Erratic behavior with NSTimers and NSRunLoops

Question

I am attempting to have a UIImageView (a picture of a finger) animate from right to left, using CGAffineTransformMakeTranslation(). This animation will repeat until the user performs a swipe, moving the user along in a tutorial. All of this is already working swimmingly as follows:

[UIView animateWithDuration:1.0 animations:^ {
    self.finger.alpha = 1.0; 
}completion:^(BOOL finished) {
    CGAffineTransform originalTransform = self.finger.transform;
    [UIView animateWithDuration:1.0 delay:0.0 options:UIViewAnimationCurveEaseInOut animations:^ {

        self.finger.transform = CGAffineTransformMakeTranslation(-200, 0);
        self.finger.alpha = 0.0;

        }completion:^(BOOL finished) {

            self.finger.transform = originalTransform;
            NSTimer *timer = [NSTimer timerWithTimeInterval:1.0 target:self selector:@selector(repeatSwipeAnimation1) userInfo:nil repeats:YES];
            self.swipeTimerForFinger1 = timer;
            NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
            [runLoop addTimer:self.swipeTimerForFinger1 forMode:NSDefaultRunLoopMode];
            [self.swipeTimerForFinger1 fire];


            }];

        }];

And the selector:

-(void)repeatSwipeAnimation1 {
    [UIView animateWithDuration:1.0 delay:0.0 options:UIViewAnimationCurveEaseIn animations:^ {
    self.finger.alpha = 1.0;
}completion:^(BOOL finished) {
    CGAffineTransform originalTransform = self.finger.transform;
    [UIView animateWithDuration:1.0 delay:0.0 options:UIViewAnimationCurveEaseInOut animations:^ {
        self.finger.transform = CGAffineTransformMakeTranslation(-200, 0);
        self.finger.alpha = 0.0;
    }completion:^(BOOL finished) {
        self.finger.transform = originalTransform;

    }];
}];
}

The finger animates and translates beautifully.

The issue comes when I want to do this with a different finger with a different timer. I have the same exact code, however it is a different finger and different selector for the timer.

What happens is that the timer's selector will not translate the UIImageView, and (more scary) the timer will not invalidate when I call the method to invalidate. Upon debugging, I see that the the 2nd timer is calling the 2nd selector, but just not behaving (e.g. not translating, and fading in the 2nd finger too rapidly).

What I am assuming is that I need to somehow turn off the NSRunLoop when I first call it? This is the first time working with NSRunLoop so I apologize for my ignorance. Any help is very much appricated.

score 1 · Accepted Answer · answered Jun 21 '12 at 01:15

1

Well, you certainly have a block retain cycle going on. You need to use either the __block or __weak specifier. See Blocks and Variables. Your problem may be related to a memory issue.

Make sure you invalidate the first timer when it is done.

For safety, you may also want to reset the transform on your UIImageView before attempting to transform it. You can do that like so:

finger.transform = CGAffineTransformIdentity;

answered Jun 21 '12 at 01:15

Michael Frederick

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Thank you for your advice Michael! I definitely do invalidate the first timer, and really excited for the resting of the transform. Would it be too much trouble for you to explain the "block retain cycle"? – Maximilian Jun 21 '12 at 03:19
1

the self object retains the block and when you reference self within the block then the block also retains self and when two objects retain each other like that you end up with a retain cycle and neither object will ever get released. see http://stackoverflow.com/questions/7853915/how-do-i-avoid-capturing-self-in-blocks-when-implementing-an-api – Michael Frederick Jun 21 '12 at 05:05
Michael, so incredibly helpful. I totally get it now, thank you so much! I am going to change some of these long animation chains and see if I can get out of this cycle. – Maximilian Jun 21 '12 at 05:52

Erratic behavior with NSTimers and NSRunLoops

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