docs/bevy/examples/ecs/generic_system.rs 2.9 K · 89 lines · raw · history

1 //! Generic types allow us to reuse logic across many related systems,
2 //! allowing us to specialize our function's behavior based on which type (or types) are passed in.
3 //!
4 //! This is commonly useful for working on related components or resources,
5 //! where we want to have unique types for querying purposes but want them all to work the same way.
6 //! This is particularly powerful when combined with user-defined traits to add more functionality to these related types.
7 //! Remember to insert a specialized copy of the system into the schedule for each type that you want to operate on!
8 //!
9 //! For more advice on working with generic types in Rust, check out <https://doc.rust-lang.org/book/ch10-01-syntax.html>
10 //! or <https://doc.rust-lang.org/rust-by-example/generics.html>
11
12 use bevy::prelude::*;
13
14 #[derive(Debug, Default, Clone, Copy, Eq, PartialEq, Hash, States)]
15 enum AppState {
16 #[default]
17 MainMenu,
18 InGame,
19 }
20
21 #[derive(Component)]
22 struct TextToPrint(String);
23
24 #[derive(Component, Deref, DerefMut)]
25 struct PrinterTick(Timer);
26
27 #[derive(Component)]
28 struct MenuClose;
29
30 #[derive(Component)]
31 struct LevelUnload;
32
33 fn main() {
34 App::new()
35 .add_plugins(DefaultPlugins)
36 .init_state::<AppState>()
37 .add_systems(Startup, setup_system)
38 .add_systems(
39 Update,
40 (
41 print_text_system,
42 transition_to_in_game_system.run_if(in_state(AppState::MainMenu)),
43 ),
44 )
45 // Cleanup systems.
46 // Pass in the types your system should operate on using the ::<T> (turbofish) syntax
47 .add_systems(OnExit(AppState::MainMenu), cleanup_system::<MenuClose>)
48 .add_systems(OnExit(AppState::InGame), cleanup_system::<LevelUnload>)
49 .run();
50 }
51
52 fn setup_system(mut commands: Commands) {
53 commands.spawn((
54 PrinterTick(Timer::from_seconds(1.0, TimerMode::Repeating)),
55 TextToPrint("I will print until you press space.".to_string()),
56 MenuClose,
57 ));
58
59 commands.spawn((
60 PrinterTick(Timer::from_seconds(1.0, TimerMode::Repeating)),
61 TextToPrint("I will always print".to_string()),
62 LevelUnload,
63 ));
64 }
65
66 fn print_text_system(time: Res<Time>, mut query: Query<(&mut PrinterTick, &TextToPrint)>) {
67 for (mut timer, text) in &mut query {
68 if timer.tick(time.delta()).just_finished() {
69 info!("{}", text.0);
70 }
71 }
72 }
73
74 fn transition_to_in_game_system(
75 mut next_state: ResMut<NextState<AppState>>,
76 keyboard_input: Res<ButtonInput<KeyCode>>,
77 ) {
78 if keyboard_input.pressed(KeyCode::Space) {
79 next_state.set(AppState::InGame);
80 }
81 }
82
83 // Type arguments on functions come after the function name, but before ordinary arguments.
84 // Here, the `Component` trait is a trait bound on T, our generic type
85 fn cleanup_system<T: Component>(mut commands: Commands, query: Query<Entity, With<T>>) {
86 for e in &query {
87 commands.entity(e).despawn();
88 }
89 }