callbacks/
callbacks.rs

1//! Sometimes, you want an extremely flexible way to store logic associated with an entity.
2//! This example demonstrates how to store arbitrary systems in components and run them on demand.
3//!
4//! This pattern trades some performance for flexibility and works well for things like cutscenes, scripted events,
5//! or one-off UI-driven interactions that don't need to run every frame.
6
7use bevy::{ecs::system::SystemId, prelude::*};
8
9fn main() {
10    let mut app = App::new();
11    app.add_systems(Startup, setup_callbacks);
12    app.add_systems(Update, run_callbacks);
13    app.run();
14}
15
16#[derive(Component)]
17struct Callback {
18    system_id: SystemId<(), ()>,
19}
20
21fn setup_callbacks(mut commands: Commands) {
22    let trivial_callback = Callback {
23        system_id: commands.register_system(|| {
24            println!("This is the trivial callback system");
25        }),
26    };
27
28    let ordinary_system_callback = Callback {
29        system_id: commands.register_system(|query: Query<&Callback>| {
30            let n_callbacks = query.iter().len();
31            println!("This is the ordinary callback system. There are currently {n_callbacks} callbacks in the world.");
32        }),
33    };
34
35    let exclusive_callback = Callback {
36        system_id: commands.register_system(|world: &mut World| {
37            let n_entities = world.entities().len();
38            println!("This is the exclusive callback system. There are currently {n_entities} entities in the world.");
39        }),
40    };
41
42    commands.spawn(trivial_callback);
43    commands.spawn(ordinary_system_callback);
44    commands.spawn(exclusive_callback);
45
46    // It is also possible to register systems that are already boxed.
47
48    let boxed_trivial_system = Box::new(IntoSystem::into_system(|| {
49        println!("This is the boxed trivial callback system");
50    }));
51
52    let boxed_trivial_callback = Callback {
53        system_id: commands.register_boxed_system(boxed_trivial_system),
54    };
55
56    let boxed_ordinary_system = Box::new(IntoSystem::into_system(|query: Query<&Callback>| {
57        let n_callbacks = query.iter().len();
58        println!("This is the boxed ordinary callback system. There are currently {n_callbacks} callbacks in the world.");
59    }));
60
61    let boxed_ordinary_system_callback = Callback {
62        system_id: commands.register_boxed_system(boxed_ordinary_system),
63    };
64
65    let boxed_exclusive_system = Box::new(IntoSystem::into_system(|world: &mut World| {
66        let n_entities = world.entities().len();
67        println!("This is the boxed exclusive callback system. There are currently {n_entities} entities in the world.");
68    }));
69
70    let boxed_exclusive_callback = Callback {
71        system_id: commands.register_boxed_system(boxed_exclusive_system),
72    };
73
74    commands.spawn(boxed_trivial_callback);
75    commands.spawn(boxed_ordinary_system_callback);
76    commands.spawn(boxed_exclusive_callback);
77}
78
79// In many cases, you might want to use an observer to detect when a callback should run,
80// triggering the callback in response to some entity-event!
81fn run_callbacks(mut commands: Commands, query: Query<&Callback>) {
82    for callback in query.iter() {
83        commands.run_system(callback.system_id);
84    }
85}
callbacks/callbacks.rs

callbacks/
callbacks.rs
