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Add removal_detection example (#945)
Co-authored-by: Carter Anderson <[email protected]>
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,74 @@ | ||
| // This example shows how you can know when a `Component` has been removed, so you can react to it. | ||
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| use bevy::prelude::*; | ||
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| 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(); | ||
| } | ||
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| // 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; | ||
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| fn setup( | ||
| commands: &mut Commands, | ||
| asset_server: Res<AssetServer>, | ||
| mut materials: ResMut<Assets<ColorMaterial>>, | ||
| ) { | ||
| let texture = asset_server.load("branding/icon.png"); | ||
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| commands | ||
| .spawn(Camera2dBundle::default()) | ||
| .spawn(SpriteBundle { | ||
| material: materials.add(texture.into()), | ||
| ..Default::default() | ||
| }) | ||
| .with(MyComponent); // Add the `Component`. | ||
| } | ||
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| 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); | ||
| } | ||
| } | ||
| } | ||
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| 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(); | ||
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| // `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); | ||
| } | ||
| } | ||
| } |