如何使用后台下载器在Windows Phone 8.1中下载图像

Question

我想从Windows Phone 8.1中的网址下载图片，我使用BackgroundDownloader，但它不起作用，有人可以帮助我吗？

var source = new Uri(source_image, UriKind.Absolute);
string LocalName = "10oswall";
var destinationFile =
    await KnownFolders.PicturesLibrary.CreateFileAsync(
        LocalName, CreationCollisionOption.ReplaceExisting);
var downloader = new BackgroundDownloader();
DownloadOperation download = downloader.CreateDownload(source, destinationFile);

download.Priority = BackgroundTransferPriority.High;
await download.StartAsync();