Walkthrough - Using Background Transfer Service and NSURLSession

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In this walkthrough, we use the Background Transfer Service and NSURLSession API to kick off downloading a large image that continues to download when the app is in the background.


A background transfer is initiated by configuring a background NSURLSession and enqueuing upload or download tasks. If tasks complete while the application is backgrounded, suspended, or terminated, iOS will notify the application by calling the completion handler in the application's AppDelegate. The following diagram demonstrates this in action:

Let's see what this looks like in code.

Configuring a Background Session

To make a background session, create a new NSUrlSession and configure it using an NSUrlSessionConfiguration object.

The configuration object determines what the session can do and the kinds of tasks it can run. Sessions configured using the CreateBackgroundSessionConfiguration method will run in a separate process, and perform discretionary (WiFi) transfers to preserve data and battery life. The following code sample demonstrates proper setup of a background transfer session using the CreateBackgroundSessionConfiguration method and a unique string identifier:

public partial class SimpleBackgroundTransferViewController : UIViewController
  NSUrlSession session = null;

  NSUrlSessionConfiguration configuration =
      NSUrlSessionConfiguration.CreateBackgroundSessionConfiguration ("com.SimpleBackgroundTransfer.BackgroundSession");
  session = NSUrlSession.FromConfiguration
      (configuration, (NSUrlSessionDelegate) new MySessionDelegate, new NSOperationQueue());


Apart from a configuration object, a session also requires a session delegate and a queue. The queue determines the order in which the tasks will complete. The session delegate chaperones the transfer process, and handles authentication, caching and other session-related issues. We will explore the session delegate in more detail next.

Working With Tasks and Delegates

Now that we have configured a background session, let's kick off tasks to handle the transfer. We can keep track of these tasks using the an NSUrlSessionDelegate instance called a session delegate. The session delegate is responsible for waking a terminated or suspended application in the background to handle authentication, errors, or transfer completion.

An NSUrlSessionDelegate provides the following basic methods to check transfer status:

  • DidFinishEventsForBackgroundSession - This method gets called when all tasks have finished, and the transfer is complete.
  • DidReceiveChallenge - Called to request credentials when authorization is required.
  • DidBecomeInvalidWithError - Called if the NSURLSession becomes invalidated.

Background sessions require more specialized delegates depending on the types of tasks that are running. Background sessions are limited to two types of tasks:

  • Upload Tasks - Tasks of type NSUrlSessionUploadTask use the NSUrlSessionTaskDelegate , which inherits from NSUrlSessionDelegate . This delegate provides additional methods to track upload progress, handle HTTP redirection, and more.
  • Download Tasks - Tasks of type NSUrlSessionDownloadTask use the NSUrlSessionDownloadDelegate , which inherits from NSUrlSessionTaskDelegate . This delegate provides all the methods for upload tasks, as well as download-specific methods to track the download progress and determine when a download task has resumed or completed.

The following code defines a task that can be used to download an image from a URL. We kick off the task by calling CreateDownloadTask on our background session, and passing in the URL request:

const string DownloadURLString = "http://cdn1.xamarin.com/webimages/images/xamarin.png";
public NSUrlSessionDownloadTask downloadTask;

NSUrl downloadURL = NSUrl.FromString (DownloadURLString);
NSUrlRequest request = NSUrlRequest.FromUrl (downloadURL);
downloadTask = session.CreateDownloadTask (request);

Next, we'll create a new session download delegate to keep track of all download tasks in this session:

public class MySessionDelegate : NSUrlSessionDownloadDelegate
  public override void DidWriteData (NSUrlSession session, NSUrlSessionDownloadTask downloadTask, long bytesWritten, long totalBytesWritten, long totalBytesExpectedToWrite)
    Console.WriteLine (string.Format ("DownloadTask: {0}  progress: {1}", downloadTask, progress));
    InvokeOnMainThread( () => {
      // update UI with progress bar, if desired

If we want to find out the progress of a download task, we can override the DidWriteData method to track progress, and even update the UI. UI updates will appear immediately if the application is in the foreground, or will be waiting for the user the next time they open the application.

The session delegate API provides a broad toolkit for interacting with tasks. For a full list of session delegate methods, refer to the NSUrlSessionDelegate API documentation.

Note: Background sessions are started on a background thread, so any calls to update the UI must be explicitely run on the UI thread by calling InvokeOnMainThread to avoid iOS terminating the app.

Handling Transfer Completion

The final step is to let the application know when all the tasks associated with the session have completed, and handle the new content.

In the AppDelegate, subscribe to the HandleEventsForBackgroundUrl event. When the application enters the background and a transfer session is running, this method is called and the system passes us a completion handler:

public System.Action backgroundSessionCompletionHandler;

public override void HandleEventsForBackgroundUrl (UIApplication application, string sessionIdentifier, System.Action completionHandler)
  this.backgroundSessionCompletionHandler = completionHandler;

We will use the completion handler to let iOS know when our application is done processing.

Recall that a session can spawn several tasks to process a transfer. When the last task completes, a suspended or terminated application is re-launched into the background. Then, the application re-connects to the NSURLSession using the unique session identifier, and calls DidFinishEventsForBackgroundSession on the session delegate. This method is the application's opportunity to handle new content, including updating the UI to reflect the results of the transfer:

public override void DidFinishEventsForBackgroundSession (NSUrlSession session) {
  // Handle new information, update UI, etc.

Once we're done handling new content, we call the completion handler to let the system know it is safe to take a snapshot of the application and go back to sleep:

public override void DidFinishEventsForBackgroundSession (NSUrlSession session) {
  var appDelegate = UIApplication.SharedApplication.Delegate as AppDelegate;

  // Handle new information, update UI, etc.

  // call completion handler when you're done
  if (appDelegate.backgroundSessionCompletionHandler != null) {
    NSAction handler = appDelegate.backgroundSessionCompletionHandler;
    appDelegate.backgroundSessionCompletionHandler = null;
    handler.Invoke ();

In this walkthrough, we covered the basic steps to implement the Background Transfer Service in iOS 7.

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