /// times since the previous pass.
    // TODO: benchmark and optimize
    pub fn check_change_ticks(&mut self) -> Option<CheckChangeTicks> {
        let change_tick = self.change_tick();
        if change_tick.relative_to(self.last_check_tick).get() < CHECK_TICK_THRESHOLD {
            return None;
        }

        let check = CheckChangeTicks(change_tick);

        let Storages {
            ref mut tables,
            ref mut sparse_sets,
            ref mut resources,
            ref mut non_send_resources,
        } = self.storages;

        #[cfg(feature = "trace")]
        let _span = tracing::info_span!("check component ticks").entered();
        tables.check_change_ticks(check);
        sparse_sets.check_change_ticks(check);
        resources.check_change_ticks(check);
        non_send_resources.check_change_ticks(check);
        self.entities.check_change_ticks(check);

        if let Some(mut schedules) = self.get_resource_mut::<Schedules>() {
            schedules.check_change_ticks(check);
        }

        self.trigger(check);
        self.flush();

        self.last_check_tick = change_tick;

        Some(check)
    }

    /// Runs both [`clear_entities`](Self::clear_entities) and [`clear_resources`](Self::clear_resources),
    /// invalidating all [`Entity`] and resource fetches such as [`Res`](crate::system::Res), [`ResMut`](crate::system::ResMut)
    pub fn clear_all(&mut self) {
        self.clear_entities();
        self.clear_resources();
    }

    /// Despawns all entities in this [`World`].
    pub fn clear_entities(&mut self) {
        self.storages.tables.clear();
        self.storages.sparse_sets.clear_entities();
        self.archetypes.clear_entities();
        self.entities.clear();
    }

    /// Clears all resources in this [`World`].
    ///
    /// **Note:** Any resource fetch to this [`World`] will fail unless they are re-initialized,
    /// including engine-internal resources that are only initialized on app/world construction.
    ///
    /// This can easily cause systems expecting certain resources to immediately start panicking.
    /// Use with caution.
    pub fn clear_resources(&mut self) {
        self.storages.resources.clear();
        self.storages.non_send_resources.clear();
    }

    /// Registers all of the components in the given [`Bundle`] and returns both the component
    /// ids and the bundle id.
    ///
    /// This is largely equivalent to calling [`register_component`](Self::register_component) on each
    /// component in the bundle.
    #[inline]
    pub fn register_bundle<B: Bundle>(&mut self) -> &BundleInfo {
        let id = self.register_bundle_info::<B>();

        // SAFETY: We just initialized the bundle so its id should definitely be valid.
        unsafe { self.bundles.get(id).debug_checked_unwrap() }
    }

    pub(crate) fn register_bundle_info<B: Bundle>(&mut self) -> BundleId {
        // SAFETY: These come from the same world. `Self.components_registrator` can't be used since we borrow other fields too.
        let mut registrator =
            unsafe { ComponentsRegistrator::new(&mut self.components, &mut self.component_ids) };

        // SAFETY: `registrator`, `self.storages` and `self.bundles` all come from this world.
        unsafe {
            self.bundles
                .register_info::<B>(&mut registrator, &mut self.storages)
        }
    }

    pub(crate) fn register_contributed_bundle_info<B: Bundle>(&mut self) -> BundleId {
        // SAFETY: These come from the same world. `Self.components_registrator` can't be used since we borrow other fields too.
        let mut registrator =
            unsafe { ComponentsRegistrator::new(&mut self.components, &mut self.component_ids) };

        // SAFETY: `registrator`, `self.bundles` and `self.storages` are all from this world.
        unsafe {
            self.bundles
                .register_contributed_bundle_info::<B>(&mut registrator, &mut self.storages)
        }
    }

    /// Registers the given [`ComponentId`]s as a dynamic bundle and returns both the required component ids and the bundle id.
    ///
    /// Note that the components need to be registered first, this function only creates a bundle combining them. Components
    /// can be registered with [`World::register_component`]/[`_with_descriptor`](World::register_component_with_descriptor).
    ///
    /// **You should prefer to use the typed API [`World::register_bundle`] where possible and only use this in cases where
    /// not all of the actual types are known at compile time.**
    ///
    /// # Panics
    /// This function will panic if any of the provided component ids do not belong to a component known to this [`World`].
    #[inline]
    pub fn register_dynamic_bundle(&mut self, component_ids: &[ComponentId]) -> &BundleInfo {
        let id =
            self.bundles
                .init_dynamic_info(&mut self.storages, &self.components, component_ids);
        // SAFETY: We just initialized the bundle so its id should definitely be valid.
        unsafe { self.bundles.get(id).debug_checked_unwrap() }
    }

    /// Convenience method for accessing the world's default error handler,
    /// which can be overwritten with [`DefaultErrorHandler`].
    #[inline]
    pub fn default_error_handler(&self) -> ErrorHandler {
        self.get_resource::<DefaultErrorHandler>()
            .copied()
            .unwrap_or_default()
            .0
    }
}

impl World {
    /// Gets a pointer to the resource with the id [`ComponentId`] if it exists.
    /// The returned pointer must not be used to modify the resource, and must not be
    /// dereferenced after the immutable borrow of the [`World`] ends.
    ///
    /// **You should prefer to use the typed API [`World::get_resource`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    #[inline]
    pub fn get_resource_by_id(&self, component_id: ComponentId) -> Option<Ptr<'_>> {
        // SAFETY:
        // - `as_unsafe_world_cell_readonly` gives permission to access the whole world immutably
        // - `&self` ensures there are no mutable borrows on world data
        unsafe {
            self.as_unsafe_world_cell_readonly()
                .get_resource_by_id(component_id)
        }
    }

    /// Gets a pointer to the resource with the id [`ComponentId`] if it exists.
    /// The returned pointer may be used to modify the resource, as long as the mutable borrow
    /// of the [`World`] is still valid.
    ///
    /// **You should prefer to use the typed API [`World::get_resource_mut`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    #[inline]
    pub fn get_resource_mut_by_id(&mut self, component_id: ComponentId) -> Option<MutUntyped<'_>> {
        // SAFETY:
        // - `&mut self` ensures that all accessed data is unaliased
        // - `as_unsafe_world_cell` provides mutable permission to the whole world
        unsafe {
            self.as_unsafe_world_cell()
                .get_resource_mut_by_id(component_id)
        }
    }

