Objective-c Long Running Task Thread Confusion

Question

My app needs to do some internet related operation then pop that view when I send the application to background. And then the root view controller fetches some data and updates the collection view. Do you have any idea how can I solve following issues related to above operations(by the way, I use local notifications to start the process):

1) UI related operations(popping current view controller) seems to fail in the background.

2) When I pop my view to root view controller, Root view has some nsurlconnection which sends data to its delegate. Since long running tasks run in global queue, nsurlconnection seems to fail sending any information to its delegate.

I use the following code for this process:

   UIBackgroundTaskIdentifier __block bgTask;
   UIApplication  *app = [UIApplication sharedApplication];
   bgTask = [app beginBackgroundTaskWithExpirationHandler:^{ 
         [app endBackgroundTask:bgTask]; 
         bgTask = UIBackgroundTaskInvalid;
    }];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
         [self performOperation];
         NSLog(@"Operation finished");
     });

Answer 1

As matt pointed out, you can't do UI operations like popping the view controller when the app is in the background. When the app comes back to the foreground (e.g. the user taps on the icon again), the pop may take place then (if the app wasn't completely terminated in the interim).

I assume matt's observation answered your second question. If it didn't, please clarify what you mean. But this use of UIBackgroundTaskIdentifier doesn't care whether you used global queues or custom queues or whatever. The only restriction is that some UI operations will not take place, so anything contingent upon, for example, viewDidAppear, won't take place.

As an aside, I wanted to point out that you really want to call endBackgroundTask when the code that is doing the task completes, which your code sample does not appear to do. See the Executing a Finite-Length Task in the Background of the App States and Multitasking chapter of the iOS App Programming Guide.

Thus, you might do something like:

UIApplication *app = [UIApplication sharedApplication];

UIBackgroundTaskIdentifier __block bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
    NSLog(@"Operation did not finish!!!");

    [app endBackgroundTask:bgTask]; 
    bgTask = UIBackgroundTaskInvalid;
}];

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    [self performOperation];
    NSLog(@"Operation finished");

    if (bgTask != UIBackgroundTaskInvalid) {
        [app endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }
});