bevy/examples/ecs/removal_detection.rs

// 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`. `Command`s are applied after a stage has
    // finished executing. So you need to react to the removal at some stage after the
    // `Component` is removed.
    //
    // With these constraints in mind we make sure to place the system that removes a `Component` on
    // the `stage::UPDATE` stage, and the system that reacts on the removal on the
    // `stage::POST_UPDATE` stage.
    App::build()
        .add_plugins(DefaultPlugins)
        .add_startup_system(setup.system())
        .add_system_to_stage(stage::UPDATE, remove_component.system())
        .add_system_to_stage(stage::POST_UPDATE, react_on_removal.system())
        .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()`.
struct MyComponent;

fn setup(
    commands: &mut Commands,
    asset_server: Res<AssetServer>,
    mut materials: ResMut<Assets<ColorMaterial>>,
) {
    let texture = asset_server.load("branding/icon.png");

    commands
        .spawn(Camera2dBundle::default())
        .spawn(SpriteBundle {
            material: materials.add(texture.into()),
            ..Default::default()
        })
        .with(MyComponent); // Add the `Component`.
}

fn remove_component(
    time: Res<Time>,
    commands: &mut Commands,
    query: Query<Entity, With<MyComponent>>,
) {
    // After two seconds have passed the `Component` is removed.
    if time.seconds_since_startup() > 2.0 {
        if let Some(entity) = query.iter().next() {
            commands.remove_one::<MyComponent>(entity);
        }
    }
}

fn react_on_removal(
    mut materials: ResMut<Assets<ColorMaterial>>,
    query: Query<(Entity, &Handle<ColorMaterial>)>,
) {
    // Note: usually this isn't how you would handle a `Query`. In this example it makes things
    // a bit easier to read.
    let (query_entity, material) = query.iter().next().unwrap();

    // `Query.removed<T>` returns an array with the `Entity`s in the `Query` that had its
    // `Component` `T` (in this case `MyComponent`) removed at some point earlier during the frame.
    for entity in query.removed::<MyComponent>() {
        // We compare the `Entity` that had its `MyComponent` `Component` removed with the `Entity`
        // in the current `Query`. If they match all red is removed from the material.
        if query_entity == *entity {
            materials.get_mut(material).unwrap().color.set_r(0.0);
        }
    }
}