How to handle background url session lost internet connection?

Question

I have background downloading zip file:

if let url = NSURL(string: urlstring) 
         {            
            let config = NSURLSessionConfiguration.backgroundSessionConfigurationWithIdentifier((NSUUID().UUIDString))
            let session = NSURLSession(configuration: config, delegate: self, delegateQueue: nil)
            let task = session.downloadTaskWithURL(url)
            session.sessionDescription = filepath
            if let sessionId = session.configuration.identifier
            {
                print("start zip session: " + sessionId)
            }

            task.resume()               
            }
        }

it works cool if you have internet connection but if you lose it during downloading app just wait and URLSession(session: NSURLSession, task: NSURLSessionTask, didCompleteWithError error: NSError?) will not be called never. How it can be handle?
Something like time for response from server

Dimitar Atanasov Dimitar Atanasov · Accepted Answer · 2018-01-11T22:12:06

The question is old, but still unanswered, so here is one workaround.

Some clarifications first

In general there is a property of URLSessionConfiguration called waitsForConnectivity which can be set to false where URLSessionTask will then directly fail on connection lost. However if true then URLSessionTaskDelegate will receive a callback on the urlSession(_:taskIsWaitingForConnectivity:) method.

However

Background tasks such as DownloadTask always wait for connectivity and ignore waitsForConnectivity property of URLSessionConfiguration. They also DON'T trigger urlSession(_:taskIsWaitingForConnectivity:) callback, so there is no official way to listen for connectivity drop-out on download task.

Workaround

If you listening for the download progress you'll notice that a call to the method is done few times in a second. Therefore we can conclude that if the progress callback is not called for more than 5 seconds, then there could be a connectivity drop issue. So the workaround is to make additional property to the URLSessionDownloadDelegate delegate and store the last update of the progress. Then have interval function to periodically check whether this property were not updated soon.

Something like:

    class Downloader: NSObject, URLSessionTaskDelegate, URLSessionDownloadDelegate {

        var lastUpdate: Date;
        var downloadTask: URLSessionDownloadTask?;

        public var session : URLSession {
            get {
                let config = URLSessionConfiguration.background(
                withIdentifier: "\(Bundle.main.bundleIdentifier!).downloader");
                config.isDiscretionary = true;

                return URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue());
            }
        }

        override init() {
            self.lastUpdate = Date();
            super.init();
        }

        func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
            // Handle download completition
            // ...
        }

        func urlSession(
            _ session: URLSession,
            downloadTask: URLSessionDownloadTask,
            didWriteData bytesWritten: Int64,
            totalBytesWritten writ: Int64,
            totalBytesExpectedToWrite exp: Int64)
        {
            let progress = 100 * writ / exp;
            // Do something with the progress
            // ...

            self.lastUpdate = Date();
        }
    }

    var downloader = Downloader();
    let url = "http://...";
    var request = URLRequest(url: URL(string: url)!);
    currentWorker.downloadTask = downloader.session.downloadTask(with: request);
    currentWorker.downloadTask!.resume();

    // Schedule timer for every 5 secs
    var timer = NSTimer.scheduledTimerWithTimeInterval(5.0, target: self, selector: "CheckInternetDrop", userInfo: nil, repeats: true);

func CheckInternetDrop(){
    let interval = Date().timeIntervalSinceDate(downloader.lastUpdate);
    if (interval > 5) {
        print("connection dropped");
    }
}

Very simple example...

How to handle background url session lost internet connection?

3 Answers