[Asset]Add dependency tracking API.

Modules/Luna/Asset/Asset.hpp

@@ -125,6 +125,13 @@ namespace Luna
             //! This can be `nullptr` if the user calls @ref set_asset_data with `data` equals to `nullptr`. In such case,
             //! this function behaves like unloading existing asset data object.
             RV(*on_set_asset_data)(object_t userdata, asset_t asset, object_t data) = nullptr;
+            //! Called when assets referred by the specified asset is required.
+            //! @param[in] userdata The userdata.
+            //! @param[in] asset The asset handle of the asset to be queried.
+            //! @param[out] referred_assets Returns the assets referred by this asset.
+            //! This vector may not be empty when this function is called. If this vector is not empty, the returned
+            //! assets shall be pushed to the end of this vector, and existing elements shall not be modified.
+            void(*on_get_referred_assets)(object_t userdata, asset_t asset, Vector<asset_t>& referred_assets) = nullptr;
         };
 
         //! Registers one asset type so the asset system can handle the asset of that type.
@@ -319,6 +326,13 @@ namespace Luna
         //! @param[in] asset The asset handle of the asset to operate.
         LUNA_ASSET_API RV save_asset(asset_t asset);
 
+        //! Gets referred assets of the specified asset.
+        //! @param[in] asset The handle of the asset to query.
+        //! @param[out] out_referred_assets Returns the referred assets of the specified asset.
+        //! If this vector is not empty, the returned assets will be pushed to the end of the vector, and existing
+        //! elements will not be modified.
+        LUNA_ASSET_API void get_referred_assets(asset_t asset, Vector<asset_t>& out_referred_assets);
+
         //! Closes the asset registry.
         //! @details This call removes all registered assets and asset types, and invalidates all asset handles.
         LUNA_ASSET_API void close();

Modules/Luna/Asset/Source/Asset.cpp

@@ -579,6 +579,21 @@ namespace Luna
             lucatchret;
             return ok;
         }
+        LUNA_ASSET_API void get_referred_assets(asset_t asset, Vector<asset_t>& out_referred_assets)
+        {
+            if(!asset.handle) return;
+            AssetEntry* entry = (AssetEntry*)asset.handle;
+            LockGuard g(entry->lock);
+            if (entry->type.empty()) return;
+            auto desc = get_asset_type_desc(entry->type);
+            if(failed(desc)) return;
+            ObjRef data = entry->data;
+            g.unlock();
+            if(desc.get().on_get_referred_assets)
+            {
+                desc.get().on_get_referred_assets(desc.get().userdata.get(), asset, out_referred_assets);
+            }
+        }
         LUNA_ASSET_API void close()
         {
             MutexGuard guard(g_assets_mutex);