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Imagine an app that downloads and plays movies. If a user begins downloading a movie, it'd be great if the file could continue downloading even if the user presses the home button and sends the app to the background. NSURLSessionDownloadTask can help. Let's take a look.

First we create a new background configuration and session. Then we use that session to create a new download task:

class ViewController: UIViewController, NSURLSessionDownloadDelegate, NSURLSessionTaskDelegate {
    func downloadMovieFile() {
    let configuration = NSURLSessionConfiguration.backgroundSessionConfigurationWithIdentifier("downloads-session")
    let session = NSURLSession(configuration: configuration, delegate: self, delegateQueue: nil)

    let task = session.downloadTaskWithURL(NSURL(string: "http://supersecret.disney.com/movies/episodeVII.mp4")!)

We can grab the downloaded data pretty easily, by implementing this function from NSURLSessionDownloadDelegate:

func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didFinishDownloadingToURL location: NSURL) {
    print("Movie data written to \(location)")

Last but not least, we'll need to implement a function from NSURLSessionTaskDelegate. In it we'll call finishTasksAndInvalidate on the session. This ensures everything is cleaned up and the session lets go of us (self) as its delegate.

func URLSession(session: NSURLSession, task: NSURLSessionTask, didCompleteWithError error: NSError?) {
  if let error = error { print("Download Error: \(error)") }