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//! Demonstrates the use of "one-shot systems", which run once when triggered.
//!
//! These can be useful to help structure your logic in a push-based fashion,
//! reducing the overhead of running extremely rarely run systems
//! and improving schedule flexibility.
//!
//! See the [`World::run_system`](World::run_system) or
//! [`World::run_system_once`](World#method.run_system_once_with)
//! docs for more details.
use bevy::{
ecs::system::{RunSystemOnce, SystemId},
prelude::*,
};
fn main() {
App::new()
.add_systems(Startup, (count_entities, setup))
.add_systems(PostUpdate, count_entities)
.add_systems(Update, evaluate_callbacks)
.run();
}
// Any ordinary system can be run via commands.run_system or world.run_system.
fn count_entities(all_entities: Query<()>) {
dbg!(all_entities.iter().count());
}
#[derive(Component)]
struct Callback(SystemId);
#[derive(Component)]
struct Triggered;
fn setup(world: &mut World) {
let button_pressed_id = world.register_system(button_pressed);
world.spawn((Callback(button_pressed_id), Triggered));
// This entity does not have a Triggered component, so its callback won't run.
let slider_toggled_id = world.register_system(slider_toggled);
world.spawn(Callback(slider_toggled_id));
world.run_system_once(count_entities);
}
fn button_pressed() {
println!("A button was pressed!");
}
fn slider_toggled() {
println!("A slider was toggled!");
}
/// Runs the systems associated with each `Callback` component if the entity also has a Triggered component.
///
/// This could be done in an exclusive system rather than using `Commands` if preferred.
fn evaluate_callbacks(query: Query<&Callback, With<Triggered>>, mut commands: Commands) {
for callback in query.iter() {
commands.run_system(callback.0);
}
}