    /// Iterates over all resources in the world.
    ///
    /// The returned iterator provides lifetimed, but type-unsafe pointers. Actually reading the contents
    /// of each resource will require the use of unsafe code.
    ///
    /// # Examples
    ///
    /// ## Printing the size of all resources
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// # #[derive(Resource)]
    /// # struct A(u32);
    /// # #[derive(Resource)]
    /// # struct B(u32);
    /// #
    /// # let mut world = World::new();
    /// # world.remove_resource::<bevy_ecs::entity_disabling::DefaultQueryFilters>();
    /// # world.insert_resource(A(1));
    /// # world.insert_resource(B(2));
    /// let mut total = 0;
    /// for (info, _) in world.iter_resources() {
    ///    println!("Resource: {}", info.name());
    ///    println!("Size: {} bytes", info.layout().size());
    ///    total += info.layout().size();
    /// }
    /// println!("Total size: {} bytes", total);
    /// # assert_eq!(total, size_of::<A>() + size_of::<B>());
    /// ```
    ///
    /// ## Dynamically running closures for resources matching specific `TypeId`s
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// # use std::collections::HashMap;
    /// # use std::any::TypeId;
    /// # use bevy_ptr::Ptr;
    /// # #[derive(Resource)]
    /// # struct A(u32);
    /// # #[derive(Resource)]
    /// # struct B(u32);
    /// #
    /// # let mut world = World::new();
    /// # world.insert_resource(A(1));
    /// # world.insert_resource(B(2));
    /// #
    /// // In this example, `A` and `B` are resources. We deliberately do not use the
    /// // `bevy_reflect` crate here to showcase the low-level [`Ptr`] usage. You should
    /// // probably use something like `ReflectFromPtr` in a real-world scenario.
    ///
    /// // Create the hash map that will store the closures for each resource type
    /// let mut closures: HashMap<TypeId, Box<dyn Fn(&Ptr<'_>)>> = HashMap::default();
    ///
    /// // Add closure for `A`
    /// closures.insert(TypeId::of::<A>(), Box::new(|ptr| {
    ///     // SAFETY: We assert ptr is the same type of A with TypeId of A
    ///     let a = unsafe { &ptr.deref::<A>() };
    /// #   assert_eq!(a.0, 1);
    ///     // ... do something with `a` here
    /// }));
    ///
    /// // Add closure for `B`
    /// closures.insert(TypeId::of::<B>(), Box::new(|ptr| {
    ///     // SAFETY: We assert ptr is the same type of B with TypeId of B
    ///     let b = unsafe { &ptr.deref::<B>() };
    /// #   assert_eq!(b.0, 2);
    ///     // ... do something with `b` here
    /// }));
    ///
    /// // Iterate all resources, in order to run the closures for each matching resource type
    /// for (info, ptr) in world.iter_resources() {
    ///     let Some(type_id) = info.type_id() else {
    ///        // It's possible for resources to not have a `TypeId` (e.g. non-Rust resources
    ///        // dynamically inserted via a scripting language) in which case we can't match them.
    ///        continue;
    ///     };
    ///
    ///     let Some(closure) = closures.get(&type_id) else {
    ///        // No closure for this resource type, skip it.
    ///        continue;
    ///     };
    ///
    ///     // Run the closure for the resource
    ///     closure(&ptr);
    /// }
    /// ```
    #[inline]
    pub fn iter_resources(&self) -> impl Iterator<Item = (&ComponentInfo, Ptr<'_>)> {
        self.storages
            .resources
            .iter()
            .filter_map(|(component_id, data)| {
                // SAFETY: If a resource has been initialized, a corresponding ComponentInfo must exist with its ID.
                let component_info = unsafe {
                    self.components
                        .get_info(component_id)
                        .debug_checked_unwrap()
                };
                Some((component_info, data.get_data()?))
            })
    }

    /// Mutably iterates over all resources in the world.
    ///
    /// The returned iterator provides lifetimed, but type-unsafe pointers. Actually reading from or writing
    /// to the contents of each resource will require the use of unsafe code.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// # use bevy_ecs::change_detection::MutUntyped;
    /// # use std::collections::HashMap;
    /// # use std::any::TypeId;
    /// # #[derive(Resource)]
    /// # struct A(u32);
    /// # #[derive(Resource)]
    /// # struct B(u32);
    /// #
    /// # let mut world = World::new();
    /// # world.insert_resource(A(1));
    /// # world.insert_resource(B(2));
    /// #
    /// // In this example, `A` and `B` are resources. We deliberately do not use the
    /// // `bevy_reflect` crate here to showcase the low-level `MutUntyped` usage. You should
    /// // probably use something like `ReflectFromPtr` in a real-world scenario.
    ///
    /// // Create the hash map that will store the mutator closures for each resource type
    /// let mut mutators: HashMap<TypeId, Box<dyn Fn(&mut MutUntyped<'_>)>> = HashMap::default();
    ///
    /// // Add mutator closure for `A`
    /// mutators.insert(TypeId::of::<A>(), Box::new(|mut_untyped| {
    ///     // Note: `MutUntyped::as_mut()` automatically marks the resource as changed
    ///     // for ECS change detection, and gives us a `PtrMut` we can use to mutate the resource.
    ///     // SAFETY: We assert ptr is the same type of A with TypeId of A
    ///     let a = unsafe { &mut mut_untyped.as_mut().deref_mut::<A>() };
    /// #   a.0 += 1;
    ///     // ... mutate `a` here
    /// }));
    ///
    /// // Add mutator closure for `B`
    /// mutators.insert(TypeId::of::<B>(), Box::new(|mut_untyped| {
    ///     // SAFETY: We assert ptr is the same type of B with TypeId of B
    ///     let b = unsafe { &mut mut_untyped.as_mut().deref_mut::<B>() };
    /// #   b.0 += 1;
    ///     // ... mutate `b` here
    /// }));
    ///
    /// // Iterate all resources, in order to run the mutator closures for each matching resource type
    /// for (info, mut mut_untyped) in world.iter_resources_mut() {
    ///     let Some(type_id) = info.type_id() else {
    ///        // It's possible for resources to not have a `TypeId` (e.g. non-Rust resources
    ///        // dynamically inserted via a scripting language) in which case we can't match them.
    ///        continue;
    ///     };
    ///
    ///     let Some(mutator) = mutators.get(&type_id) else {
    ///        // No mutator closure for this resource type, skip it.
    ///        continue;
    ///     };
    ///
    ///     // Run the mutator closure for the resource
    ///     mutator(&mut mut_untyped);
    /// }
    /// # assert_eq!(world.resource::<A>().0, 2);
    /// # assert_eq!(world.resource::<B>().0, 3);
    /// ```
    #[inline]
    pub fn iter_resources_mut(&mut self) -> impl Iterator<Item = (&ComponentInfo, MutUntyped<'_>)> {
        self.storages
            .resources
            .iter()
            .filter_map(|(component_id, data)| {
                // SAFETY: If a resource has been initialized, a corresponding ComponentInfo must exist with its ID.
                let component_info = unsafe {
                    self.components
                        .get_info(component_id)
                        .debug_checked_unwrap()
                };
                let (ptr, ticks, caller) = data.get_with_ticks()?;

