revert 'free_unused_asset_system' ordering to allow 1 frame delay for asset removal

Author: NathanSWard

crates/bevy_asset/src/asset_server.rs

@@ -525,14 +525,13 @@ impl AssetServer {
     }
 }
 
-fn free_unused_assets_system_ref(asset_server: &AssetServer) {
-    // note: order of these function is important.
-    asset_server.mark_unused_assets();
+fn free_unused_assets_system_impl(asset_server: &AssetServer) {
     asset_server.free_unused_assets();
+    asset_server.mark_unused_assets();
 }
 
 pub fn free_unused_assets_system(asset_server: Res<AssetServer>) {
-    free_unused_assets_system_ref(&asset_server);
+    free_unused_assets_system_impl(&asset_server);
 }
 
 #[cfg(test)]
@@ -761,18 +760,24 @@ mod test {
 
         // mimics one full frame
         let tick = |server: &AssetServer, assets: &mut Assets<PngAsset>| {
-            free_unused_assets_system_ref(server);
+            free_unused_assets_system_impl(server);
             server.update_asset_storage(assets);
         };
 
         tick(&asset_server, &mut assets);
         // asset should exist and be loaded at this point
         assert_eq!(LoadState::Loaded, asset_server.get_load_state(id));
+        assert!(assets.get(&handle).is_some());
 
-        // after dropping the handle, next call to `tick` should
-        // remove the asset and reset the `LoadState`.
+        // after dropping the handle, next call to `tick` will prepare the assets for removal.
         drop(handle);
         tick(&asset_server, &mut assets);
+        assert_eq!(LoadState::Loaded, asset_server.get_load_state(id));
+        assert!(assets.get(id).is_some());
+
+        // second call to tick will actually remove the asset.
+        tick(&asset_server, &mut assets);
         assert_eq!(LoadState::NotLoaded, asset_server.get_load_state(id));
+        assert!(assets.get(id).is_none());
     }
 }