deferred_world.rs

use core::ops::Deref;

use bevy_utils::prelude::DebugName;

use crate::{
    archetype::Archetype,
    change_detection::{MaybeLocation, MutUntyped},
    component::{ComponentId, Mutable},
    entity::Entity,
    event::{
        BufferedEvent, EntityComponentsTrigger, Event, EventId, EventKey, Events, Trigger,
        WriteBatchIds,
    },
    lifecycle::{HookContext, Insert, Replace, INSERT, REPLACE},
    observer::TriggerContext,
    prelude::{Component, QueryState},
    query::{QueryData, QueryFilter},
    relationship::RelationshipHookMode,
    resource::Resource,
    system::{Commands, Query},
    world::{error::EntityMutableFetchError, EntityFetcher, WorldEntityFetch},
};

use super::{unsafe_world_cell::UnsafeWorldCell, Mut, World};

/// A [`World`] reference that disallows structural ECS changes.
/// This includes initializing resources, registering components or spawning entities.
///
/// This means that in order to add entities, for example, you will need to use commands instead of the world directly.
pub struct DeferredWorld<'w> {
    // SAFETY: Implementers must not use this reference to make structural changes
    world: UnsafeWorldCell<'w>,
}

impl<'w> Deref for DeferredWorld<'w> {
    type Target = World;

    fn deref(&self) -> &Self::Target {
        // SAFETY: Structural changes cannot be made through &World
        unsafe { self.world.world() }
    }
}

impl<'w> UnsafeWorldCell<'w> {
    /// Turn self into a [`DeferredWorld`]
    ///
    /// # Safety
    /// Caller must ensure there are no outstanding mutable references to world and no
    /// outstanding references to the world's command queue, resource or component data
    #[inline]
    pub unsafe fn into_deferred(self) -> DeferredWorld<'w> {
        DeferredWorld { world: self }
    }
}

impl<'w> From<&'w mut World> for DeferredWorld<'w> {
    fn from(world: &'w mut World) -> DeferredWorld<'w> {
        DeferredWorld {
            world: world.as_unsafe_world_cell(),
        }
    }
}

impl<'w> DeferredWorld<'w> {
    /// Reborrow self as a new instance of [`DeferredWorld`]
    #[inline]
    pub fn reborrow(&mut self) -> DeferredWorld<'_> {
        DeferredWorld { world: self.world }
    }

    /// Creates a [`Commands`] instance that pushes to the world's command queue
    #[inline]
    pub fn commands(&mut self) -> Commands<'_, '_> {
        // SAFETY: &mut self ensure that there are no outstanding accesses to the queue
        let command_queue = unsafe { self.world.get_raw_command_queue() };
        // SAFETY: command_queue is stored on world and always valid while the world exists
        unsafe { Commands::new_raw_from_entities(command_queue, self.world.entities()) }
    }

    /// 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.
    #[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`](DeferredWorld::get_mut).
    #[inline]
    #[track_caller]
    pub(crate) fn modify_component_with_relationship_hook_mode<T: Component, R>(
        &mut self,
        entity: Entity,
        relationship_hook_mode: RelationshipHookMode,
        f: impl FnOnce(&mut T) -> R,
    ) -> Result<Option<R>, EntityMutableFetchError> {
        // If the component is not registered, then it doesn't exist on this entity, so no action required.
        let Some(component_id) = self.component_id::<T>() else {
            return Ok(None);
        };

        self.modify_component_by_id_with_relationship_hook_mode(
            entity,
            component_id,
            relationship_hook_mode,
            move |component| {
                // SAFETY: component matches the component_id collected in the above line
                let mut component = unsafe { component.with_type::<T>() };

                f(&mut component)
            },
        )
    }

    /// 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`](DeferredWorld::get_mut_by_id).
    ///
    /// You should prefer the typed [`modify_component_with_relationship_hook_mode`](DeferredWorld::modify_component_with_relationship_hook_mode)
    /// whenever possible.
    #[inline]
    #[track_caller]
    pub(crate) fn modify_component_by_id_with_relationship_hook_mode<R>(
        &mut self,
        entity: Entity,
        component_id: ComponentId,
        relationship_hook_mode: RelationshipHookMode,
        f: impl for<'a> FnOnce(MutUntyped<'a>) -> R,
    ) -> Result<Option<R>, EntityMutableFetchError> {
        let entity_cell = self.get_entity_mut(entity)?;

        if !entity_cell.contains_id(component_id) {
            return Ok(None);
        }

        let archetype = &raw const *entity_cell.archetype();