                // SAFETY:
                // - We have exclusive access to the world, so no other code can be aliasing the `TickCells`
                // - We only hold one `TicksMut` at a time, and we let go of it before getting the next one
                let ticks = unsafe {
                    TicksMut::from_tick_cells(
                        ticks,
                        self.last_change_tick(),
                        self.read_change_tick(),
                    )
                };

                let mut_untyped = MutUntyped {
                    // SAFETY:
                    // - We have exclusive access to the world, so no other code can be aliasing the `Ptr`
                    // - We iterate one resource at a time, and we let go of each `PtrMut` before getting the next one
                    value: unsafe { ptr.assert_unique() },
                    ticks,
                    // SAFETY:
                    // - We have exclusive access to the world, so no other code can be aliasing the `Ptr`
                    // - We iterate one resource at a time, and we let go of each `PtrMut` before getting the next one
                    changed_by: unsafe { caller.map(|caller| caller.deref_mut()) },
                };

                Some((component_info, mut_untyped))
            })
    }

    /// Gets a `!Send` resource to the resource with the id [`ComponentId`] if it exists.
    /// The returned pointer must not be used to modify the resource, and must not be
    /// dereferenced after the immutable borrow of the [`World`] ends.
    ///
    /// **You should prefer to use the typed API [`World::get_resource`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    ///
    /// # Panics
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    #[inline]
    pub fn get_non_send_by_id(&self, component_id: ComponentId) -> Option<Ptr<'_>> {
        // SAFETY:
        // - `as_unsafe_world_cell_readonly` gives permission to access the whole world immutably
        // - `&self` ensures there are no mutable borrows on world data
        unsafe {
            self.as_unsafe_world_cell_readonly()
                .get_non_send_resource_by_id(component_id)
        }
    }

    /// Gets a `!Send` resource to the resource with the id [`ComponentId`] if it exists.
    /// The returned pointer may be used to modify the resource, as long as the mutable borrow
    /// of the [`World`] is still valid.
    ///
    /// **You should prefer to use the typed API [`World::get_resource_mut`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    ///
    /// # Panics
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    #[inline]
    pub fn get_non_send_mut_by_id(&mut self, component_id: ComponentId) -> Option<MutUntyped<'_>> {
        // SAFETY:
        // - `&mut self` ensures that all accessed data is unaliased
        // - `as_unsafe_world_cell` provides mutable permission to the whole world
        unsafe {
            self.as_unsafe_world_cell()
                .get_non_send_resource_mut_by_id(component_id)
        }
    }

    /// Removes the resource of a given type, if it exists. Otherwise returns `None`.
    ///
    /// **You should prefer to use the typed API [`World::remove_resource`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    pub fn remove_resource_by_id(&mut self, component_id: ComponentId) -> Option<()> {
        self.storages
            .resources
            .get_mut(component_id)?
            .remove_and_drop();
        Some(())
    }

    /// Removes the resource of a given type, if it exists. Otherwise returns `None`.
    ///
    /// **You should prefer to use the typed API [`World::remove_resource`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    ///
    /// # Panics
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    pub fn remove_non_send_by_id(&mut self, component_id: ComponentId) -> Option<()> {
        self.storages
            .non_send_resources
            .get_mut(component_id)?
            .remove_and_drop();
        Some(())
    }

    /// Retrieves an immutable untyped reference to the given `entity`'s [`Component`] of the given [`ComponentId`].
    /// Returns `None` if the `entity` does not have a [`Component`] of the given type.
    ///
    /// **You should prefer to use the typed API [`World::get_mut`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    ///
    /// # Panics
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    #[inline]
    pub fn get_by_id(&self, entity: Entity, component_id: ComponentId) -> Option<Ptr<'_>> {
        self.get_entity(entity).ok()?.get_by_id(component_id).ok()
    }

    /// Retrieves a mutable untyped reference to the given `entity`'s [`Component`] of the given [`ComponentId`].
    /// Returns `None` if the `entity` does not have a [`Component`] of the given type.
    ///
    /// **You should prefer to use the typed API [`World::get_mut`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    #[inline]
    pub fn get_mut_by_id(
        &mut self,
        entity: Entity,
        component_id: ComponentId,
    ) -> Option<MutUntyped<'_>> {
        self.get_entity_mut(entity)
            .ok()?
            .into_mut_by_id(component_id)
            .ok()
    }
}

// Schedule-related methods
impl World {
    /// Adds the specified [`Schedule`] to the world.
    /// If a schedule already exists with the same [label](Schedule::label), it will be replaced.
    ///
    /// The schedule can later be run
    /// by calling [`.run_schedule(label)`](Self::run_schedule) or by directly
    /// accessing the [`Schedules`] resource.
    ///
    /// The `Schedules` resource will be initialized if it does not already exist.
    ///
    /// An alternative to this is to call [`Schedules::add_systems()`] with some
    /// [`ScheduleLabel`] and let the schedule for that label be created if it
    /// does not already exist.
    pub fn add_schedule(&mut self, schedule: Schedule) {
        let mut schedules = self.get_resource_or_init::<Schedules>();
        schedules.insert(schedule);
    }

