Background Downloader. Has the same API as WebClient but uses the underlying NSUrlRequest to download in the background.

gistfile1.txt

public class BackgroundDownload
	{
		public BackgroundDownload ()
		{
		}
		NSUrlSessionDownloadTask downloadTask;
		static NSUrlSession session;
		public async Task DownloadFileAsync(Uri url, string destination)
		{
			if (downloadTask != null)
				return;
			if(session == null)
				session = InitBackgroundSession ();
			Tcs = new TaskCompletionSource<bool> ();
			Destination = destination;
			using (var request = new NSUrlRequest(new NSUrl(url.AbsoluteUri))) {
				downloadTask = session.CreateDownloadTask (request);
				downloadTask.Resume ();
			}
			await Tcs.Task;
		}
		public event Action<float> ProgressChanged;
		float progress;
		public float Progress {
			get {
				return progress;
			}
			private set {
				if (Math.Abs (progress - value) < float.Epsilon)
					return;
				progress = value;
				if (ProgressChanged != null)
					ProgressChanged (progress);

			}
		}
		NSUrlSession InitBackgroundSession ()
		{
			Console.WriteLine ("InitBackgroundSession");
			using (var configuration = NSUrlSessionConfiguration.BackgroundSessionConfiguration ("async.background.downloader")) {
				return NSUrlSession.FromConfiguration (configuration, new UrlSessionDelegate (this), null);
			}
		}
		protected string Destination;
		protected TaskCompletionSource<bool> Tcs;
		public class UrlSessionDelegate : NSUrlSessionDownloadDelegate
		{
			public BackgroundDownload controller;

			public UrlSessionDelegate (BackgroundDownload controller)
			{
				this.controller = controller;
			}

			public override void DidWriteData (NSUrlSession session, NSUrlSessionDownloadTask downloadTask, long bytesWritten, long totalBytesWritten, long totalBytesExpectedToWrite)
			{
				Console.WriteLine ("Set Progress");
				if (downloadTask == controller.downloadTask) {
					float progress = totalBytesWritten / (float)totalBytesExpectedToWrite;
					Console.WriteLine (string.Format ("DownloadTask: {0}  progress: {1}", downloadTask, progress));
					InvokeOnMainThread(() => controller.Progress = progress);
				}
			}

			public override void DidFinishDownloading (NSUrlSession session, NSUrlSessionDownloadTask downloadTask, NSUrl location)
			{
				Console.WriteLine ("Finished");
				Console.WriteLine ("File downloaded in : {0}", location);
				NSFileManager fileManager = NSFileManager.DefaultManager;

				var URLs = fileManager.GetUrls (NSSearchPathDirectory.DocumentDirectory, NSSearchPathDomain.User);
				NSUrl documentsDictionry = URLs [0];

				NSUrl originalURL = downloadTask.OriginalRequest.Url;
				NSUrl destinationURL = NSUrl.FromFilename(controller.Destination);
				NSError removeCopy;
				NSError errorCopy;

				fileManager.Remove (destinationURL, out removeCopy);
				bool success = fileManager.Copy (location, destinationURL, out errorCopy);

				if (success) {
					controller.Tcs.TrySetResult (true);
				} else {

					controller.Tcs.TrySetException(new Exception(string.Format("Error during the copy: {0}", errorCopy.LocalizedDescription)));
				}

			}

			public override void DidCompleteWithError (NSUrlSession session, NSUrlSessionTask task, NSError error)
			{

				float progress = task.BytesReceived / (float)task.BytesExpectedToReceive;
				InvokeOnMainThread (() => controller.Progress = progress);

				controller.downloadTask = null;

				Console.WriteLine ("DidComplete");
				if (error == null)
					controller.Tcs.TrySetResult (true);
				else {

					controller.Tcs.TrySetException(new Exception(string.Format("Error during the copy: {0}", error.LocalizedDescription)));
				}

			}

			public override void DidResume (NSUrlSession session, NSUrlSessionDownloadTask downloadTask, long resumeFileOffset, long expectedTotalBytes)
			{
				Console.WriteLine ("DidResume");
			}

			public override void DidFinishEventsForBackgroundSession (NSUrlSession session)
			{
				AppDelegate appDelegate = UIApplication.SharedApplication.Delegate as AppDelegate;
				var handler = appDelegate.BackgroundSessionCompletionHandler;
				if (handler != null) {
					appDelegate.BackgroundSessionCompletionHandler = null;
					handler ();
				}
				BackgroundDownload.session = null;
				Console.WriteLine ("All tasks are finished");
			}
		}
	}