TransferStatusChanged and TransferProgressChanged events of BackgroundTransferRequest in Windows Phone not getting called

Question

I am implementing an app where the user will have the option of downloading the assets(approx 750MB in images, each of which is around 50KB in size). I cannot do this using the regular HttpClient since if the user presses the home button, the SendAsync/GetAsync APIs don't work in the background and crash when the user comes back to the application.

My only option here is to use the BackgroundTransferService which I am using as follows:-

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);
    }

The downloads are completed successfully but for some reason, the backgroundTransferRequest_TransferProgressChanged and backgroundTransferRequest_TransferStatusChanged events are not getting called and I cannot free up the download queue for any additional downloads.

Please help!

Thanks

Answer 1

The problem was that I was running the loop that was calling the DownloadImageToIsoStore on the UI thread.

The transfer status changed and progress changed events also apparently need the UI thread. As soon as I moved the calling function to a separate thread, the status and progress changed events started firing as expected.

Answer 2

That's not the reason I believe. you added the request to the background service with out attaching the handlers. handlers must be attached before you add the request to the background service.