Add the AssetChanged query filter

Solves bevyengine#5069.

`AssetChanged<T>` is just a thin wrapper around the `&Handle<T>` query,
with additional logic to check in `Assets<T>` if the corresponding
`HandleId` was changed between the last and current runs of the system.

This adds some overhead in the `asset_event_system` for keeping track of
when which handle got updated. Otherwise, the overhead is constrained to
the `AssetChanged` query, which is ripe for optimizations.

NOTE: We chose to update the `change_ticks` hash map during asset event
propagation rather than when accessing the asset, because we need access
somehow to the current `change_tick`. Updating the tick hash map during
modification would require passing the `change_tick` to all methods of
`Assets` that accepts a `&mut self`. Which would be a huge ergonomic
loss.

As a result, the asset change expressed by `AssetChanged` is only
effective after `asset_event_system` ran. This is in line with how
`AssetEvents` currently works, but might cause a 1 frame lag for asset
change detection.

A potential future update could implement some sort of specialization so
that when `Assets` is accessed through a `ResMut`, the called methods
are called with the `ResMut`'s change_tick. But with current rust, this
is either impossible or causing huge ergonomic loss on the `Assets` API.

crates/bevy_asset/src/assets.rs

@@ -5,7 +5,7 @@ use crate::{
 use bevy_app::App;
 use bevy_ecs::{
     event::{EventWriter, Events},
-    system::ResMut,
+    system::{ResMut, SystemChangeTick},
     world::FromWorld,
 };
 use bevy_utils::HashMap;
@@ -65,6 +65,7 @@ impl<T: Asset> Debug for AssetEvent<T> {
 #[derive(Debug)]
 pub struct Assets<T: Asset> {
     assets: HashMap<HandleId, T>,
+    pub(crate) change_ticks: HashMap<HandleId, u32>,
     events: Events<AssetEvent<T>>,
     pub(crate) ref_change_sender: Sender<RefChange>,
 }
@@ -73,6 +74,7 @@ impl<T: Asset> Assets<T> {
     pub(crate) fn new(ref_change_sender: Sender<RefChange>) -> Self {
         Assets {
             assets: HashMap::default(),
+            change_ticks: HashMap::default(),
             events: Events::default(),
             ref_change_sender,
         }
@@ -239,13 +241,29 @@ impl<T: Asset> Assets<T> {
     }
 
     pub fn asset_event_system(
-        mut events: EventWriter<AssetEvent<T>>,
+        ticks: SystemChangeTick,
+        mut event_writer: EventWriter<AssetEvent<T>>,
         mut assets: ResMut<Assets<T>>,
     ) {
         // Check if the events are empty before calling `drain`.
         // As `drain` triggers change detection.
         if !assets.events.is_empty() {
-            events.send_batch(assets.events.drain());
+            let current_tick = ticks.change_tick();
+            let Assets {
+                change_ticks,
+                events,
+                ..
+            } = &mut *assets;
+            let update_change_map = |event| {
+                use AssetEvent::{Created, Modified, Removed};
+                match &event {
+                    Created { handle } => change_ticks.insert(handle.id, current_tick),
+                    Modified { handle } => change_ticks.insert(handle.id, current_tick),
+                    Removed { handle } => change_ticks.remove(&handle.id),
+                };
+                event
+            };
+            event_writer.send_batch(events.drain().map(update_change_map));
         }
     }
 

crates/bevy_asset/src/lib.rs

@@ -13,8 +13,11 @@ mod info;
 mod io;
 mod loader;
 mod path;
+pub mod query;
 
 pub mod prelude {
+    #[doc(hidden)]
+    pub use crate::query::AssetChanged;
     #[doc(hidden)]
     pub use crate::{AddAsset, AssetEvent, AssetServer, Assets, Handle, HandleUntyped};
 }

