Deprecate ChangeTrackers<T> in favor of Ref<T> (#7306)

# 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`.
        }
    }
}
```

crates/bevy_ecs/src/change_detection.rs

@@ -696,10 +696,9 @@ mod tests {
     use crate::{
         self as bevy_ecs,
         change_detection::{
-            Mut, NonSendMut, ResMut, TicksMut, CHECK_TICK_THRESHOLD, MAX_CHANGE_AGE,
+            Mut, NonSendMut, Ref, ResMut, TicksMut, CHECK_TICK_THRESHOLD, MAX_CHANGE_AGE,
         },
         component::{Component, ComponentTicks, Tick},
-        query::ChangeTrackers,
         system::{IntoSystem, Query, System},
         world::World,
     };
@@ -718,11 +717,11 @@ mod tests {
 
     #[test]
     fn change_expiration() {
-        fn change_detected(query: Query<ChangeTrackers<C>>) -> bool {
+        fn change_detected(query: Query<Ref<C>>) -> bool {
             query.single().is_changed()
         }
 
-        fn change_expired(query: Query<ChangeTrackers<C>>) -> bool {
+        fn change_expired(query: Query<Ref<C>>) -> bool {
             query.single().is_changed()
         }
 
@@ -753,7 +752,7 @@ mod tests {
 
     #[test]
     fn change_tick_wraparound() {
-        fn change_detected(query: Query<ChangeTrackers<C>>) -> bool {
+        fn change_detected(query: Query<Ref<C>>) -> bool {
             query.single().is_changed()
         }
 
@@ -784,10 +783,10 @@ mod tests {
         *world.change_tick.get_mut() += MAX_CHANGE_AGE + CHECK_TICK_THRESHOLD;
         let change_tick = world.change_tick();
 
-        let mut query = world.query::<ChangeTrackers<C>>();
+        let mut query = world.query::<Ref<C>>();
         for tracker in query.iter(&world) {
-            let ticks_since_insert = change_tick.wrapping_sub(tracker.component_ticks.added.tick);
-            let ticks_since_change = change_tick.wrapping_sub(tracker.component_ticks.changed.tick);
+            let ticks_since_insert = change_tick.wrapping_sub(tracker.ticks.added.tick);
+            let ticks_since_change = change_tick.wrapping_sub(tracker.ticks.changed.tick);
             assert!(ticks_since_insert > MAX_CHANGE_AGE);
             assert!(ticks_since_change > MAX_CHANGE_AGE);
         }
@@ -796,8 +795,8 @@ mod tests {
         world.check_change_ticks();
 
         for tracker in query.iter(&world) {
-            let ticks_since_insert = change_tick.wrapping_sub(tracker.component_ticks.added.tick);
-            let ticks_since_change = change_tick.wrapping_sub(tracker.component_ticks.changed.tick);
+            let ticks_since_insert = change_tick.wrapping_sub(tracker.ticks.added.tick);
+            let ticks_since_change = change_tick.wrapping_sub(tracker.ticks.changed.tick);
             assert!(ticks_since_insert == MAX_CHANGE_AGE);
             assert!(ticks_since_change == MAX_CHANGE_AGE);
         }

crates/bevy_ecs/src/lib.rs

@@ -25,6 +25,9 @@ pub use bevy_ptr as ptr;
 
 /// Most commonly used re-exported types.
 pub mod prelude {
+    #[doc(hidden)]
+    #[allow(deprecated)]
+    pub use crate::query::ChangeTrackers;
     #[doc(hidden)]
     #[cfg(feature = "bevy_reflect")]
     pub use crate::reflect::{ReflectComponent, ReflectResource};
@@ -35,7 +38,7 @@ pub mod prelude {
         component::Component,
         entity::Entity,
         event::{Event, EventReader, EventWriter, Events},
-        query::{Added, AnyOf, ChangeTrackers, Changed, Or, QueryState, With, Without},
+        query::{Added, AnyOf, Changed, Or, QueryState, With, Without},
         removal_detection::RemovedComponents,
         schedule::{
             apply_state_transition, apply_system_buffers, common_conditions::*, IntoSystemConfig,
@@ -63,11 +66,10 @@ mod tests {
     use crate::prelude::Or;
     use crate::{
         bundle::Bundle,
+        change_detection::Ref,
         component::{Component, ComponentId},
         entity::Entity,
-        query::{
-            Added, ChangeTrackers, Changed, FilteredAccess, ReadOnlyWorldQuery, With, Without,
-        },
+        query::{Added, Changed, FilteredAccess, ReadOnlyWorldQuery, With, Without},
         system::Resource,
         world::{Mut, World},
     };
@@ -1297,7 +1299,10 @@ mod tests {
     }
 
     #[test]
+    #[allow(deprecated)]
     fn trackers_query() {
+        use crate::prelude::ChangeTrackers;
+
         let mut world = World::default();
         let e1 = world.spawn((A(0), B(0))).id();
         world.spawn(B(0));
@@ -1541,7 +1546,7 @@ mod tests {
         assert_eq!(1, query_min_size![&B, (With<A>, With<C>)],);
         assert_eq!(1, query_min_size![(&A, &B), With<C>],);
         assert_eq!(4, query_min_size![&A, ()], "Simple Archetypal");
-        assert_eq!(4, query_min_size![ChangeTrackers<A>, ()],);
+        assert_eq!(4, query_min_size![Ref<A>, ()],);
         // All the following should set minimum size to 0, as it's impossible to predict
         // how many entities the filters will trim.
