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removal_detection.rs
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removal_detection.rs
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//! This example shows how you can know when a [`Component`] has been removed, so you can react to it.
use bevy::prelude::*;
fn main() {
// Information regarding removed `Component`s is discarded at the end of each frame, so you need
// to react to the removal before the frame is over.
//
// Also, `Components` are removed via a `Command`, which are not applied immediately.
// So you need to react to the removal at some stage after `apply_deferred` has run,
// and the Component` is removed.
//
// With these constraints in mind we make sure to place the system that removes a `Component` in
// `Update', and the system that reacts on the removal in `PostUpdate`.
App::new()
.add_plugins(DefaultPlugins)
.add_systems(Startup, setup)
.add_systems(Update, remove_component)
.add_systems(PostUpdate, react_on_removal)
.run();
}
// This `Struct` is just used for convenience in this example. This is the `Component` we'll be
// giving to the `Entity` so we have a `Component` to remove in `remove_component()`.
#[derive(Component)]
struct MyComponent;
fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
commands.spawn(Camera2dBundle::default());
commands.spawn((
SpriteBundle {
texture: asset_server.load("branding/icon.png"),
..default()
},
// Add the `Component`.
MyComponent,
));
}
fn remove_component(
time: Res<Time>,
mut commands: Commands,
query: Query<Entity, With<MyComponent>>,
) {
// After two seconds have passed the `Component` is removed.
if time.elapsed_seconds() > 2.0 {
if let Some(entity) = query.iter().next() {
commands.entity(entity).remove::<MyComponent>();
}
}
}
fn react_on_removal(mut removed: RemovedComponents<MyComponent>, mut query: Query<&mut Sprite>) {
// `RemovedComponents<T>::read()` returns an iterator with the `Entity`s that had their
// `Component` `T` (in this case `MyComponent`) removed at some point earlier during the frame.
for entity in removed.read() {
if let Ok(mut sprite) = query.get_mut(entity) {
sprite.color.set_r(0.0);
}
}
}