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.

is the code shown in `periodicTimer` above really all that happens periodically? you just need to update a label that displays a time? or is there more involved background processing? — Nate, May 26 '12 at 10:26
It is all that happens periodically (NSLog was just a test to see if the method at least gets called) — Fr4ncis, May 26 '12 at 11:22

score 27 · Accepted Answer · edited May 23 '17 at 11:53

27

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

edited May 23 '17 at 11:53

Community

1
1

answered May 26 '12 at 11:58

Joris Kluivers

11,894
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Yes, this is the right way to handle this problem. I started prototyping my solutions before the poster replied that his periodicTimer function was not a simplification for the sake of asking the question, but that he truly didn't have anything going on besides changing a text label. So, no need to use background work as the UI update is all there is. +1 – Nate May 26 '12 at 12:11
6

FYI - NSRunLoopCommonModes includes UITrackRunLoopMode as of iOS 7 – Matt S. Nov 04 '13 at 19:41

score -2 · Answer 2 · answered May 26 '12 at 10:02

-2

Not sure about this one, but my first guess would be that the main thread on which the interface is being rendered your timer just doesn't get a chance to do anything while its updating the interface.

You could create a new thread with a new run loop for your timer, but that is a bit of an ugly solution maybe. What functionality in your app are you trying to achieve? Maybe we can advise a better strategy than using a timer.

answered May 26 '12 at 10:02

Antwan van Houdt

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The functionality is the timer itself. I think it's a problem about NSTimer not being called while the main thread is taking care of the UI scrolling. – Fr4ncis May 26 '12 at 11:24

NSTimer periodic task doesn't get called while scrolling

2 Answers2