Windows Phone中BackgroundTransferRequest的TransferStatusChange

我正在实现一个应用程序，用户可以选择下载资产(大约750MB的图像，每个大小约为50KB)。我不能使用常规的HttpClient做到这一点，因为如果用户按下home键，SendAsync/GetAsync api在后台不起作用，当用户回到应用程序时崩溃。

我唯一的选择是使用BackgroundTransferService，我使用如下:-

for(...)//calling the download function here
    DownloadImageToIsoStore(planImageUri, cemetery_id + "_plan_1.gif", countryName);

public void DownloadImageToIsoStore(Uri imageUri, string imageName,string countryName)
    {
        while ((BackgroundTransferService.Requests.Count()) >= 25) { Thread.Sleep(100); }//do not add if count is 25 since that is the limit. added this to wait till space frees up in the queue
        BackgroundTransferRequest backgroundTransferRequest = new BackgroundTransferRequest(imageUri);
        backgroundTransferRequest.Method = "GET";
        backgroundTransferRequest.DownloadLocation = new Uri("shared/transfers/" + imageName, UriKind.RelativeOrAbsolute);//shared/transfers is the required directory
        backgroundTransferRequest.Tag = countryName;//will check this when download is completed
        backgroundTransferRequest.TransferPreferences = TransferPreferences.AllowCellularAndBattery;
        BackgroundTransferService.Add(backgroundTransferRequest);
        backgroundTransferRequest.TransferStatusChanged +=  new EventHandler<BackgroundTransferEventArgs>(backgroundTransferRequest_TransferStatusChanged);
        backgroundTransferRequest.TransferProgressChanged +=  new EventHandler<BackgroundTransferEventArgs>(backgroundTransferRequest_TransferProgressChanged);
    }
void backgroundTransferRequest_TransferProgressChanged(object sender, BackgroundTransferEventArgs e)
    {
        long bytes = e.Request.BytesReceived;
        Debug.WriteLine(bytes);
    }
void backgroundTransferRequest_TransferStatusChanged(object sender, BackgroundTransferEventArgs e)
    {
        BackgroundTransferRequest backgroundTransferRequest = e.Request;
        BackgroundTransferService.Remove(backgroundTransferRequest);
    }

下载成功完成，但由于某种原因，backgroundTransferRequest_TransferProgressChanged和backgroundTransferRequest_TransferStatusChanged事件没有被调用，我无法为任何额外的下载释放下载队列。

请帮忙!

谢谢