/// <summary>
        /// 在Android平台上,从(apk包中的) Assets目录复制文件至指定目录
        /// 注意:只能在主线程中调用
        /// </summary>
        /// <param name="sourceDir">源目录,是以 assets 为根目录的相对路径</param>
        /// <param name="destDir">目标目录</param>
        /// <param name="title">进度对话框标题</param>
        /// <returns></returns>
        public static void CopyAssetDirectory()
        {
            ColaLoom.QueueOnMainThread(() =>
            {
                Debug.Log("CopyAssetDirectory, sourceDir = " + sourceDir + ", destDir = " + destDir);

                AndroidJavaClass unityPlayer      = new AndroidJavaClass("com.unity3d.player.UnityPlayer");
                AndroidJavaObject currentActitivy = unityPlayer.GetStatic <AndroidJavaObject>("currentActivity");

                Debug.Log("CopyAssetDirectory before execute");

                bool ret = currentActitivy.Call <bool>("copyAssets", sourceDir, destDir);
                callback(ret);
                Debug.Log("CopyAssetDirectory finished");
            });
        }