064e5e48b4
# Objective `Entity::PLACEHOLDER` acts as a magic number that will *probably* never really exist, but it certainly could. And, `Entity` has a niche, so the only reason to use `PLACEHOLDER` is as an alternative to `MaybeUninit` that trades safety risks for logic risks. As a result, bevy has generally advised against using `PLACEHOLDER`, but we still use if for a lot internally. This pr starts removing internal uses of it, starting from observers. ## Solution Change all trigger target related types from `Entity` to `Option<Entity>` Small migration guide to come. ## Testing CI ## Future Work This turned a lot of code from ```rust trigger.target() ``` to ```rust trigger.target().unwrap() ``` The extra panic is no worse than before; it's just earlier than panicking after passing the placeholder to something else. But this is kinda annoying. I would like to add a `TriggerMode` or something to `Event` that would restrict what kinds of targets can be used for that event. Many events like `Removed` etc, are always triggered with a target. We can make those have a way to assume Some, etc. But I wanted to save that for a future pr.
58 lines
2.2 KiB
Rust
58 lines
2.2 KiB
Rust
//! This example shows how you can know when a [`Component`] has been removed, so you can react to it.
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//!
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//! When a [`Component`] is removed from an [`Entity`], all [`Observer`] with an [`OnRemove`] trigger for
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//! that [`Component`] will be notified. These observers will be called immediately after the
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//! [`Component`] is removed. For more info on observers, see the
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//! [observers example](https://github.com/bevyengine/bevy/blob/main/examples/ecs/observers.rs).
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//!
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//! Advanced users may also consider using a lifecycle hook
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//! instead of an observer, as it incurs less overhead for a case like this.
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//! See the [component hooks example](https://github.com/bevyengine/bevy/blob/main/examples/ecs/component_hooks.rs).
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use bevy::prelude::*;
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fn main() {
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App::new()
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.add_plugins(DefaultPlugins)
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.add_systems(Startup, setup)
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// This system will remove a component after two seconds.
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.add_systems(Update, remove_component)
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// This observer will react to the removal of the component.
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.add_observer(react_on_removal)
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.run();
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}
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/// This `struct` is just used for convenience in this example. This is the [`Component`] we'll be
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/// giving to the `Entity` so we have a [`Component`] to remove in `remove_component()`.
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#[derive(Component)]
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struct MyComponent;
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fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
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commands.spawn(Camera2d);
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commands.spawn((
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Sprite::from_image(asset_server.load("branding/icon.png")),
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// Add the `Component`.
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MyComponent,
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));
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}
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fn remove_component(
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time: Res<Time>,
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mut commands: Commands,
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query: Query<Entity, With<MyComponent>>,
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) {
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// After two seconds have passed the `Component` is removed.
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if time.elapsed_secs() > 2.0 {
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if let Some(entity) = query.iter().next() {
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commands.entity(entity).remove::<MyComponent>();
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}
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}
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}
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fn react_on_removal(trigger: Trigger<OnRemove, MyComponent>, mut query: Query<&mut Sprite>) {
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// The `OnRemove` trigger was automatically called on the `Entity` that had its `MyComponent` removed.
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let entity = trigger.target().unwrap();
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if let Ok(mut sprite) = query.get_mut(entity) {
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sprite.color = Color::srgb(0.5, 1., 1.);
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}
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}
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