0.14.1, tracking change detection, and more rendering examples

0.14.1 is out, Bevy Jam 5 voting is ongoing for another week, and there's plenty of submissions to showcase this week.

track_change_detection is probably my favorite merged feature this week alongside some new quality-of-life APIs, and a fantastic new example for mid-level rendering APIs. I'm a huge fan of new rendering examples because even when Bevy's rendering changes, the examples are always kept up to date as a solid resource.

We see a bunch of game jam submissions showcase'd this week. They will be mentioned in the issue if the author's chose to showcase their work but we won't be covering games in depth until after voting has closed.

What Changed my Component or Resource?

#14034 includes an incredible new change makes it possible to know what changed your Component or Resource. This is a new changed_by function on Components and Resources that is behind a new track_change_detection feature flag.

The change_detection example can now be run with the feature enabled.

cargo run --example change_detection --features track_change_detection

resulting in more informative output!

2024-07-28T06:57:48.946022Z  INFO component_change_detection: Entity { index: 1, generation: 1 }: New value: MyComponent(0.0)
2024-07-28T06:57:49.004371Z  INFO component_change_detection: Entity { index: 1, generation: 1 }: New value: MyComponent(1.0)
2024-07-28T06:57:49.012738Z  WARN component_change_detection: Change detected!
        -> value: Ref(MyComponent(1.0))
        -> added: false
        -> changed: true
        -> changed by: examples/ecs/component_change_detection.rs:36:23

The actual API usage is similar to is_changed.

fn change_detection(
    changed_components: Query<Ref<MyComponent>, Changed<MyComponent>>,
    my_resource: Res<MyResource>,
) {
    for component in &changed_components {
        warn!(
            "Change detected!\n\t-> value: {:?}\n\t-> added: {}\n\t-> changed: {}\n\t-> changed by: {}",
            component,
            component.is_added(),
            component.is_changed(),
            component.changed_by()
        );
    }

    if my_resource.is_changed() {
        warn!(
            "Change detected!\n\t-> value: {:?}\n\t-> added: {}\n\t-> changed: {}\n\t-> changed by: {}",
            my_resource,
            my_resource.is_added(),
            my_resource.is_changed(),
            my_resource.changed_by()
        );
    }
}

and this information also shows up when using a debugger.

AccumulatedMouseMotion

Early in July #14044 added support for two new resources: AccumulatedMouseMotion and AccumulatedMouseScroll, which store accumulated mouse movement and make it easier to use the accumulated mouse event values. This week in #14488, these resources were used in more examples, so check this out if you want to replace some MouseMotion event usage.

More Observers

As of #13859 the SceneInstanceReady event is now an observer event! This means that when you spawn a scene the event will be triggered either globally or on the parent of the scene if one exists.

SpecializedMeshPipeline

#14370 adds a new example showing off how to use SpecializedMeshPipeline, which is a mid-level rendering API. The example spawn 3 entities with a custom TriangleList mesh, including vertex colors, and introduces some Bevy-specific abstractions for draw commands and pipelines. This example also starts to expose some wgpu concepts as well, although many types in the example are still bevy_render-specific, such as FragmentState which is a bevy-specialized version of wgpu's FragmentState.

This is a great example for anyone looking to move beyond Material and AsBindGroup

BuildChildren::with_child

commands
    .spawn(TextBundle::default())
    .with_child(TextSection::from("Hello"));

#14594 introduces a new quality-of-life API in with_child on the BuildChildren trait. This improves the workflow for spawning a Bundle as a child, which contrasts against the already-existing add_child, that accepts an Entity.

New Camera functions

There are a whole bunch of camera-related functions for converting between world space, viewport, and ndc (normalized device coordinates). #14590 adds two more in depth_ndc_to_view_z and world_to_viewport_with_depth, which enables working with depth values.

Alice's Merge Train is a maintainer-level view into active PRs, both those that are merging and those that need work.