    /// Temporarily removes the schedule associated with `label` from the world,
    /// runs user code, and finally re-adds the schedule.
    /// This returns a [`TryRunScheduleError`] if there is no schedule
    /// associated with `label`.
    ///
    /// The [`Schedule`] is fetched from the [`Schedules`] resource of the world by its label,
    /// and system state is cached.
    ///
    /// For simple cases where you just need to call the schedule once,
    /// consider using [`World::try_run_schedule`] instead.
    /// For other use cases, see the example on [`World::schedule_scope`].
    pub fn try_schedule_scope<R>(
        &mut self,
        label: impl ScheduleLabel,
        f: impl FnOnce(&mut World, &mut Schedule) -> R,
    ) -> Result<R, TryRunScheduleError> {
        let label = label.intern();
        let Some(mut schedule) = self
            .get_resource_mut::<Schedules>()
            .and_then(|mut s| s.remove(label))
        else {
            return Err(TryRunScheduleError(label));
        };

        let value = f(self, &mut schedule);

        let old = self.resource_mut::<Schedules>().insert(schedule);
        if old.is_some() {
            warn!("Schedule `{label:?}` was inserted during a call to `World::schedule_scope`: its value has been overwritten");
        }

        Ok(value)
    }

    /// Temporarily removes the schedule associated with `label` from the world,
    /// runs user code, and finally re-adds the schedule.
    ///
    /// The [`Schedule`] is fetched from the [`Schedules`] resource of the world by its label,
    /// and system state is cached.
    ///
    /// # Examples
    ///
    /// ```
    /// # use bevy_ecs::{prelude::*, schedule::ScheduleLabel};
    /// # #[derive(ScheduleLabel, Debug, Clone, Copy, PartialEq, Eq, Hash)]
    /// # pub struct MySchedule;
    /// # #[derive(Resource)]
    /// # struct Counter(usize);
    /// #
    /// # let mut world = World::new();
    /// # world.insert_resource(Counter(0));
    /// # let mut schedule = Schedule::new(MySchedule);
    /// # schedule.add_systems(tick_counter);
    /// # world.init_resource::<Schedules>();
    /// # world.add_schedule(schedule);
    /// # fn tick_counter(mut counter: ResMut<Counter>) { counter.0 += 1; }
    /// // Run the schedule five times.
    /// world.schedule_scope(MySchedule, |world, schedule| {
    ///     for _ in 0..5 {
    ///         schedule.run(world);
    ///     }
    /// });
    /// # assert_eq!(world.resource::<Counter>().0, 5);
    /// ```
    ///
    /// For simple cases where you just need to call the schedule once,
    /// consider using [`World::run_schedule`] instead.
    ///
    /// # Panics
    ///
    /// If the requested schedule does not exist.
    pub fn schedule_scope<R>(
        &mut self,
        label: impl ScheduleLabel,
        f: impl FnOnce(&mut World, &mut Schedule) -> R,
    ) -> R {
        self.try_schedule_scope(label, f)
            .unwrap_or_else(|e| panic!("{e}"))
    }

    /// Attempts to run the [`Schedule`] associated with the `label` a single time,
    /// and returns a [`TryRunScheduleError`] if the schedule does not exist.
    ///
    /// The [`Schedule`] is fetched from the [`Schedules`] resource of the world by its label,
    /// and system state is cached.
    ///
    /// For simple testing use cases, call [`Schedule::run(&mut world)`](Schedule::run) instead.
    pub fn try_run_schedule(
        &mut self,
        label: impl ScheduleLabel,
    ) -> Result<(), TryRunScheduleError> {
        self.try_schedule_scope(label, |world, sched| sched.run(world))
    }

    /// Runs the [`Schedule`] associated with the `label` a single time.
    ///
    /// The [`Schedule`] is fetched from the [`Schedules`] resource of the world by its label,
    /// and system state is cached.
    ///
    /// For simple testing use cases, call [`Schedule::run(&mut world)`](Schedule::run) instead.
    /// This avoids the need to create a unique [`ScheduleLabel`].
    ///
    /// # Panics
    ///
    /// If the requested schedule does not exist.
    pub fn run_schedule(&mut self, label: impl ScheduleLabel) {
        self.schedule_scope(label, |world, sched| sched.run(world));
    }

    /// Ignore system order ambiguities caused by conflicts on [`Component`]s of type `T`.
    pub fn allow_ambiguous_component<T: Component>(&mut self) {
        let mut schedules = self.remove_resource::<Schedules>().unwrap_or_default();
        schedules.allow_ambiguous_component::<T>(self);
        self.insert_resource(schedules);
    }

    /// Ignore system order ambiguities caused by conflicts on [`Resource`]s of type `T`.
    pub fn allow_ambiguous_resource<T: Resource>(&mut self) {
        let mut schedules = self.remove_resource::<Schedules>().unwrap_or_default();
        schedules.allow_ambiguous_resource::<T>(self);
        self.insert_resource(schedules);
    }
}

impl fmt::Debug for World {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        // SAFETY: `UnsafeWorldCell` requires that this must only access metadata.
        // Accessing any data stored in the world would be unsound.
        f.debug_struct("World")
            .field("id", &self.id)
            .field("entity_count", &self.entities.len())
            .field("archetype_count", &self.archetypes.len())
            .field("component_count", &self.components.len())
            .field("resource_count", &self.storages.resources.len())
            .finish()
    }
}

// SAFETY: all methods on the world ensure that non-send resources are only accessible on the main thread
unsafe impl Send for World {}
// SAFETY: all methods on the world ensure that non-send resources are only accessible on the main thread
unsafe impl Sync for World {}

/// Creates an instance of the type this trait is implemented for
/// using data from the supplied [`World`].
///
/// This can be helpful for complex initialization or context-aware defaults.
///
/// [`FromWorld`] is automatically implemented for any type implementing [`Default`]
/// and may also be derived for:
/// - any struct whose fields all implement `FromWorld`
/// - any enum where one variant has the attribute `#[from_world]`
///
/// ```rs
///
/// #[derive(Default)]
/// struct A;
///
/// #[derive(Default)]
/// struct B(Option<u32>)
///
/// struct C;
///
/// impl FromWorld for C {
///     fn from_world(_world: &mut World) -> Self {
///         Self
///     }
/// }
///
/// #[derive(FromWorld)]
/// struct D(A, B, C);
///
/// #[derive(FromWorld)]
/// enum E {
///     #[from_world]
///     F,
///     G
/// }
/// ```
pub trait FromWorld {
    /// Creates `Self` using data from the given [`World`].
    fn from_world(world: &mut World) -> Self;
}

impl<T: Default> FromWorld for T {
    /// Creates `Self` using [`default()`](`Default::default`).
    fn from_world(_world: &mut World) -> Self {
        T::default()
    }
}

