Question

I have an app that is slowing down the entire OS after using it for about 30-35 minutes. The lag is gradual and I can see it builds up over time as I repeat the same operation over and over again. It’s a music app and after streaming about 15-20 tracks, the lag is unbearable.

While scrolling items in a UIScrollView, the frame rate drops below 10. A lot of frames are skipped and the UI almost locks up. If I background the app, I see this lag in the OS in the SpringBoard and basically everywhere. Scrolling the app icons in SpringBoard becomes choppy. The slide to unlock becomes choppy, etc.

How would I go about solving this issue? What could be the probably causes. I cannot cut down the code to create a minimal reproducible example as the codebase is fairly complex. I need help with understanding what could cause the OS to almost lock up. It’s not a deadlock as the UI still responds but just takes an awful long time.

What profiling tools could help shine a light on the cause of this problem? I’m suspecting it may be because of a memory leak, but surprisingly the OS hasn’t sent the app a memory warning, so I’m not entirely sure about that either.

Any help is greatly appreciated.

Answer 1

The problem was with a build up of animations. Activity Monitor was most useful in debugging this issue. The CPU usage of Springboard kept rising up until it shot up to 100%. So the time was clearly not being spent in my application, but in the render server residing in Springboard.

I created two animations with a huge repeat count to let them run forever. Then I added each animation to a separate layer. To create the animation, I used a lazy check, and asked the layer for any existing animations with the given key. If the layer returned nothing, I created the animation. The problem was that the layer always returned nothing, so I kept creating these forever repeating animations.

This was the problematic code.

// This call always returned nil.
CABasicAnimation *innerRotationAnimation = (CABasicAnimation *)[self.spinnerViewInner.layer animationForKey:@"rotationAnimation"];

// So I kept on creating animations and piling them up.
if (innerRotationAnimation == nil)
{
    CATransform3D innerRotationTransform = CATransform3DMakeRotation(0.25f * M_PI * -1, 0, 0, 1.0);
    innerRotationAnimation = [CABasicAnimation animationWithKeyPath:@"transform"];
    innerRotationAnimation.toValue = [NSValue valueWithCATransform3D:innerRotationTransform];
    innerRotationAnimation.duration = 0.25f;
    innerRotationAnimation.cumulative = YES;
    innerRotationAnimation.repeatCount = HUGE_VALF;
    [self.spinnerViewInner.layer addAnimation:innerRotationAnimation forKey:@"rotationAnimation"];
}

To fix the issue, I started removing the existing animation. I could have done this at two points, either in the animationDidStop:finished: callback, or in my setAnimating: method, and both worked fine. Here’s the change in the setAnimating: method.

- (void)setAnimating:(BOOL)animating
{
    if (animating == NO) {
        // Remove all existing animations now.
        [self.layer removeAllAnimations];
    }
    else {
        CABasicAnimation *animation = // Create animation;
        [self.layer addAnimation:animation forKey:@"rotationAnimation"];
    }
}

Here’s the original BROKEN code if anyone’s interested.

- (void)setAnimating:(BOOL)animating
{
    if (self.isAnimating == animating)
    {
        return;
    }

    _animating = animating;

    if (self.isAnimating == YES)
    {
        CABasicAnimation *innerRotationAnimation = (CABasicAnimation *)[self.spinnerViewInner.layer animationForKey:@"rotationAnimation"];
        CABasicAnimation *outerRotationAnimation = (CABasicAnimation *)[self.spinnerViewOuter.layer animationForKey:@"rotationAnimation"];

        if (innerRotationAnimation == nil)
        {
            CATransform3D innerRotationTransform = CATransform3DMakeRotation(0.25f * M_PI * -1, 0, 0, 1.0);
            innerRotationAnimation = [CABasicAnimation animationWithKeyPath:@"transform"];
            innerRotationAnimation.toValue = [NSValue valueWithCATransform3D:innerRotationTransform];
            innerRotationAnimation.duration = 0.25f;
            innerRotationAnimation.cumulative = YES;
            innerRotationAnimation.repeatCount = HUGE_VALF;
            [self.spinnerViewInner.layer addAnimation:innerRotationAnimation forKey:@"rotationAnimation"];
        }
        if (outerRotationAnimation == nil)
        {
            CATransform3D outerRotationTransform = CATransform3DMakeRotation(0.25f * M_PI * -1, 0, 0, -1.0);
            outerRotationAnimation = [CABasicAnimation animationWithKeyPath:@"transform"];
            outerRotationAnimation.toValue = [NSValue valueWithCATransform3D:outerRotationTransform];
            outerRotationAnimation.duration = 0.25f;
            outerRotationAnimation.cumulative = YES;
            outerRotationAnimation.repeatCount = HUGE_VALF;
            [self.spinnerViewOuter.layer addAnimation:outerRotationAnimation forKey:@"rotationAnimation"];
        }
    }
}

- (void)animationDidStop:(CAAnimation *)theAnimation finished:(BOOL)flag
{
    self.spinnerViewInner.layer.opacity = (self.isAnimating ? 1.0 : 0.0);
    self.spinnerViewOuter.layer.opacity = (self.isAnimating ? 1.0 : 0.0);
}

There is one thing I’m still curious about though. Since there can only be one active animation for a given key, shouldn’t Core Animation have removed my existing animation when I tried to add a new one with the same key? And why did animationForKey: return nil at all.