AsyncOperation 可以对其执行 yield 以等待操作完成的对象。
卸载未使用的资源。
如果在遍历整个游戏对象层级视图后未访问到某资源（包括脚本组件），则将其视为未使用的资源。该操作也检查静态变量。
The script execution stack, however, is not examined so an asset referenced only from within the script stack will be unloaded. All assets other than ScriptableObjects are loaded back in the next time one of its properties or methods is used. This requires extra care for assets which have been modified in memory. Make sure to call EditorUtility.SetDirty before an asset garbage collection is triggered.
另请参阅：EditorUtility.UnloadUnusedAssetsImmediate。