        // SAFETY:
        // - DeferredWorld ensures archetype pointer will remain valid as no
        //   relocations will occur.
        // - component_id exists on this world and this entity
        // - REPLACE is able to accept ZST events
        unsafe {
            let archetype = &*archetype;
            self.trigger_on_replace(
                archetype,
                entity,
                [component_id].into_iter(),
                MaybeLocation::caller(),
                relationship_hook_mode,
            );
            if archetype.has_replace_observer() {
                // SAFETY: the REPLACE event_key corresponds to the Replace event's type
                self.trigger_raw(
                    REPLACE,
                    &mut Replace { entity },
                    &mut EntityComponentsTrigger {
                        components: &[component_id],
                    },
                    MaybeLocation::caller(),
                );
            }
        }

        let mut entity_cell = self
            .get_entity_mut(entity)
            .expect("entity access confirmed above");

        // SAFETY: we will run the required hooks to simulate removal/replacement.
        let mut component = unsafe {
            entity_cell
                .get_mut_assume_mutable_by_id(component_id)
                .expect("component access confirmed above")
        };

        let result = f(component.reborrow());

        // Simulate adding this component by updating the relevant ticks
        *component.ticks.added = *component.ticks.changed;

        // SAFETY:
        // - DeferredWorld ensures archetype pointer will remain valid as no
        //   relocations will occur.
        // - component_id exists on this world and this entity
        // - REPLACE is able to accept ZST events
        unsafe {
            let archetype = &*archetype;
            self.trigger_on_insert(
                archetype,
                entity,
                [component_id].into_iter(),
                MaybeLocation::caller(),
                relationship_hook_mode,
            );
            if archetype.has_insert_observer() {
                // SAFETY: the INSERT event_key corresponds to the Insert event's type
                self.trigger_raw(
                    INSERT,
                    &mut Insert { entity },
                    &mut EntityComponentsTrigger {
                        components: &[component_id],
                    },
                    MaybeLocation::caller(),
                );
            }
        }

        Ok(Some(result))
    }

    /// 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 [`EntityMut`].
    /// - 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 an [`&EntityHashSet`] to receive an [`EntityHashMap<EntityMut>`].
    ///
    /// **As [`DeferredWorld`] does not allow structural changes, all returned
    /// references are [`EntityMut`]s, which do not allow structural changes
    /// (i.e. adding/removing components or despawning the entity).**
    ///
    /// # 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 [`DeferredWorld::entity_mut`].
    ///
    /// [`EntityMut`]: crate::world::EntityMut
    /// [`&EntityHashSet`]: crate::entity::EntityHashSet
    /// [`EntityHashMap<EntityMut>`]: crate::entity::EntityHashMap
    /// [`Vec<EntityMut>`]: alloc::vec::Vec
    #[inline]
    pub fn get_entity_mut<F: WorldEntityFetch>(
        &mut self,
        entities: F,
    ) -> Result<F::DeferredMut<'_>, 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_deferred_mut(cell) }
    }

    /// 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 [`DeferredWorld::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 [`EntityMut`].
    /// - 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 an [`&EntityHashSet`] to receive an [`EntityHashMap<EntityMut>`].
    ///
    /// **As [`DeferredWorld`] does not allow structural changes, all returned
    /// references are [`EntityMut`]s, which do not allow structural changes
    /// (i.e. adding/removing components or despawning the entity).**
    ///
    /// # Panics
    ///
    /// If any of the given `entities` do not exist in the world.
    ///
    /// # Examples
    ///
    /// ## Single [`Entity`]
    ///
    /// ```
    /// # use bevy_ecs::{prelude::*, world::DeferredWorld};
    /// #[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 world: DeferredWorld = // ...
    /// #   DeferredWorld::from(&mut world);
    ///
    /// 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);
    /// ```
    ///
    /// ## Array of [`Entity`]s
    ///
    /// ```
    /// # use bevy_ecs::{prelude::*, world::DeferredWorld};
    /// #[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 world: DeferredWorld = // ...
    /// #   DeferredWorld::from(&mut world);
    ///
    /// 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::*, world::DeferredWorld};
    /// #[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 mut world: DeferredWorld = // ...
    /// #   DeferredWorld::from(&mut world);
    ///
    /// 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`]
    ///
    /// ```
    /// # use bevy_ecs::{prelude::*, entity::EntityHashSet, world::DeferredWorld};
    /// #[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 mut world: DeferredWorld = // ...
    /// #   DeferredWorld::from(&mut world);
    ///
    /// 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);
    /// }
    /// ```
    ///
    /// [`EntityMut`]: crate::world::EntityMut
    /// [`&EntityHashSet`]: crate::entity::EntityHashSet
    /// [`EntityHashMap<EntityMut>`]: crate::entity::EntityHashMap
    /// [`Vec<EntityMut>`]: alloc::vec::Vec
    #[inline]
    pub fn entity_mut<F: WorldEntityFetch>(&mut self, entities: F) -> F::DeferredMut<'_> {
        self.get_entity_mut(entities).unwrap()
    }