         assert_eq!(0, query_min_size![(), Added<A>], "Simple Added");

crates/bevy_ecs/src/query/fetch.rs

@@ -1115,6 +1115,7 @@ unsafe impl<T: ReadOnlyWorldQuery> ReadOnlyWorldQuery for Option<T> {}
 /// }
 /// # bevy_ecs::system::assert_is_system(print_moving_objects_system);
 /// ```
+#[deprecated = "`ChangeTrackers<T>` will be removed in bevy 0.11. Use `bevy_ecs::prelude::Ref<T>` instead."]
 pub struct ChangeTrackers<T: Component> {
     pub(crate) component_ticks: ComponentTicks,
     pub(crate) last_change_tick: u32,
@@ -1122,18 +1123,17 @@ pub struct ChangeTrackers<T: Component> {
     marker: PhantomData<T>,
 }
 
+#[allow(deprecated)]
 impl<T: Component> Clone for ChangeTrackers<T> {
     fn clone(&self) -> Self {
-        Self {
-            component_ticks: self.component_ticks,
-            last_change_tick: self.last_change_tick,
-            change_tick: self.change_tick,
-            marker: PhantomData,
-        }
+        *self
     }
 }
+
+#[allow(deprecated)]
 impl<T: Component> Copy for ChangeTrackers<T> {}
 
+#[allow(deprecated)]
 impl<T: Component> std::fmt::Debug for ChangeTrackers<T> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         f.debug_struct("ChangeTrackers")
@@ -1144,6 +1144,7 @@ impl<T: Component> std::fmt::Debug for ChangeTrackers<T> {
     }
 }
 
+#[allow(deprecated)]
 impl<T: Component> ChangeTrackers<T> {
     /// Returns true if this component has been added since the last execution of this system.
     pub fn is_added(&self) -> bool {
@@ -1171,6 +1172,7 @@ pub struct ChangeTrackersFetch<'w, T> {
     change_tick: u32,
 }
 
+#[allow(deprecated)]
 // SAFETY: `ROQueryFetch<Self>` is the same as `QueryFetch<Self>`
 unsafe impl<T: Component> WorldQuery for ChangeTrackers<T> {
     type Fetch<'w> = ChangeTrackersFetch<'w, T>;
@@ -1317,6 +1319,7 @@ unsafe impl<T: Component> WorldQuery for ChangeTrackers<T> {
     }
 }
 
+#[allow(deprecated)]
 /// SAFETY: access is read only
 unsafe impl<T: Component> ReadOnlyWorldQuery for ChangeTrackers<T> {}
 

crates/bevy_ecs/src/query/filter.rs

@@ -572,7 +572,7 @@ impl_tick_filter!(
     /// A common use for this filter is one-time initialization.
     ///
     /// To retain all results without filtering but still check whether they were added after the
-    /// system last ran, use [`ChangeTrackers<T>`](crate::query::ChangeTrackers).
+    /// system last ran, use [`Ref<T>`](crate::change_detection::Ref).
     ///
     /// # Examples
     ///

examples/ecs/component_change_detection.rs

@@ -1,6 +1,6 @@
 //! This example illustrates how to react to component change.
 
-use bevy::prelude::*;
+use bevy::{ecs::world::Ref, prelude::*};
 use rand::Rng;
 
 fn main() {
@@ -43,15 +43,15 @@ fn change_detection(query: Query<(Entity, &MyComponent), Changed<MyComponent>>)
     }
 }
 
-// By looking at trackers, the query is not filtered but the information is available
-fn tracker_monitoring(
-    query: Query<(
-        Entity,
-        Option<&MyComponent>,
-        Option<ChangeTrackers<MyComponent>>,
-    )>,
-) {
-    for (entity, component, trackers) in &query {
-        info!("{:?}: {:?} -> {:?}", entity, component, trackers);
+// 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()
+        );
     }
 }