Remove Interaction, and replace it with bevy_picking based fields on Button

[alice-i-cecile]

Objective

Fixes #15550. See that issue and the linked issues within it about why the existing Interaction component is inadequate.

Solution

• remove Interaction and all engine systems that use it
• add pressed and hovered fields to Button
• updated these fields using observers powered by bevy_picking
• migrate all existing examples
• figure out what's going wrong with hovering in the button example
  • pointer presses are getting propagated correctly, but pointer hovering seems to be blocked by Text children
• make Text entities not pickable by default
• swap to HoverMap based detection for hovering to avoid blocking by children
• remove FocusPolicy in favor of Pickable
• remove RelativeCursorPosition. Widgets should compute this information themselves based on bevy_picking input.
• fix slider widgets in the animation_graph example
• consider how to improve the migration path for the existing dynamic-styling pattern

Note to reviewers: the previous solution has special-cased logic for hidden nodes to ensure that they can't be pressed. I would prefer not to implement that in this PR, as that's fundamentally a picking backend concern.

Testing

For basic functionality, run the button example.

For something more holistic, try the game_menu example.

Migration Guide

The Interaction component has been removed. Instead, use the new pressed and hovered fields on Button. Bear in mind that you can also use observers to listen for bevy_picking events such as Pointer<Down> and Pointer<Over> directly as well.

To migrate code that matches on Interaction to change some property, follow the example below:

Bevy 0.14:

outline.color = match *button {
    Interaction::Pressed => RED.into(),
    Interaction::Hovered => WHITE.into(),
    Interaction::None => Color::NONE,
};

Bevy 0.15:

outline.color = match (button.pressed, button.hovered) {
    (true, _) => RED.into(),
    (false, true) => WHITE.into(),
    (false, false) => Color::NONE,
};

NodeBundle (and their Bevy 0.15 equivalents) no longer track whether or not they are pressed / hovered by default. If you require this functionality, add the Button component to your entity.

The FocusPolicy component has been removed, and replaced with the universal Pickable component.

The RelativeCursorPosition component has been removed. If you need this information, you should compute it from bevy_picking events in your widget's logic. See the animation_graph example for a demonstration of how this can be done.

[Leinnan]

From game maker perspective- how to most easily get the "is this button just released" info?

[alice-i-cecile]

From game maker perspective- how to most easily get the "is this button just released" info?

I think that the best strategy is to use an observer listening to Pointer<Up> event, and check if it matches the entities you care about. Can you say more about why you want this information? Things like dragging should use the specialized bevy_picking APIs for that purpose.

[MiniaczQ]

I like the idea in general,
not super sure about the separation (button has pressed & hovered) / naming (button) here.
There can be label widgets that get highlighted when hovered over, does that make them a button?
If not, should hovered be a separate component or should button have a better name

[alice-i-cecile]

There can be label widgets that get highlighted when hovered over, does that make them a button?

The idea from @NthTensor is that each widget type should track its own interaction state independently, as its rules may differ. I agree with this design. I would have showcased it here, but uh, we only have one widget type 😅

[github-actions]

The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

[tim-blackbird]

Suggested change

	pickable: Pickable {
	should_block_lower: true,
	..Default::default()
	},
	pickable: Pickable::default(),

should_block_lower is true by default

[alice-i-cecile]

Great; that means we can swap to a pure derive then.

[NthTensor]

Pickable::default() is basically a no-op. We should discourage users from adding it imo, or make Pickable required by Node.

[mnmaita]

Isn't it possible to use any() with DescendantsIter?