Move observe helper to ecs observer module

[jaminhaber]

Objective

The observe() helper in bevy_ui_widgets is extremely useful in a bunch of contexts beyond just UI. I've ended up copying the original file from bevy_ui_widgets into multiple projects to get this functionality without adding the whole feature.

Solution

Move the helper out of bevy_ui_widgets and into bevy_ecs

Testing

Moving code without changes

---

Showcase

EX: Adding colliders to a GLTF scene

commands.spawn((
    SceneRoot(gltf_scene_handle),
    RigidBody::Static,
    observe(add_colliders)
))


fn add_colliders(
    trigger: On<SceneInstanceReady>,
    children: Query<&Children>,
    mut commands: Commands,
) {
...
}

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[alice-i-cecile]

This was deliberately squirrelled away because it's a bit scuffed and unidiomatic.

I agree that it's extremely useful, and I want a real solution here, but I want to be sure we act carefully before fully letting that genie out on the bottle.

[jaminhaber]

@alice-i-cecile Ah I see, makes sense. Were you thinking of something like the children! macro, but for observers?

[alice-i-cecile]

We're not entirely sure of the final shape. I believe @cart might have some plans here with respect to BSN, but I'd also like to be able to define observers directly via relations per #17607 to unify the whole thing.