    /// 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 [`World::entities_and_commands`] for the non-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::new();
    /// # let e1 = _world.spawn_empty().id();
    /// # let e2 = _world.spawn_empty().id();
    /// # let eid = _world.spawn(Targets(vec![e1, e2])).id();
    /// let mut world: DeferredWorld = // ...
    /// #   DeferredWorld::from(&mut _world);
    /// 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)
    }

    /// Returns [`Query`] for the given [`QueryState`], which is used to efficiently
    /// run queries on the [`World`] by storing and reusing the [`QueryState`].
    ///
    /// # Panics
    /// If state is from a different world then self
    #[inline]
    pub fn query<'s, D: QueryData, F: QueryFilter>(
        &mut self,
        state: &'s mut QueryState<D, F>,
    ) -> Query<'_, 's, D, F> {
        // SAFETY: We have mutable access to the entire world
        unsafe { state.query_unchecked(self.world) }
    }

    /// Gets a mutable reference to the resource of the given type
    ///
    /// # Panics
    ///
    /// Panics if the resource does not exist.
    /// Use [`get_resource_mut`](DeferredWorld::get_resource_mut) instead if you want to handle this case.
    #[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 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: &mut self ensure that there are no outstanding accesses to the resource
        unsafe { self.world.get_resource_mut() }
    }

    /// 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 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: &mut self ensure that there are no outstanding accesses to the resource
        unsafe { self.world.get_non_send_resource_mut() }
    }

    /// 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 `DeferredWorld::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 `DeferredWorld::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 `DeferredWorld::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)
    }

    /// 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 ensure that there are no outstanding accesses to the resource
        unsafe { self.world.get_resource_mut_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 ensure that there are no outstanding accesses to the resource
        unsafe { self.world.get_non_send_resource_mut_by_id(component_id) }
    }

    /// 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()
    }

    /// Triggers all `on_add` hooks for [`ComponentId`] in target.
    ///
    /// # Safety
    /// Caller must ensure [`ComponentId`] in target exist in self.
    #[inline]
    pub(crate) unsafe fn trigger_on_add(
        &mut self,
        archetype: &Archetype,
        entity: Entity,
        targets: impl Iterator<Item = ComponentId>,
        caller: MaybeLocation,
    ) {
        if archetype.has_add_hook() {
            for component_id in targets {
                // SAFETY: Caller ensures that these components exist
                let hooks = unsafe { self.components().get_info_unchecked(component_id) }.hooks();
                if let Some(hook) = hooks.on_add {
                    hook(
                        DeferredWorld { world: self.world },
                        HookContext {
                            entity,
                            component_id,
                            caller,
                            relationship_hook_mode: RelationshipHookMode::Run,
                        },
                    );
                }
            }
        }
    }

    /// Triggers all `on_insert` hooks for [`ComponentId`] in target.
    ///
    /// # Safety
    /// Caller must ensure [`ComponentId`] in target exist in self.
    #[inline]
    pub(crate) unsafe fn trigger_on_insert(
        &mut self,
        archetype: &Archetype,
        entity: Entity,
        targets: impl Iterator<Item = ComponentId>,
        caller: MaybeLocation,
        relationship_hook_mode: RelationshipHookMode,
    ) {
        if archetype.has_insert_hook() {
            for component_id in targets {
                // SAFETY: Caller ensures that these components exist
                let hooks = unsafe { self.components().get_info_unchecked(component_id) }.hooks();
                if let Some(hook) = hooks.on_insert {
                    hook(
                        DeferredWorld { world: self.world },
                        HookContext {
                            entity,
                            component_id,
                            caller,
                            relationship_hook_mode,
                        },
                    );
                }
            }
        }
    }

