NSTimer periodic task doesn't get called while scrolling

Question

I have an NSTimer

timer = [NSTimer scheduledTimerWithTimeInterval:1 
                                         target:self 
                                       selector:@selector(periodicTimer) 
                                       userInfo:nil 
                                        repeats:YES];

which does

- (void)periodicTimer
{
    NSLog(@"Bang!");
    if (timerStart != nil)
        [timerLabel setText:[[NSDate date] timeDifference:timerStart]];        
}

The problem is that while scrolling a tableview (or doing other tasks) the label doesn't get updated, furthermore, "Bang!" doesn't appear, so I supposed the method doesn't get called.

My question is how to update the label periodically even when the user is playing around with the app interface.

Answer 1

You'll need to add your timer to the UITrackingRunLoopMode to make sure your timer also fires during scrolling.

NSRunLoop *runloop = [NSRunLoop currentRunLoop];
NSTimer *timer = [NSTimer timerWithTimeInterval:0.1 target:self selector:@selector(myTimerAction:) userInfo:nil repeats:YES];
[runloop addTimer:timer forMode:NSRunLoopCommonModes];
[runloop addTimer:timer forMode:UITrackingRunLoopMode];

From: https://stackoverflow.com/a/1997018/474896