crates/bevy_asset/src/query.rs

@@ -0,0 +1,165 @@
+//! Filter query to limit iteration over [`Handle`]s which content was updated recently.
+use std::marker::PhantomData;
+
+use bevy_ecs::{
+    archetype::{Archetype, ArchetypeComponentId},
+    change_detection::MAX_CHANGE_AGE,
+    component::ComponentId,
+    prelude::World,
+    query::{
+        Access, ComponentIdState, Fetch, FetchState, FilteredAccess, QueryItem, ReadFetch,
+        ReadOnlyWorldQuery, WorldQuery, WorldQueryGats,
+    },
+    storage::{Table, Tables},
+};
+use bevy_utils::HashMap;
+
+use crate::{Asset, Assets, Handle, HandleId};
+
+struct AssetChangeCheck<'w> {
+    handle_change_map: Option<&'w HashMap<HandleId, u32>>,
+    prev_system_tick: u32,
+    current_tick: u32,
+}
+impl<'w> AssetChangeCheck<'w> {
+    fn new<T: Asset>(assets: Option<&'w Assets<T>>, last_change: u32, current: u32) -> Self {
+        let handle_change_map = assets.map(|a| &a.change_ticks);
+        Self {
+            handle_change_map,
+            prev_system_tick: last_change,
+            current_tick: current,
+        }
+    }
+    fn ticks_since(&self, event_tick: u32) -> u32 {
+        self.current_tick
+            .wrapping_sub(event_tick)
+            .min(MAX_CHANGE_AGE)
+    }
+    // TODO: some sort of caching? Each check has two levels of indirection,
+    // which is not optimal.
+    fn has_changed<T: Asset>(&self, handle: &Handle<T>) -> bool {
+        let is_changed = || {
+            let map = self.handle_change_map?;
+            let handle_change_tick = *map.get(&handle.id)?;
+            // This replicates the logic used in `ComponentTicks::is_changed`
+            let ticks_since_change = self.ticks_since(handle_change_tick);
+            let ticks_since_system = self.ticks_since(self.prev_system_tick);
+            Some(ticks_since_system >= ticks_since_change)
+        };
+        is_changed().unwrap_or(true)
+    }
+}
+
+/// Query filter to get all entities which `Handle<T>` component underlying asset changed.
+pub struct AssetChanged<T>(PhantomData<T>);
+
+/// Fetch for [`AssetChanged`]
+#[doc(hidden)]
+pub struct AssetChangedFetch<'w, T: Asset> {
+    inner: ReadFetch<'w, Handle<T>>,
+    check: AssetChangeCheck<'w>,
+}
+
+/// State for [`AssetChanged`]
+pub struct AssetChangedState<T: Asset> {
+    inner: ComponentIdState<Handle<T>>,
+}
+
+// SAFETY: `ROQueryFetch<Self>` is the same as `QueryFetch<Self>`
+unsafe impl<T: Asset> WorldQuery for AssetChanged<T> {
+    type ReadOnly = Self;
+    type State = AssetChangedState<T>;
+
+    fn shrink<'wlong: 'wshort, 'wshort>(item: QueryItem<'wlong, Self>) -> QueryItem<'wshort, Self> {
+        item
+    }
+}
+
+impl<T: Asset> FetchState for AssetChangedState<T> {
+    fn init(world: &mut World) -> Self {
+        Self {
+            inner: ComponentIdState::init(world),
+        }
+    }
+
+    fn matches_component_set(&self, set_contains_id: &impl Fn(ComponentId) -> bool) -> bool {
+        self.inner.matches_component_set(set_contains_id)
+    }
+}
+
+impl<'w, T: Asset> WorldQueryGats<'w> for AssetChanged<T> {
+    type Fetch = AssetChangedFetch<'w, T>;
+    type _State = AssetChangedState<T>;
+}
+
+// SAFETY: this reads the T component. archetype component access and component access are updated to reflect that
+unsafe impl<'w, T: Asset> Fetch<'w> for AssetChangedFetch<'w, T> {
+    type State = AssetChangedState<T>;
+    type Item = bool;
+
+    unsafe fn init(
+        world: &'w World,
+        state: &AssetChangedState<T>,
+        last_change_tick: u32,
+        change_tick: u32,
+    ) -> Self {
+        Self {
+            inner: ReadFetch::init(world, &state.inner, last_change_tick, change_tick),
+            check: AssetChangeCheck::new::<T>(world.get_resource(), last_change_tick, change_tick),
+        }
+    }
+
+    const IS_DENSE: bool = <ReadFetch<'w, Handle<T>>>::IS_DENSE;
+
+    const IS_ARCHETYPAL: bool = <ReadFetch<'w, Handle<T>>>::IS_ARCHETYPAL;
+
+    unsafe fn set_table(&mut self, state: &Self::State, table: &'w Table) {
+        self.inner.set_table(&state.inner, table);
+    }
+
+    unsafe fn set_archetype(
+        &mut self,
+        state: &Self::State,
+        archetype: &'w Archetype,
+        tables: &'w Tables,
+    ) {
+        self.inner.set_archetype(&state.inner, archetype, tables);
+    }
+
+    unsafe fn table_fetch(&mut self, table_row: usize) -> Self::Item {
+        let handle = self.inner.table_fetch(table_row);
+        self.check.has_changed(handle)
+    }
+
+    unsafe fn archetype_fetch(&mut self, archetype_index: usize) -> Self::Item {
+        let handle = self.inner.archetype_fetch(archetype_index);
+        self.check.has_changed(handle)
+    }
+
+    #[inline]
+    unsafe fn table_filter_fetch(&mut self, table_row: usize) -> bool {
+        self.table_fetch(table_row)
+    }
+
+    #[inline]
+    unsafe fn archetype_filter_fetch(&mut self, archetype_index: usize) -> bool {
+        self.archetype_fetch(archetype_index)
+    }
+
+    #[inline]
+    fn update_component_access(state: &Self::State, access: &mut FilteredAccess<ComponentId>) {
+        ReadFetch::<Handle<T>>::update_component_access(&state.inner, access);
+    }
+
+    #[inline]
+    fn update_archetype_component_access(
+        state: &Self::State,
+        archetype: &Archetype,
+        access: &mut Access<ArchetypeComponentId>,
+    ) {
+        ReadFetch::<Handle<T>>::update_archetype_component_access(&state.inner, archetype, access);
+    }
+}
+
+/// SAFETY: read-only access
+unsafe impl<T: Asset> ReadOnlyWorldQuery for AssetChanged<T> {}

