Add change detection to `Manifest`

common/src/assets/manifest.rs

@@ -76,6 +76,11 @@ pub struct Manifest<'w, T: ManifestEntry> {
 }
 
 impl<T: ManifestEntry> Manifest<'_, T> {
+    /// System condition which returns `true` if this manifest was added or changed.
+    pub fn changed(res: Option<Res<ManifestResource<T>>>) -> bool {
+        resource_exists_and_changed(res)
+    }
+
     /// Looks up an [`Asset`] of type `T` from its associated manifest.
     ///
     /// Returns `None` if an asset with the given identifier is missing from
@@ -90,6 +95,31 @@ impl<T: ManifestEntry> Manifest<'_, T> {
         let entry = self.entry_assets.get(entry_handle).unwrap();
         Some(entry)
     }
+
+    pub fn iter(&self) -> impl Iterator<Item = &T> {
+        let Some(manifest_handle) = self.resource.as_deref().map(|res| &res.handle) else {
+            return ManifestIter(None);
+        };
+        // // SAFETY: Manifest loads this as a dependency, so it should exist.
+        let manifest = self.manifest_assets.get(manifest_handle).unwrap();
+        let values = manifest.map.values();
+        // // SAFETY: Entry loads this as a dependency, so they should exist.
+        let iter = values.map(|h| self.entry_assets.get(h).unwrap());
+        ManifestIter(Some(iter))
+    }
+}
+
+struct ManifestIter<I>(Option<I>);
+
+impl<'a, T, I> Iterator for ManifestIter<I>
+where
+    T: ManifestEntry,
+    I: Iterator<Item = &'a T>,
+{
+    type Item = &'a T;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.as_mut().and_then(|i| i.next())
+    }
 }
 
 pub trait InitManifest {
@@ -105,5 +135,19 @@ impl InitManifest for App {
         // Insert a `Resource` that holds a handle to the `ManifestAsset` once it's loaded.
         let handle = self.world_mut().load_asset(path);
         self.load_resource_with(ManifestResource::<T> { handle });
+
+        self.add_systems(PostUpdate, trigger_change_on_loaded::<T>);
+    }
+}
+
+fn trigger_change_on_loaded<T: ManifestEntry>(
+    mut events: MessageReader<AssetEvent<ManifestAsset<T>>>,
+    mut resource: Option<ResMut<ManifestResource<T>>>,
+) {
+    for event in events.read() {
+        if let AssetEvent::LoadedWithDependencies { id: _ } = event {
+            // Trigger change detection on the `ManifestResource`.
+            resource.as_deref_mut();
+        }
     }
 }