    /// Triggers all `on_replace` hooks for [`ComponentId`] in target.
    ///
    /// # Safety
    /// Caller must ensure [`ComponentId`] in target exist in self.
    #[inline]
    pub(crate) unsafe fn trigger_on_replace(
        &mut self,
        archetype: &Archetype,
        entity: Entity,
        targets: impl Iterator<Item = ComponentId>,
        caller: MaybeLocation,
        relationship_hook_mode: RelationshipHookMode,
    ) {
        if archetype.has_replace_hook() {
            for component_id in targets {
                // SAFETY: Caller ensures that these components exist
                let hooks = unsafe { self.components().get_info_unchecked(component_id) }.hooks();
                if let Some(hook) = hooks.on_replace {
                    hook(
                        DeferredWorld { world: self.world },
                        HookContext {
                            entity,
                            component_id,
                            caller,
                            relationship_hook_mode,
                        },
                    );
                }
            }
        }
    }

    /// Triggers all `on_remove` hooks for [`ComponentId`] in target.
    ///
    /// # Safety
    /// Caller must ensure [`ComponentId`] in target exist in self.
    #[inline]
    pub(crate) unsafe fn trigger_on_remove(
        &mut self,
        archetype: &Archetype,
        entity: Entity,
        targets: impl Iterator<Item = ComponentId>,
        caller: MaybeLocation,
    ) {
        if archetype.has_remove_hook() {
            for component_id in targets {
                // SAFETY: Caller ensures that these components exist
                let hooks = unsafe { self.components().get_info_unchecked(component_id) }.hooks();
                if let Some(hook) = hooks.on_remove {
                    hook(
                        DeferredWorld { world: self.world },
                        HookContext {
                            entity,
                            component_id,
                            caller,
                            relationship_hook_mode: RelationshipHookMode::Run,
                        },
                    );
                }
            }
        }
    }

    /// Triggers all `on_despawn` hooks for [`ComponentId`] in target.
    ///
    /// # Safety
    /// Caller must ensure [`ComponentId`] in target exist in self.
    #[inline]
    pub(crate) unsafe fn trigger_on_despawn(
        &mut self,
        archetype: &Archetype,
        entity: Entity,
        targets: impl Iterator<Item = ComponentId>,
        caller: MaybeLocation,
    ) {
        if archetype.has_despawn_hook() {
            for component_id in targets {
                // SAFETY: Caller ensures that these components exist
                let hooks = unsafe { self.components().get_info_unchecked(component_id) }.hooks();
                if let Some(hook) = hooks.on_despawn {
                    hook(
                        DeferredWorld { world: self.world },
                        HookContext {
                            entity,
                            component_id,
                            caller,
                            relationship_hook_mode: RelationshipHookMode::Run,
                        },
                    );
                }
            }
        }
    }

    /// Triggers all `event` observers for the given `targets`
    ///
    /// # Safety
    /// - Caller must ensure `E` is accessible as the type represented by `event_key`
    #[inline]
    pub unsafe fn trigger_raw<'a, E: Event>(
        &mut self,
        event_key: EventKey,
        event: &mut E,
        trigger: &mut E::Trigger<'a>,
        caller: MaybeLocation,
    ) {
        // SAFETY: You cannot get a mutable reference to `observers` from `DeferredWorld`
        let (mut world, observers) = unsafe {
            let world = self.as_unsafe_world_cell();
            let observers = world.observers();
            let Some(observers) = observers.try_get_observers(event_key) else {
                return;
            };
            // SAFETY: The only outstanding reference to world is `observers`
            (world.into_deferred(), observers)
        };
        let context = TriggerContext { event_key, caller };

        // SAFETY:
        // - `observers` comes from `world`, and corresponds to the `event_key`, as it was looked up above
        // - trigger_context contains the correct event_key for `event`, as enforced by the call to `trigger_raw`
        // - This method is being called for an `event` whose `Event::Trigger` matches, as the input trigger is E::Trigger.
        unsafe {
            trigger.trigger(world.reborrow(), observers, &context, event);
        }
    }

    /// Sends a global [`Event`] without any targets.
    ///
    /// This will run any [`Observer`] of the given [`Event`] that isn't scoped to specific targets.
    ///
    /// [`Observer`]: crate::observer::Observer
    pub fn trigger<'a>(&mut self, event: impl Event<Trigger<'a>: Default>) {
        self.commands().trigger(event);
    }

    /// Gets an [`UnsafeWorldCell`] containing the underlying world.
    ///
    /// # Safety
    /// - must only be used to make non-structural ECS changes
    #[inline]
    pub(crate) fn as_unsafe_world_cell(&mut self) -> UnsafeWorldCell<'_> {
        self.world
    }
}