#[cfg(test)]
#[expect(clippy::print_stdout, reason = "Allowed in tests.")]
mod tests {
    use super::{FromWorld, World};
    use crate::{
        change_detection::{DetectChangesMut, MaybeLocation},
        component::{ComponentCloneBehavior, ComponentDescriptor, ComponentInfo, StorageType},
        entity::EntityHashSet,
        entity_disabling::{DefaultQueryFilters, Disabled},
        ptr::OwningPtr,
        resource::Resource,
        world::{error::EntityMutableFetchError, DeferredWorld},
    };
    use alloc::{
        borrow::ToOwned,
        string::{String, ToString},
        sync::Arc,
        vec,
        vec::Vec,
    };
    use bevy_ecs_macros::Component;
    use bevy_platform::collections::{HashMap, HashSet};
    use bevy_utils::prelude::DebugName;
    use core::{
        any::TypeId,
        panic,
        sync::atomic::{AtomicBool, AtomicU32, Ordering},
    };
    use std::{println, sync::Mutex};

    type ID = u8;

    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
    enum DropLogItem {
        Create(ID),
        Drop(ID),
    }

    #[derive(Resource, Component)]
    struct MayPanicInDrop {
        drop_log: Arc<Mutex<Vec<DropLogItem>>>,
        expected_panic_flag: Arc<AtomicBool>,
        should_panic: bool,
        id: u8,
    }

    impl MayPanicInDrop {
        fn new(
            drop_log: &Arc<Mutex<Vec<DropLogItem>>>,
            expected_panic_flag: &Arc<AtomicBool>,
            should_panic: bool,
            id: u8,
        ) -> Self {
            println!("creating component with id {id}");
            drop_log.lock().unwrap().push(DropLogItem::Create(id));

            Self {
                drop_log: Arc::clone(drop_log),
                expected_panic_flag: Arc::clone(expected_panic_flag),
                should_panic,
                id,
            }
        }
    }

    impl Drop for MayPanicInDrop {
        fn drop(&mut self) {
            println!("dropping component with id {}", self.id);

            {
                let mut drop_log = self.drop_log.lock().unwrap();
                drop_log.push(DropLogItem::Drop(self.id));
                // Don't keep the mutex while panicking, or we'll poison it.
                drop(drop_log);
            }

            if self.should_panic {
                self.expected_panic_flag.store(true, Ordering::SeqCst);
                panic!("testing what happens on panic inside drop");
            }
        }
    }

    struct DropTestHelper {
        drop_log: Arc<Mutex<Vec<DropLogItem>>>,
        /// Set to `true` right before we intentionally panic, so that if we get
        /// a panic, we know if it was intended or not.
        expected_panic_flag: Arc<AtomicBool>,
    }

    impl DropTestHelper {
        pub fn new() -> Self {
            Self {
                drop_log: Arc::new(Mutex::new(Vec::<DropLogItem>::new())),
                expected_panic_flag: Arc::new(AtomicBool::new(false)),
            }
        }

        pub fn make_component(&self, should_panic: bool, id: ID) -> MayPanicInDrop {
            MayPanicInDrop::new(&self.drop_log, &self.expected_panic_flag, should_panic, id)
        }

        pub fn finish(self, panic_res: std::thread::Result<()>) -> Vec<DropLogItem> {
            let drop_log = self.drop_log.lock().unwrap();
            let expected_panic_flag = self.expected_panic_flag.load(Ordering::SeqCst);

            if !expected_panic_flag {
                match panic_res {
                    Ok(()) => panic!("Expected a panic but it didn't happen"),
                    Err(e) => std::panic::resume_unwind(e),
                }
            }

            drop_log.to_owned()
        }
    }

    #[test]
    fn panic_while_overwriting_component() {
        let helper = DropTestHelper::new();

        let res = std::panic::catch_unwind(|| {
            let mut world = World::new();
            world
                .spawn_empty()
                .insert(helper.make_component(true, 0))
                .insert(helper.make_component(false, 1));

            println!("Done inserting! Dropping world...");
        });

        let drop_log = helper.finish(res);

        assert_eq!(
            &*drop_log,
            [
                DropLogItem::Create(0),
                DropLogItem::Create(1),
                DropLogItem::Drop(0),
                DropLogItem::Drop(1),
            ]
        );
    }

    #[derive(Resource)]
    struct TestResource(u32);

    #[derive(Resource)]
    struct TestResource2(String);

    #[derive(Resource)]
    struct TestResource3;

    #[test]
    fn get_resource_by_id() {
        let mut world = World::new();
        world.insert_resource(TestResource(42));
        let component_id = world
            .components()
            .get_valid_resource_id(TypeId::of::<TestResource>())
            .unwrap();

        let resource = world.get_resource_by_id(component_id).unwrap();
        // SAFETY: `TestResource` is the correct resource type
        let resource = unsafe { resource.deref::<TestResource>() };

        assert_eq!(resource.0, 42);
    }

    #[test]
    fn get_resource_mut_by_id() {
        let mut world = World::new();
        world.insert_resource(TestResource(42));
        let component_id = world
            .components()
            .get_valid_resource_id(TypeId::of::<TestResource>())
            .unwrap();

        {
            let mut resource = world.get_resource_mut_by_id(component_id).unwrap();
            resource.set_changed();
            // SAFETY: `TestResource` is the correct resource type
            let resource = unsafe { resource.into_inner().deref_mut::<TestResource>() };
            resource.0 = 43;
        }

        let resource = world.get_resource_by_id(component_id).unwrap();
        // SAFETY: `TestResource` is the correct resource type
        let resource = unsafe { resource.deref::<TestResource>() };

        assert_eq!(resource.0, 43);
    }

    #[test]
    fn iter_resources() {
        let mut world = World::new();
        // Remove DefaultQueryFilters so it doesn't show up in the iterator
        world.remove_resource::<DefaultQueryFilters>();
        world.insert_resource(TestResource(42));
        world.insert_resource(TestResource2("Hello, world!".to_string()));
        world.insert_resource(TestResource3);
        world.remove_resource::<TestResource3>();

        let mut iter = world.iter_resources();

        let (info, ptr) = iter.next().unwrap();
        assert_eq!(info.name(), DebugName::type_name::<TestResource>());
        // SAFETY: We know that the resource is of type `TestResource`
        assert_eq!(unsafe { ptr.deref::<TestResource>().0 }, 42);

