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component_change_detection.rs
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component_change_detection.rs
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//! This example illustrates how to react to component change.
use bevy::{ecs::world::Ref, prelude::*};
use rand::Rng;
fn main() {
App::new()
.add_plugins(DefaultPlugins)
.add_systems(Startup, setup)
.add_systems(
Update,
(change_component, change_detection, tracker_monitoring),
)
.run();
}
#[derive(Component, PartialEq, Debug)]
struct MyComponent(f32);
fn setup(mut commands: Commands) {
commands.spawn(MyComponent(0.));
commands.spawn(Transform::IDENTITY);
}
fn change_component(time: Res<Time>, mut query: Query<(Entity, &mut MyComponent)>) {
for (entity, mut component) in &mut query {
if rand::thread_rng().gen_bool(0.1) {
info!("changing component {:?}", entity);
let new_component = MyComponent(time.elapsed_seconds().round());
// Change detection occurs on mutable dereference,
// and does not consider whether or not a value is actually equal.
// To avoid triggering change detection when nothing has actually changed,
// you can use the `set_if_neq` method on any component or resource that implements PartialEq
component.set_if_neq(new_component);
}
}
}
// There are query filters for `Changed<T>` and `Added<T>`
// Only entities matching the filters will be in the query
fn change_detection(query: Query<(Entity, &MyComponent), Changed<MyComponent>>) {
for (entity, component) in &query {
info!("{:?} changed: {:?}", entity, component);
}
}
// By using `Ref`, the query is not filtered but the information is available
fn tracker_monitoring(query: Query<(Entity, Ref<MyComponent>)>) {
for (entity, component) in &query {
info!(
"{:?}: {:?} -> {{is_added: {}, is_changed: {}}}",
entity,
component,
component.is_added(),
component.is_changed()
);
}
}