IOs7 app crashing when in background

Question

My app sometimes crashes when in background and shows the following crash log:

Nov  7 12:33:31 iPad backboardd[29] <Warning>: MyApp[3096] has active assertions beyond permitted time: 
    {(
        <BKProcessAssertion: 0x14680c60> identifier: Called by MyApp, from -[AppDelegate applicationDidEnterBackground:] process: MyApp[3096] permittedBackgroundDuration: 180.000000 reason: finishTask owner pid:3096 preventSuspend  preventIdleSleep  preventSuspendOnSleep 
    )}

Looking through other questions I found out that the crash message indicates that I didn't end a task properly, so when its time expired the OS ended it and crashed my app.

So I added some NSLogs:

- (void)applicationDidEnterBackground:(UIApplication *)application
{       
    [self saveContext];
    [Settings setCallLock:YES];

    [self saveStuff];

    if ([self isBackgroundTaskNeeded]) 
    {

        UIApplication*  app = [UIApplication sharedApplication];

        bgTask = [app beginBackgroundTaskWithExpirationHandler:^{

            [self pauseDownloads];

            NSLog(@"Debug - Ending background task %d",bgTask);
            [app endBackgroundTask:bgTask];
            NSLog(@"Debug -  Background task %d ended",bgTask);
            bgTask = UIBackgroundTaskInvalid;


        }];
        NSLog(@"Debug - Starting background task %d",bgTask);
        [self initBackground];

    }

}

and found out that the task was called twice when it crashed:

Nov  7 12:30:02 iPad MyApp[3096] <Warning>: Debug - Starting background task 7
Nov  7 12:30:30 iPad MyApp[3096] <Warning>: Debug - Starting background task 8
Nov  7 12:33:26 iPad MyApp[3096] <Warning>: Debug - Ending background task 8
Nov  7 12:33:26 iPad MyApp[3096] <Warning>: Debug -  Background task 8 ended
Nov  7 12:33:26 iPad MyApp[3096] <Warning>: Debug - Ending background task 0
Nov  7 12:33:26 iPad MyApp[3096] <Warning>: Debug -  Background task 0 ended

I wasn't able to tell when the double call happens and why. So question is why is the task being called twice and is there a way to avoid it?

Edit:

- (void) pauseDownloads
{
    for (AFDownloadRequestOperation *operation in self.operationQueue.operations)
    {
        if(![operation isPaused])
        {
            [operation pause];
        }
    }
}

Answer 1

You should endBackground without expiration..

    UIApplication*  app = [UIApplication sharedApplication];

    bgTask = [app beginBackgroundTaskWithExpirationHandler:^{

        [self pauseDownloads];

        NSLog(@"Debug - Ending background task %d",bgTask);
        [app endBackgroundTask:bgTask];
        NSLog(@"Debug -  Background task %d ended",bgTask);
        bgTask = UIBackgroundTaskInvalid;


    }];
    NSLog(@"Debug - Starting background task %d",bgTask);

    [self initBackground];

    NSLog(@"Debug - Ending background task %d without expiration",bgTask);
    [app endBackgroundTask:bgTask];
    bgTask = UIBackgroundTaskInvalid;

When app enters in foreground, put this balancing endBackgroundTask call -

- (void)applicationWillEnterForeground:(UIApplication *)application
{
    if (bgTask != UIBackgroundTaskInvalid) {
       NSLog(@"Debug - Ending background task %d without expiration (when applicationWillEnterForeground)",bgTask);
       [app endBackgroundTask:bgTask];
       bgTask = UIBackgroundTaskInvalid;
    }

    // Also, to counter expiration handler pauseDownloads call, consider resuming the downloads here (or applicationDidBecomeActive app delegate).
}

Because - The expiration handler block {} is executed when the background task expire (usually up to 10 min, but not guaranteed). I assume your actual background task is [self initBackground]; , otherwise you should put it outside the block and before endBackgroundTask call.

(Another potential issue)

Also, I noticed your pauseDownloads code now, it seems you are performing downloads using NSOperationQueue when app goes in background. In such a case, ensure that the endBackgroundTask is not called until you have finished downloading or expiration handler is executed. In other words, the control doesn't return too early which means you might need to monitor these download NSOperation(s).

Answer 2

The short answer is that your crash was happening because you created two background tasks: 7 & 8 but only ended 8.

With that knowledge, you should apply Ashok's solution to ensure you always end any background task that you create.