        let (info, ptr) = iter.next().unwrap();
        assert_eq!(info.name(), DebugName::type_name::<TestResource2>());
        assert_eq!(
            // SAFETY: We know that the resource is of type `TestResource2`
            unsafe { &ptr.deref::<TestResource2>().0 },
            &"Hello, world!".to_string()
        );

        assert!(iter.next().is_none());
    }

    #[test]
    fn iter_resources_mut() {
        let mut world = World::new();
        // Remove DefaultQueryFilters so it doesn't show up in the iterator
        world.remove_resource::<DefaultQueryFilters>();
        world.insert_resource(TestResource(42));
        world.insert_resource(TestResource2("Hello, world!".to_string()));
        world.insert_resource(TestResource3);
        world.remove_resource::<TestResource3>();

        let mut iter = world.iter_resources_mut();

        let (info, mut mut_untyped) = iter.next().unwrap();
        assert_eq!(info.name(), DebugName::type_name::<TestResource>());
        // SAFETY: We know that the resource is of type `TestResource`
        unsafe {
            mut_untyped.as_mut().deref_mut::<TestResource>().0 = 43;
        };

        let (info, mut mut_untyped) = iter.next().unwrap();
        assert_eq!(info.name(), DebugName::type_name::<TestResource2>());
        // SAFETY: We know that the resource is of type `TestResource2`
        unsafe {
            mut_untyped.as_mut().deref_mut::<TestResource2>().0 = "Hello, world?".to_string();
        };

        assert!(iter.next().is_none());
        drop(iter);

        assert_eq!(world.resource::<TestResource>().0, 43);
        assert_eq!(
            world.resource::<TestResource2>().0,
            "Hello, world?".to_string()
        );
    }

    #[test]
    fn dynamic_resource() {
        let mut world = World::new();

        let descriptor = ComponentDescriptor::new_resource::<TestResource>();

        let component_id = world.register_resource_with_descriptor(descriptor);

        let value = 0;
        OwningPtr::make(value, |ptr| {
            // SAFETY: value is valid for the layout of `TestResource`
            unsafe {
                world.insert_resource_by_id(component_id, ptr, MaybeLocation::caller());
            }
        });

        // SAFETY: We know that the resource is of type `TestResource`
        let resource = unsafe {
            world
                .get_resource_by_id(component_id)
                .unwrap()
                .deref::<TestResource>()
        };
        assert_eq!(resource.0, 0);

        assert!(world.remove_resource_by_id(component_id).is_some());
    }

    #[test]
    fn custom_resource_with_layout() {
        static DROP_COUNT: AtomicU32 = AtomicU32::new(0);

        let mut world = World::new();

        // SAFETY: the drop function is valid for the layout and the data will be safe to access from any thread
        let descriptor = unsafe {
            ComponentDescriptor::new_with_layout(
                "Custom Test Component".to_string(),
                StorageType::Table,
                core::alloc::Layout::new::<[u8; 8]>(),
                Some(|ptr| {
                    let data = ptr.read::<[u8; 8]>();
                    assert_eq!(data, [0, 1, 2, 3, 4, 5, 6, 7]);
                    DROP_COUNT.fetch_add(1, Ordering::SeqCst);
                }),
                true,
                ComponentCloneBehavior::Default,
            )
        };

        let component_id = world.register_resource_with_descriptor(descriptor);

        let value: [u8; 8] = [0, 1, 2, 3, 4, 5, 6, 7];
        OwningPtr::make(value, |ptr| {
            // SAFETY: value is valid for the component layout
            unsafe {
                world.insert_resource_by_id(component_id, ptr, MaybeLocation::caller());
            }
        });

        // SAFETY: [u8; 8] is the correct type for the resource
        let data = unsafe {
            world
                .get_resource_by_id(component_id)
                .unwrap()
                .deref::<[u8; 8]>()
        };
        assert_eq!(*data, [0, 1, 2, 3, 4, 5, 6, 7]);

        assert!(world.remove_resource_by_id(component_id).is_some());

        assert_eq!(DROP_COUNT.load(Ordering::SeqCst), 1);
    }

    #[derive(Resource)]
    struct TestFromWorld(u32);
    impl FromWorld for TestFromWorld {
        fn from_world(world: &mut World) -> Self {
            let b = world.resource::<TestResource>();
            Self(b.0)
        }
    }

    #[test]
    fn init_resource_does_not_overwrite() {
        let mut world = World::new();
        world.insert_resource(TestResource(0));
        world.init_resource::<TestFromWorld>();
        world.insert_resource(TestResource(1));
        world.init_resource::<TestFromWorld>();

        let resource = world.resource::<TestFromWorld>();

        assert_eq!(resource.0, 0);
    }

    #[test]
    fn init_non_send_resource_does_not_overwrite() {
        let mut world = World::new();
        world.insert_resource(TestResource(0));
        world.init_non_send_resource::<TestFromWorld>();
        world.insert_resource(TestResource(1));
        world.init_non_send_resource::<TestFromWorld>();

        let resource = world.non_send_resource::<TestFromWorld>();

        assert_eq!(resource.0, 0);
    }

    #[derive(Component)]
    struct Foo;

    #[derive(Component)]
    struct Bar;

    #[derive(Component)]
    struct Baz;

    #[test]
    fn inspect_entity_components() {
        let mut world = World::new();
        let ent0 = world.spawn((Foo, Bar, Baz)).id();
        let ent1 = world.spawn((Foo, Bar)).id();
        let ent2 = world.spawn((Bar, Baz)).id();
        let ent3 = world.spawn((Foo, Baz)).id();
        let ent4 = world.spawn(Foo).id();
        let ent5 = world.spawn(Bar).id();
        let ent6 = world.spawn(Baz).id();

        fn to_type_ids(component_infos: Vec<&ComponentInfo>) -> HashSet<Option<TypeId>> {
            component_infos
                .into_iter()
                .map(ComponentInfo::type_id)
                .collect()
        }

