Action Extension - Multiple NSURLSession

Question

Im trying to upload images from the Photo app through Action Extension. I use NSURLSession to upload it in background. Here is the code i use.

var configName = "com.myapp.uploadImage"
var config  = NSURLSessionConfiguration.backgroundSessionConfigurationWithIdentifier(configName)
config.sharedContainerIdentifier = "group.myApp.sample"

var session = NSURLSession(configuration: config)
let task = session.dataTaskWithRequest(request)
task.resume()

self.extensionContext!.completeRequestReturningItems(self.extensionContext!.inputItems, completionHandler: nil)

It works fine.

The question is when i upload an image and dismiss the view once and then again try uploading a second image while the initial process is still running in the background, the initial NSURLSession isn't completed. Only the second process gets completed. In short, the second session overcomes the first session.

I tried using NSOperationQueue. But action extension once dismissed and opened again for the second session, it just creates a new NSOperationQueue and hence the problem still persists.

Any suggestion will be helpful. Thanks in advance.

Answer 1

Make sure you don't attempt to instantiate a second background session with the same identifier while the first is still running. Save your background session so you can use it later.

As the "Background Session Considerations" of the URL Loading System Programming Guide: Using NSURLSession says:

Note: You must create exactly one session per identifier (specified when you create the configuration object). The behavior of multiple sessions sharing the same identifier is undefined.

Note, this document also mentions that one "must" specify and implement the delegate. (If nothing else, how will you know about failure if you don't do that?) The example provided in the Performing Uploads and Downloads section of the App Extension Programming Guide specifies the delegate, too.

Also, has your main app's app delegate implemented the handleEventsForBackgroundURLSession method? You have to capture the completionHandler and call it when the NSURLSessionDelegate method URLSessionDidFinishEventsForBackgroundURLSession is called.

Finally, I notice that you're using data task. The NSURLSession documentation is specific that one should not be using data tasks with background sessions, only upload/download tasks. I always assumed that was just so you don't try to use didReceiveData delegate method, but I might try using upload task just in case there's some other issue associated with data tasks with background sessions.