/// <summary>
 /// 要下载调用这个方法
 /// </summary>
 /// <param name="url">下载的文件网址的来源</param>
 /// <returns></returns>
 public static async Task Start(string filename,string url,DownloadType type,StorageFolder folder=null)
 {
     try
     {
         Uri uri = new Uri(Uri.EscapeUriString(url), UriKind.RelativeOrAbsolute);
         BackgroundDownloader downloader = new BackgroundDownloader();
         if (folder==null)
         {
             folder = await KnownFolders.MusicLibrary.CreateFolderAsync("kgdownload", CreationCollisionOption.OpenIfExists);
             switch (type)
             {
                 case DownloadType.song:
                     folder = await folder.CreateFolderAsync("song", CreationCollisionOption.OpenIfExists);
                     downloader.TransferGroup = BackgroundTransferGroup.CreateGroup("song");
                     break;
                 case DownloadType.mv:
                     folder = await folder.CreateFolderAsync("mv", CreationCollisionOption.OpenIfExists);
                     downloader.TransferGroup = BackgroundTransferGroup.CreateGroup("mv");
                     break;
                 case DownloadType.other:
                     folder = await folder.CreateFolderAsync("other", CreationCollisionOption.OpenIfExists);
                     downloader.TransferGroup = BackgroundTransferGroup.CreateGroup("other");
                     break;
                 default:
                     break;
             }
         }else
         {
             downloader.TransferGroup = BackgroundTransferGroup.CreateGroup("other");
         }
         //string name = uri.ToString().Substring(uri.ToString().LastIndexOf("/"), uri.ToString().Length);
         string name = filename;
         StorageFile file = await folder.CreateFileAsync(name, CreationCollisionOption.GenerateUniqueName);
         downloader.FailureToastNotification = DownloadedToast.Done(filename, DownloadedToast.DownResult.Fa);
         downloader.SuccessToastNotification = DownloadedToast.Done(filename, DownloadedToast.DownResult.Su);
         var download = downloader.CreateDownload(new Uri(url), file);
         TransferModel transfer = new TransferModel();
         transfer.DownloadOperation = download;
         transfer.Source = download.RequestedUri.ToString();
         transfer.Destination = download.ResultFile.Path;
         transfer.BytesReceived = download.Progress.BytesReceived;
         transfer.TotalBytesToReceive = download.Progress.TotalBytesToReceive;
         transfer.Progress = 0;
         transfers.Add(transfer);
         Progress<DownloadOperation> progressCallback = new Progress<DownloadOperation>(DownloadProgress);
         download.StartAsync().AsTask(cancelToken.Token, progressCallback);
     }
     catch
     {
         await new MessageDialog("链接已失效!").ShowAsync();
     }
 }