generic component propagation

[robtfm]

Objective

add functionality to allow propagating components to children. requested originally for RenderLayers but can be useful more generally.

Solution

• add HierarchyPropagatePlugin<C, F=()> which schedules systems to propagate components through entities matching F
• add Propagate<C: Component + Clone + PartialEq> which will cause C to be added to all children more niche features:
• add PropagateStop<C> which stops the propagation at this entity
• add PropagateOver<C> which allows the propagation to continue to children, but doesn't add/remove/modify a C on this entity itself

Testing

see tests inline

Notes

• could happily be an out-of-repo plugin
• not sure where it lives: ideally it would be in bevy_ecs but it requires a Plugin so I put it in bevy_app, doesn't really belong there though.
• i'm not totally up-to-date on triggers and observers so possibly this could be done more cleanly, would be very happy to take review comments
• perf: this is pretty cheap except for update_reparented which has to check the parent of every moved entity. since the entirety is opt-in i think it's acceptable but i could possibly use (Changed<Children>, With<Inherited<C>>) instead if it's a concern

[robtfm]

@viridia iirc you requested this originally. if you're not interested i think this pr is going to be stuck in limbo so i may as well close it (i'm using it out of repo which is fine).

[viridia]

I have worked around the problem for now, but the solution isn't as elegant as I'd like.

The original motivation was to be able to spawn GLTF models on a render layer. Currently I manage this by using an observer and traversing the scene.

[robtfm]

in case it's unclear from the tests, the way to do that would be app.add_plugins(HierarchyPropagationPlugin::<RenderLayers>::default()); then commands.entity(gltf_root).insert(Propagate(your_renderlayer));.

it would have no effect on entities in other parts of the scene, and would set the renderlayer on the gltf_root and children (including removing it if they move out, adding it if they move in).

[robtfm]

I’ll see if I can do 3 over the weekend, and try and have a look at 4 as well

[robtfm]

i couldn't see much scope to remove trait bounds (just testing by trial and error), i removed them from PropagateStop and PropagateOver only. if there's something in particular that looks wrong please let me know.

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