Question

This is my code:

- (void) printCount {
    NSLog(@"Current second: %i", i++);
}


- (void)applicationDidEnterBackground:(UIApplication *)application
{    
    UIBackgroundTaskIdentifier taskId = [application beginBackgroundTaskWithExpirationHandler:^{NSLog(@"Task expired");}];
    [NSTimer scheduledTimerWithTimeInterval:1.0
                                     target:self
                                   selector:@selector(printCount) userInfo:nil repeats:YES];
    if (i == 10000) {
        [application endBackgroundTask:taskId];
    }
}

And this is log that I get:

....
Current second: 579
Current second: 580
Current second: 581
Current second: 582
Current second: 583
Task expired
Current second: 584
Current second: 585
Current second: 586
Current second: 587
Current second: 588
Current second: 589
Current second: 590
....

As I know using beginBackgroundTaskWithExpirationHandler allowed only 10 minutes. I mean this UIBackgroundTaskIdentifier has to “expired” after 10 minutes. And this is really happened (“Task expired” in log)! But why NSTimer continue run? o_O
Regards!

Answer 1

From the documentation for beginBackgroundTaskWithExpirationHandler:

A handler to be called shortly before the application’s remaining background time reaches 0. You should use this handler to clean up and mark the end of the background task. Failure to end the task explicitly will result in the termination of the application. The handler is called synchronously on the main thread, thus blocking the application’s suspension momentarily while the application is notified.

I.e. this is just a notification that the task will expire, it is still your responsibility to close the task (and if you don’t then they system will just kill it after a while).