crates/bevy_ecs/src/change_detection.rs

@@ -206,6 +206,13 @@ change_detection_impl!(NonSendMut<'a, T>, T,);
 impl_into_inner!(NonSendMut<'a, T>, T,);
 impl_debug!(NonSendMut<'a, T>,);
 
+// Note on how change detection works.
+//
+// The `World` maintains a `Table` for each `StorageType::Table` component,
+// When accessing a `Mut` from a query, a somewhat mutable reference to the
+// tick stored in the table is passed to the `Mut`'s `ticks.component_ticks`
+// field, going from a `&UnsafeCell<ComponentTicks>` to a `&mut ComponentTicks`
+// using unsafe (in a context where the reference is guaranteed to be unique).
 /// Unique mutable borrow of an entity's component
 pub struct Mut<'a, T> {
     pub(crate) value: &'a mut T,

examples/asset/hot_asset_reloading.rs

@@ -12,10 +12,17 @@ fn main() {
             ..default()
         })
         .add_plugins(DefaultPlugins)
+        .add_system(asset_updated)
         .add_startup_system(setup)
         .run();
 }
 
+fn asset_updated(query: Query<&Name, AssetChanged<Mesh>>) {
+    for name in query.iter() {
+        println!("{name} mesh got updated");
+    }
+}
+
 fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
     // Load our mesh:
     let scene_handle = asset_server.load("models/monkey/Monkey.gltf#Scene0");