Prevent dispatch_after() background task from being executed

Question

This is my issue. When my application enters background I want it to perform a function after certain period of time. This is what I do:

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    isRunningInBackground = YES;

    taskIdentifier = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:nil];

    int64_t delayInSeconds = 30;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
    dispatch_after(popTime, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void)
    {
        [self doSomething];
    });
}

- (void)doSomething
{
   NSLog(@"HELLO");
}

taskIdentifier variable is declared in myAppDelegate.h file like this:

UIBackgroundTaskIdentifier taskIdentifier;

Everything works as it supposed to, I see that console prints HELLO just right after 30 seconds are gone. But I don't want doSomething to be executed if the app enters foreground until 30 seconds are over. So I need to cancel it. This is how i do that:

- (void)applicationWillEnterForeground:(UIApplication *)application
{    
    isRunningInBackground = NO;
    [self stopBackgroundExecution];
}

- (void)stopBackgroundExecution
{
    [[UIApplication sharedApplication] endBackgroundTask:taskIdentifier];
    taskIdentifier = UIBackgroundTaskInvalid;
}

But unfortunately it doesn't cancel doSomething, it is still performed. What am I doing wrong? How do I cancel that function?

Answer 1

Why even use GCD? You could just use an NSTimer and invalidate it when your app returns to the foregound.

Answer 2

A bit different approach OK, so, with all answers collected, and possible solutions, seems like the best one for this case (preserving simplicity) is calling performSelector:withObject:afterDelay: and cancelling it with cancelPreviousPerformRequestsWithTarget: call when desired. In my case - just before scheduling next delayed call:

[NSObject cancelPreviousPerformRequestsWithTarget: self selector:@selector(myDelayedMethod) object: self];

[self performSelector:@selector(myDelayedMethod) withObject: self afterDelay: desiredDelay];