error_handling.rs

//! Showcases how fallible systems and observers can make use of Rust's powerful result handling
//! syntax.

use bevy::ecs::{error::warn, world::DeferredWorld};
use bevy::math::sampling::UniformMeshSampler;
use bevy::prelude::*;

use rand::distr::Distribution;
use rand::SeedableRng;
use rand_chacha::ChaCha8Rng;

fn main() {
    let mut app = App::new();
    // By default, fallible systems that return an error will panic.
    //
    // We can change this by setting a custom error handler, which applies to the entire app
    // (you can also set it for specific `World`s).
    // Here we are using one of the built-in error handlers.
    // Bevy provides built-in handlers for `panic`, `error`, `warn`, `info`,
    // `debug`, `trace` and `ignore`.
    app.set_error_handler(warn);

    app.add_plugins(DefaultPlugins);

    #[cfg(feature = "bevy_mesh_picking_backend")]
    app.add_plugins(MeshPickingPlugin);

    // Fallible systems can be used the same way as regular systems. The only difference is they
    // return a `Result<(), BevyError>` instead of a `()` (unit) type. Bevy will handle both
    // types of systems the same way, except for the error handling.
    app.add_systems(Startup, setup);

    // Commands can also return `Result`s, which are automatically handled by the global error handler
    // if not explicitly handled by the user.
    app.add_systems(Startup, failing_commands);

    // Individual systems can also be handled by piping the output result:
    app.add_systems(
        PostStartup,
        failing_system.pipe(|result: In<Result>| {
            let _ = result.0.inspect_err(|err| info!("captured error: {err}"));
        }),
    );

    // Fallible observers are also supported.
    app.add_observer(fallible_observer);

    // If we run the app, we'll see the following output at startup:
    //
    //  WARN Encountered an error in system `fallible_systems::failing_system`: Resource not initialized
    // ERROR fallible_systems::failing_system failed: Resource not initialized
    //  INFO captured error: Resource not initialized
    app.run();
}

/// An example of a system that calls several fallible functions with the question mark operator.
///
/// See: <https://doc.rust-lang.org/reference/expressions/operator-expr.html#the-question-mark-operator>
fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) -> Result {
    let mut seeded_rng = ChaCha8Rng::seed_from_u64(19878367467712);

    // Make a plane for establishing space.
    commands.spawn((
        Mesh3d(meshes.add(Plane3d::default().mesh().size(12.0, 12.0))),
        MeshMaterial3d(materials.add(Color::srgb(0.3, 0.5, 0.3))),
        Transform::from_xyz(0.0, -2.5, 0.0),
    ));

    // Spawn a light:
    commands.spawn((
        PointLight {
            shadows_enabled: true,
            ..default()
        },
        Transform::from_xyz(4.0, 8.0, 4.0),
    ));

    // Spawn a camera:
    commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(-2.0, 3.0, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
    ));

    // Create a new sphere mesh:
    let mut sphere_mesh = Sphere::new(1.0).mesh().ico(7)?;
    sphere_mesh.generate_tangents()?;

    // Spawn the mesh into the scene:
    let mut sphere = commands.spawn((
        Mesh3d(meshes.add(sphere_mesh.clone())),
        MeshMaterial3d(materials.add(StandardMaterial::default())),
        Transform::from_xyz(-1.0, 1.0, 0.0),
    ));

    // Generate random sample points:
    let triangles = sphere_mesh.triangles()?;
    let distribution = UniformMeshSampler::try_new(triangles)?;

    // Setup sample points:
    let point_mesh = meshes.add(Sphere::new(0.01).mesh().ico(3)?);
    let point_material = materials.add(StandardMaterial {
        base_color: Srgba::RED.into(),
        emissive: LinearRgba::rgb(1.0, 0.0, 0.0),
        ..default()
    });

    // Add sample points as children of the sphere:
    for point in distribution.sample_iter(&mut seeded_rng).take(10000) {
        sphere.with_child((
            Mesh3d(point_mesh.clone()),
            MeshMaterial3d(point_material.clone()),
            Transform::from_translation(point),
        ));
    }

    // Indicate the system completed successfully:
    Ok(())
}

// Observer systems can also return a `Result`.
fn fallible_observer(
    event: On<Pointer<Move>>,
    mut world: DeferredWorld,
    mut step: Local<f32>,
) -> Result {
    let mut transform = world
        .get_mut::<Transform>(event.entity())
        .ok_or("No transform found.")?;

    *step = if transform.translation.x > 3. {
        -0.1
    } else if transform.translation.x < -3. || *step == 0. {
        0.1
    } else {
        *step
    };

    transform.translation.x += *step;

    Ok(())
}

#[derive(Resource)]
struct UninitializedResource;

fn failing_system(world: &mut World) -> Result {
    world
        // `get_resource` returns an `Option<T>`, so we use `ok_or` to convert it to a `Result` on
        // which we can call `?` to propagate the error.
        .get_resource::<UninitializedResource>()
        // We can provide a `str` here because `BevyError` implements `From<&str>`.
        .ok_or("Resource not initialized")?;

    Ok(())
}

fn failing_commands(mut commands: Commands) {
    commands
        // This entity doesn't exist!
        .entity(Entity::from_raw_u32(12345678).unwrap())
        // Normally, this failed command would panic,
        // but since we've set the global error handler to `warn`
        // it will log a warning instead.
        .insert(Transform::default());

    // The error handlers for commands can be set individually as well,
    // by using the queue_handled method.
    commands.queue_handled(
        |world: &mut World| -> Result {
            world
                .get_resource::<UninitializedResource>()
                .ok_or("Resource not initialized when accessed in a command")?;

            Ok(())
        },
        |error, context| {
            error!("{error}, {context}");
        },
    );
}