        let foo_id = TypeId::of::<Foo>();
        let bar_id = TypeId::of::<Bar>();
        let baz_id = TypeId::of::<Baz>();
        assert_eq!(
            to_type_ids(world.inspect_entity(ent0).unwrap().collect()),
            [Some(foo_id), Some(bar_id), Some(baz_id)]
                .into_iter()
                .collect::<HashSet<_>>()
        );
        assert_eq!(
            to_type_ids(world.inspect_entity(ent1).unwrap().collect()),
            [Some(foo_id), Some(bar_id)]
                .into_iter()
                .collect::<HashSet<_>>()
        );
        assert_eq!(
            to_type_ids(world.inspect_entity(ent2).unwrap().collect()),
            [Some(bar_id), Some(baz_id)]
                .into_iter()
                .collect::<HashSet<_>>()
        );
        assert_eq!(
            to_type_ids(world.inspect_entity(ent3).unwrap().collect()),
            [Some(foo_id), Some(baz_id)]
                .into_iter()
                .collect::<HashSet<_>>()
        );
        assert_eq!(
            to_type_ids(world.inspect_entity(ent4).unwrap().collect()),
            [Some(foo_id)].into_iter().collect::<HashSet<_>>()
        );
        assert_eq!(
            to_type_ids(world.inspect_entity(ent5).unwrap().collect()),
            [Some(bar_id)].into_iter().collect::<HashSet<_>>()
        );
        assert_eq!(
            to_type_ids(world.inspect_entity(ent6).unwrap().collect()),
            [Some(baz_id)].into_iter().collect::<HashSet<_>>()
        );
    }

    #[test]
    fn iterate_entities() {
        let mut world = World::new();
        let mut entity_counters = <HashMap<_, _>>::default();

        let iterate_and_count_entities = |world: &World, entity_counters: &mut HashMap<_, _>| {
            entity_counters.clear();
            #[expect(deprecated, reason = "remove this test in in 0.17.0")]
            for entity in world.iter_entities() {
                let counter = entity_counters.entry(entity.id()).or_insert(0);
                *counter += 1;
            }
        };

        // Adding one entity and validating iteration
        let ent0 = world.spawn((Foo, Bar, Baz)).id();

        iterate_and_count_entities(&world, &mut entity_counters);
        assert_eq!(entity_counters[&ent0], 1);
        assert_eq!(entity_counters.len(), 1);

        // Spawning three more entities and then validating iteration
        let ent1 = world.spawn((Foo, Bar)).id();
        let ent2 = world.spawn((Bar, Baz)).id();
        let ent3 = world.spawn((Foo, Baz)).id();

        iterate_and_count_entities(&world, &mut entity_counters);

        assert_eq!(entity_counters[&ent0], 1);
        assert_eq!(entity_counters[&ent1], 1);
        assert_eq!(entity_counters[&ent2], 1);
        assert_eq!(entity_counters[&ent3], 1);
        assert_eq!(entity_counters.len(), 4);

        // Despawning first entity and then validating the iteration
        assert!(world.despawn(ent0));

        iterate_and_count_entities(&world, &mut entity_counters);

        assert_eq!(entity_counters[&ent1], 1);
        assert_eq!(entity_counters[&ent2], 1);
        assert_eq!(entity_counters[&ent3], 1);
        assert_eq!(entity_counters.len(), 3);

        // Spawning three more entities, despawning three and then validating the iteration
        let ent4 = world.spawn(Foo).id();
        let ent5 = world.spawn(Bar).id();
        let ent6 = world.spawn(Baz).id();

        assert!(world.despawn(ent2));
        assert!(world.despawn(ent3));
        assert!(world.despawn(ent4));

        iterate_and_count_entities(&world, &mut entity_counters);

        assert_eq!(entity_counters[&ent1], 1);
        assert_eq!(entity_counters[&ent5], 1);
        assert_eq!(entity_counters[&ent6], 1);
        assert_eq!(entity_counters.len(), 3);

        // Despawning remaining entities and then validating the iteration
        assert!(world.despawn(ent1));
        assert!(world.despawn(ent5));
        assert!(world.despawn(ent6));

        iterate_and_count_entities(&world, &mut entity_counters);

        assert_eq!(entity_counters.len(), 0);
    }

    #[test]
    fn iterate_entities_mut() {
        #[derive(Component, PartialEq, Debug)]
        struct A(i32);

        #[derive(Component, PartialEq, Debug)]
        struct B(i32);

        let mut world = World::new();

        let a1 = world.spawn(A(1)).id();
        let a2 = world.spawn(A(2)).id();
        let b1 = world.spawn(B(1)).id();
        let b2 = world.spawn(B(2)).id();

        #[expect(deprecated, reason = "remove this test in 0.17.0")]
        for mut entity in world.iter_entities_mut() {
            if let Some(mut a) = entity.get_mut::<A>() {
                a.0 -= 1;
            }
        }
        assert_eq!(world.entity(a1).get(), Some(&A(0)));
        assert_eq!(world.entity(a2).get(), Some(&A(1)));
        assert_eq!(world.entity(b1).get(), Some(&B(1)));
        assert_eq!(world.entity(b2).get(), Some(&B(2)));

        #[expect(deprecated, reason = "remove this test in in 0.17.0")]
        for mut entity in world.iter_entities_mut() {
            if let Some(mut b) = entity.get_mut::<B>() {
                b.0 *= 2;
            }
        }
        assert_eq!(world.entity(a1).get(), Some(&A(0)));
        assert_eq!(world.entity(a2).get(), Some(&A(1)));
        assert_eq!(world.entity(b1).get(), Some(&B(2)));
        assert_eq!(world.entity(b2).get(), Some(&B(4)));

        #[expect(deprecated, reason = "remove this test in in 0.17.0")]
        let mut entities = world.iter_entities_mut().collect::<Vec<_>>();
        entities.sort_by_key(|e| e.get::<A>().map(|a| a.0).or(e.get::<B>().map(|b| b.0)));
        let (a, b) = entities.split_at_mut(2);
        core::mem::swap(
            &mut a[1].get_mut::<A>().unwrap().0,
            &mut b[0].get_mut::<B>().unwrap().0,
        );
        assert_eq!(world.entity(a1).get(), Some(&A(0)));
        assert_eq!(world.entity(a2).get(), Some(&A(2)));
        assert_eq!(world.entity(b1).get(), Some(&B(1)));
        assert_eq!(world.entity(b2).get(), Some(&B(4)));
    }

    #[test]
    fn spawn_empty_bundle() {
        let mut world = World::new();
        world.spawn(());
    }

