/// <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:只能在子线程中调用
        /// </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 CopyAssetDirectoryInThread(String sourceDir, String destDir, Action <bool> callback)
        {
            StreamingAssetHelper.sourceDir = sourceDir;
            StreamingAssetHelper.destDir   = assetPath != null?Path.Combine(assetPath, destDir) : null;

            StreamingAssetHelper.callback = callback;
            if (StreamingAssetHelper.destDir != null)
            {
                ColaLoom.RunAsync(CopyAssetDirectory);
            }
        }