Remove extra call to clear_trackers

[chrisjuchem]

Fixes #13758.

Objective

Calling update on the main app already calls clear_trackers. Calling it again in SubApps::update caused RemovedCompenet Events to be cleared earlier than they should be.

Solution

• Don't call clear_trackers an extra time.

Testing

I manually tested the fix with this unit test:

#[cfg(test)]
mod test {
    use crate::core::{FrameCount, FrameCountPlugin};
    use crate::prelude::*;

    #[test]
    fn test_next_frame_removal() {
        #[derive(Component)]
        struct Foo;

        #[derive(Resource)]
        struct RemovedCount(usize);

        let mut app = App::new();
        app.add_plugins(FrameCountPlugin);
        app.add_systems(Startup, |mut commands: Commands| {
            for _ in 0..100 {
                commands.spawn(Foo);
            }
            commands.insert_resource(RemovedCount(0));
        });

        app.add_systems(First, |counter: Res<FrameCount>| {
            println!("Frame {}:", counter.0)
        });

        fn detector_system(
            mut removals: RemovedComponents<Foo>,
            foos: Query<Entity, With<Foo>>,
            mut removed_c: ResMut<RemovedCount>,
        ) {
            for e in removals.read() {
                println!("  Detected removed Foo component for {e:?}");
                removed_c.0 += 1;
            }
            let c = foos.iter().count();
            println!("  Total Foos: {}", c);
            assert_eq!(c + removed_c.0, 100);
        }
        fn deleter_system(foos: Query<Entity, With<Foo>>, mut commands: Commands) {
            foos.iter().next().map(|e| {
                commands.entity(e).remove::<Foo>();
            });
        }
        app.add_systems(Update, (detector_system, deleter_system).chain());

        app.update();
        app.update();
        app.update();
        app.update();
    }
}

[chrisjuchem]

I'd like to add a proper unit test to bevy_app. I should be able to get to that later on tonight.

[NthTensor]

Fix makes sense to me, as does the test. Looking good.

[hymm]

Does removing this clear trackers rather than the other one mean that you can't use RemovedComponents in sub apps?

[chrisjuchem]

RemovedComponents should still work the same for SubApps. clear_trackers is called once here per SubApp (the main app is also a SubApp internally): https://github.com/bevyengine/bevy/pull/13762/files/5888bf276daa1959eeb62671d1a8d6dd230eb6db#diff-1c2be7dcf5607cc6b3a4a9e664e9de0712a5ea313079b5451464513be643e747R136

[hymm]

There was some confusion in the past that you couldn't use Extract<RemovedComponents<T>> in the extraction subapp. I believe this extra call is here, because the intention was to not clear the trackers at the end of the main app schedule to enable this.

[chrisjuchem]

Hmmm. If that's the case, then I think we would need another version of update For the main app that doesn't call clear_trackers.

[hymm]

That sounds reasonable. But if it ends up being too involved, it might be better to merge this for 0.14 and fix it in a followup.

[chrisjuchem]

Wasn't too bad at all once I came up with a test that caught the issue. I couldn't find any problems using RemovedComponents since it has a two frame buffer, but this this was an issue with regular change detection.

[alice-i-cecile]

@chrisjuchem I'll merge this as soon as merge conflicts are resolved :)