    #[test]
    fn get_entity() {
        let mut world = World::new();

        let e1 = world.spawn_empty().id();
        let e2 = world.spawn_empty().id();

        assert!(world.get_entity(e1).is_ok());
        assert!(world.get_entity([e1, e2]).is_ok());
        assert!(world
            .get_entity(&[e1, e2] /* this is an array not a slice */)
            .is_ok());
        assert!(world.get_entity(&vec![e1, e2][..]).is_ok());
        assert!(world
            .get_entity(&EntityHashSet::from_iter([e1, e2]))
            .is_ok());

        world.entity_mut(e1).despawn();

        assert_eq!(
            Err(e1),
            world.get_entity(e1).map(|_| {}).map_err(|e| e.entity)
        );
        assert_eq!(
            Err(e1),
            world.get_entity([e1, e2]).map(|_| {}).map_err(|e| e.entity)
        );
        assert_eq!(
            Err(e1),
            world
                .get_entity(&[e1, e2] /* this is an array not a slice */)
                .map(|_| {})
                .map_err(|e| e.entity)
        );
        assert_eq!(
            Err(e1),
            world
                .get_entity(&vec![e1, e2][..])
                .map(|_| {})
                .map_err(|e| e.entity)
        );
        assert_eq!(
            Err(e1),
            world
                .get_entity(&EntityHashSet::from_iter([e1, e2]))
                .map(|_| {})
                .map_err(|e| e.entity)
        );
    }

    #[test]
    fn get_entity_mut() {
        let mut world = World::new();

        let e1 = world.spawn_empty().id();
        let e2 = world.spawn_empty().id();

        assert!(world.get_entity_mut(e1).is_ok());
        assert!(world.get_entity_mut([e1, e2]).is_ok());
        assert!(world
            .get_entity_mut(&[e1, e2] /* this is an array not a slice */)
            .is_ok());
        assert!(world.get_entity_mut(&vec![e1, e2][..]).is_ok());
        assert!(world
            .get_entity_mut(&EntityHashSet::from_iter([e1, e2]))
            .is_ok());

        assert_eq!(
            Err(EntityMutableFetchError::AliasedMutability(e1)),
            world.get_entity_mut([e1, e2, e1]).map(|_| {})
        );
        assert_eq!(
            Err(EntityMutableFetchError::AliasedMutability(e1)),
            world
                .get_entity_mut(&[e1, e2, e1] /* this is an array not a slice */)
                .map(|_| {})
        );
        assert_eq!(
            Err(EntityMutableFetchError::AliasedMutability(e1)),
            world.get_entity_mut(&vec![e1, e2, e1][..]).map(|_| {})
        );
        // Aliased mutability isn't allowed by HashSets
        assert!(world
            .get_entity_mut(&EntityHashSet::from_iter([e1, e2, e1]))
            .is_ok());

        world.entity_mut(e1).despawn();

        assert!(matches!(
            world.get_entity_mut(e1).map(|_| {}),
            Err(EntityMutableFetchError::EntityDoesNotExist(e)) if e.entity == e1
        ));
        assert!(matches!(
            world.get_entity_mut([e1, e2]).map(|_| {}),
            Err(EntityMutableFetchError::EntityDoesNotExist(e)) if e.entity == e1));
        assert!(matches!(
            world
                .get_entity_mut(&[e1, e2] /* this is an array not a slice */)
                .map(|_| {}),
            Err(EntityMutableFetchError::EntityDoesNotExist(e)) if e.entity == e1));
        assert!(matches!(
            world.get_entity_mut(&vec![e1, e2][..]).map(|_| {}),
            Err(EntityMutableFetchError::EntityDoesNotExist(e)) if e.entity == e1,
        ));
        assert!(matches!(
            world
                .get_entity_mut(&EntityHashSet::from_iter([e1, e2]))
                .map(|_| {}),
            Err(EntityMutableFetchError::EntityDoesNotExist(e)) if e.entity == e1));
    }

    #[test]
    #[track_caller]
    fn entity_spawn_despawn_tracking() {
        use core::panic::Location;

        let mut world = World::new();
        let entity = world.spawn_empty().id();
        assert_eq!(
            world.entities.entity_get_spawned_or_despawned_by(entity),
            MaybeLocation::new(Some(Location::caller()))
        );
        assert_eq!(
            world.entities.entity_get_spawned_or_despawned_at(entity),
            Some(world.change_tick())
        );
        world.despawn(entity);
        assert_eq!(
            world.entities.entity_get_spawned_or_despawned_by(entity),
            MaybeLocation::new(Some(Location::caller()))
        );
        assert_eq!(
            world.entities.entity_get_spawned_or_despawned_at(entity),
            Some(world.change_tick())
        );
        let new = world.spawn_empty().id();
        assert_eq!(entity.index(), new.index());
        assert_eq!(
            world.entities.entity_get_spawned_or_despawned_by(entity),
            MaybeLocation::new(None)
        );
        assert_eq!(
            world.entities.entity_get_spawned_or_despawned_at(entity),
            None
        );
        world.despawn(new);
        assert_eq!(
            world.entities.entity_get_spawned_or_despawned_by(entity),
            MaybeLocation::new(None)
        );
        assert_eq!(
            world.entities.entity_get_spawned_or_despawned_at(entity),
            None
        );
    }

    #[test]
    fn new_world_has_disabling() {
        let mut world = World::new();
        world.spawn(Foo);
        world.spawn((Foo, Disabled));
        assert_eq!(1, world.query::<&Foo>().iter(&world).count());

        // If we explicitly remove the resource, no entities should be filtered anymore
        world.remove_resource::<DefaultQueryFilters>();
        assert_eq!(2, world.query::<&Foo>().iter(&world).count());
    }

    #[test]
    fn entities_and_commands() {
        #[derive(Component, PartialEq, Debug)]
        struct Foo(u32);

        let mut world = World::new();

        let eid = world.spawn(Foo(35)).id();

        let (mut fetcher, mut commands) = world.entities_and_commands();
        let emut = fetcher.get_mut(eid).unwrap();
        commands.entity(eid).despawn();
        assert_eq!(emut.get::<Foo>().unwrap(), &Foo(35));

        world.flush();

        assert!(world.get_entity(eid).is_err());
    }

    #[test]
    fn entities_and_commands_deferred() {
        #[derive(Component, PartialEq, Debug)]
        struct Foo(u32);

        let mut world = World::new();

        let eid = world.spawn(Foo(1)).id();

        let mut dworld = DeferredWorld::from(&mut world);

        let (mut fetcher, mut commands) = dworld.entities_and_commands();
        let emut = fetcher.get_mut(eid).unwrap();
        commands.entity(eid).despawn();
        assert_eq!(emut.get::<Foo>().unwrap(), &Foo(1));

        world.flush();

        assert!(world.get_entity(eid).is_err());
    }
}