[assets] fix Assets being set as 'changed' each frame (#2280)

## Objective
- Fixes: #2275 
- `Assets` were being flagged as 'changed' each frame regardless of if the assets were actually being updated. 

## Solution
- Only have `Assets` change detection be triggered when the collection is actually modified. 
- This includes utilizing `ResMut` further down the stack instead of a `&mut Assets` directly.

crates/bevy_asset/src/asset_server.rs

@@ -5,7 +5,7 @@ use crate::{
     RefChange, RefChangeChannel, SourceInfo, SourceMeta,
 };
 use anyhow::Result;
-use bevy_ecs::system::Res;
+use bevy_ecs::system::{Res, ResMut};
 use bevy_log::warn;
 use bevy_tasks::TaskPool;
 use bevy_utils::{HashMap, Uuid};
@@ -466,7 +466,9 @@ impl AssetServer {
         }
     }
 
-    pub(crate) fn update_asset_storage<T: Asset>(&self, assets: &mut Assets<T>) {
+    // Note: this takes a `ResMut<Assets<T>>` to ensure change detection does not get
+    // triggered unless the `Assets` collection is actually updated.
+    pub(crate) fn update_asset_storage<T: Asset>(&self, mut assets: ResMut<Assets<T>>) {
         let asset_lifecycles = self.server.asset_lifecycles.read();
         let asset_lifecycle = asset_lifecycles.get(&T::TYPE_UUID).unwrap();
         let mut asset_sources_guard = None;

crates/bevy_asset/src/assets.rs

@@ -177,7 +177,11 @@ impl<T: Asset> Assets<T> {
         mut events: EventWriter<AssetEvent<T>>,
         mut assets: ResMut<Assets<T>>,
     ) {
-        events.send_batch(assets.events.drain())
+        // 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())
+        }
     }
 
     pub fn len(&self) -> usize {

crates/bevy_asset/src/loader.rs

@@ -203,7 +203,7 @@ impl<T: Component> Default for AssetLifecycleChannel<T> {
 /// Updates the [Assets] collection according to the changes queued up by [AssetServer].
 pub fn update_asset_storage_system<T: Asset + AssetDynamic>(
     asset_server: Res<AssetServer>,
-    mut assets: ResMut<Assets<T>>,
+    assets: ResMut<Assets<T>>,
 ) {
-    asset_server.update_asset_storage(&mut assets);
+    asset_server.update_asset_storage(assets);
 }

crates/bevy_ecs/src/event.rs

@@ -342,6 +342,12 @@ impl<T: Component> Events<T> {
         self.events_b.clear();
     }
 
+    /// Returns true if there are no events in this collection.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.events_a.is_empty() && self.events_b.is_empty()
+    }
+
     /// Creates a draining iterator that removes all events.
     pub fn drain(&mut self) -> impl Iterator<Item = T> + '_ {
         self.reset_start_event_count();
@@ -557,4 +563,21 @@ mod tests {
             .iter(&events)
             .eq([TestEvent { i: 0 }, TestEvent { i: 1 }].iter()));
     }
+
+    #[test]
+    fn test_events_empty() {
+        let mut events = Events::<TestEvent>::default();
+        assert!(events.is_empty());
+
+        events.send(TestEvent { i: 0 });
+        assert!(!events.is_empty());
+
+        events.update();
+        assert!(!events.is_empty());
+
+        // events are only empty after the second call to update
+        // due to double buffering.
+        events.update();
+        assert!(events.is_empty());
+    }
 }