5a5125671b
# Objective `ChangeTrackers<>` is a `WorldQuery` type that lets you access the change ticks for a component. #7097 has added `Ref<>`, which gives access to a component's value in addition to its change ticks. Since bevy's access model does not separate a component's value from its change ticks, there is no benefit to using `ChangeTrackers<T>` over `Ref<T>`. ## Solution Deprecate `ChangeTrackers<>`. --- ## Changelog * `ChangeTrackers<T>` has been deprecated. It will be removed in Bevy 0.11. ## Migration Guide `ChangeTrackers<T>` has been deprecated, and will be removed in the next release. Any usage should be replaced with `Ref<T>`. ```rust // Before (0.9) fn my_system(q: Query<(&MyComponent, ChangeTrackers<MyComponent>)>) { for (value, trackers) in &q { if trackers.is_changed() { // Do something with `value`. } } } // After (0.10) fn my_system(q: Query<Ref<MyComponent>>) { for value in &q { if value.is_changed() { // Do something with `value`. } } } ```
58 lines
1.9 KiB
Rust
58 lines
1.9 KiB
Rust
//! This example illustrates how to react to component change.
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use bevy::{ecs::world::Ref, prelude::*};
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use rand::Rng;
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fn main() {
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App::new()
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.add_plugins(DefaultPlugins)
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.add_startup_system(setup)
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.add_system(change_component)
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.add_system(change_detection)
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.add_system(tracker_monitoring)
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.run();
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}
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#[derive(Component, PartialEq, Debug)]
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struct MyComponent(f32);
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fn setup(mut commands: Commands) {
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commands.spawn(MyComponent(0.));
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commands.spawn(Transform::IDENTITY);
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}
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fn change_component(time: Res<Time>, mut query: Query<(Entity, &mut MyComponent)>) {
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for (entity, mut component) in &mut query {
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if rand::thread_rng().gen_bool(0.1) {
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info!("changing component {:?}", entity);
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let new_component = MyComponent(time.elapsed_seconds().round());
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// Change detection occurs on mutable dereference,
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// and does not consider whether or not a value is actually equal.
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// To avoid triggering change detection when nothing has actually changed,
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// you can use the `set_if_neq` method on any component or resource that implements PartialEq
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component.set_if_neq(new_component);
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}
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}
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}
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// There are query filters for `Changed<T>` and `Added<T>`
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// Only entities matching the filters will be in the query
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fn change_detection(query: Query<(Entity, &MyComponent), Changed<MyComponent>>) {
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for (entity, component) in &query {
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info!("{:?} changed: {:?}", entity, component,);
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}
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}
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// By using `Ref`, the query is not filtered but the information is available
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fn tracker_monitoring(query: Query<(Entity, Ref<MyComponent>)>) {
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for (entity, component) in &query {
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info!(
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"{:?}: {:?} -> {{is_added: {}, is_changed: {}}}",
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entity,
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component,
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component.is_added(),
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component.is_changed()
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);
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}
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}
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