docs/bevy/examples/ecs/one_shot_systems.rs 3.4 K · 111 lines · raw · history

1 //! Demonstrates the use of "one-shot systems", which run once when triggered.
2 //!
3 //! These can be useful to help structure your logic in a push-based fashion,
4 //! reducing the overhead of running extremely rarely run systems
5 //! and improving schedule flexibility.
6 //!
7 //! See the [`World::run_system`](World::run_system) or
8 //! [`World::run_system_once`](World#method.run_system_once_with)
9 //! docs for more details.
10
11 use bevy::{
12 ecs::system::{RunSystemOnce, SystemId},
13 prelude::*,
14 };
15
16 fn main() {
17 App::new()
18 .add_plugins(DefaultPlugins)
19 .add_systems(
20 Startup,
21 (
22 setup_ui,
23 setup_with_commands,
24 setup_with_world.after(setup_ui), // since we run `system_b` once in world it needs to run after `setup_ui`
25 ),
26 )
27 .add_systems(Update, (trigger_system, evaluate_callbacks).chain())
28 .run();
29 }
30
31 #[derive(Component)]
32 struct Callback(SystemId);
33
34 #[derive(Component)]
35 struct Triggered;
36
37 #[derive(Component)]
38 struct A;
39 #[derive(Component)]
40 struct B;
41
42 fn setup_with_commands(mut commands: Commands) {
43 let system_id = commands.register_system(system_a);
44 commands.spawn((Callback(system_id), A));
45 }
46
47 fn setup_with_world(world: &mut World) {
48 // We can run it once manually
49 world.run_system_once(system_b).unwrap();
50 // Or with a Callback
51 let system_id = world.register_system(system_b);
52 world.spawn((Callback(system_id), B));
53 }
54
55 /// Tag entities that have callbacks we want to run with the `Triggered` component.
56 fn trigger_system(
57 mut commands: Commands,
58 query_a: Single<Entity, With<A>>,
59 query_b: Single<Entity, With<B>>,
60 input: Res<ButtonInput<KeyCode>>,
61 ) {
62 if input.just_pressed(KeyCode::KeyA) {
63 let entity = *query_a;
64 commands.entity(entity).insert(Triggered);
65 }
66 if input.just_pressed(KeyCode::KeyB) {
67 let entity = *query_b;
68 commands.entity(entity).insert(Triggered);
69 }
70 }
71
72 /// Runs the systems associated with each `Callback` component if the entity also has a `Triggered` component.
73 ///
74 /// This could be done in an exclusive system rather than using `Commands` if preferred.
75 fn evaluate_callbacks(query: Query<(Entity, &Callback), With<Triggered>>, mut commands: Commands) {
76 for (entity, callback) in query.iter() {
77 commands.run_system(callback.0);
78 commands.entity(entity).remove::<Triggered>();
79 }
80 }
81
82 fn system_a(entity_a: Single<Entity, With<Text>>, mut writer: TextUiWriter) {
83 *writer.text(*entity_a, 3) = String::from("A");
84 info!("A: One shot system registered with Commands was triggered");
85 }
86
87 fn system_b(entity_b: Single<Entity, With<Text>>, mut writer: TextUiWriter) {
88 *writer.text(*entity_b, 3) = String::from("B");
89 info!("B: One shot system registered with World was triggered");
90 }
91
92 fn setup_ui(mut commands: Commands) {
93 commands.spawn(Camera2d);
94 commands.spawn((
95 Text::default(),
96 TextLayout::justify(Justify::Center),
97 Node {
98 align_self: AlignSelf::Center,
99 justify_self: JustifySelf::Center,
100 ..default()
101 },
102 children![
103 (TextSpan::new("Press A or B to trigger a one-shot system\n")),
104 (TextSpan::new("Last Triggered: ")),
105 (
106 TextSpan::new("-"),
107 TextColor(bevy::color::palettes::css::ORANGE.into()),
108 )
109 ],
110 ));
111 }