private async void writeStringToFile(string fileName, string content)
        {
            //创建新文件,如果该文件存在则自动在末尾追加一个Unique名称。
            StorageFile file = await DownloadFolder.CreateFileAsync(fileName, options : CreationCollisionOption.GenerateUniqueName);

            //把文字写入文件
            await FileIO.WriteTextAsync(file, content);;
        }