On one of my projects I discovered a bug in a never-ending animation I had set up. Whenever the app was suspended (such as when you multitask and open another app), on relaunching the app the animation was frozen. After some investigating, I discovered that with Core Animation you need to set a flag on the CABasicAnimation class called removedOnCompletion to NO otherwise the animation will get cleaned up when it gets suspended.

Objective-C:

CABasicAnimation *dartWiggleAnimation = [CABasicAnimation animationWithKeyPath:@"position"];
dartWiggleAnimation.fromValue =
   [NSValue valueWithCGPoint:CGPointMake(self.center.x-DART_WIGGLE_HALF, self.center.y+DART_CENTER_H_OFFSET)];
dartWiggleAnimation.toValue =
   [NSValue valueWithCGPoint:CGPointMake(self.center.x+DART_WIGGLE_HALF, self.center.y+DART_CENTER_H_OFFSET)];
dartWiggleAnimation.timingFunction =
   [CAMediaTimingFunction functionWithName: kCAMediaTimingFunctionEaseInEaseOut]; 
dartWiggleAnimation.duration = 0.4;
dartWiggleAnimation.repeatCount = HUGE_VALF;
dartWiggleAnimation.autoreverses = YES;
dartWiggleAnimation.removedOnCompletion = NO;
[[nextDart layer] addAnimation:dartWiggleAnimation forKey:@"dartWiggle"];

Swift:

        let dartWiggleAnimation = CABasicAnimation(keyPath: #keyPath(CALayer.position))
        dartWiggleAnimation.fromValue = CGPoint(x: self.center.x - DART_WIGGLE_HALF, y: self.center.y + DART_CENTER_H_OFFSET)
        dartWiggleAnimation.toValue = CGPoint(x: self.center.x + DART_WIGGLE_HALF, y: self.center.y + DART_CENTER_H_OFFSET)
        dartWiggleAnimation.timingFunction = CAMediaTimingFunction(name: CAMediaTimingFunctionName.easeInEaseOut)
        dartWiggleAnimation.duration = 0.4
        dartWiggleAnimation.repeatCount = Float.greatestFiniteMagnitude
        dartWiggleAnimation.autoreverses = true
        dartWiggleAnimation.isRemovedOnCompletion = false
        nextDart.layer.add(dartWiggleAnimation, forKey: "dartWiggle")

Does this seem like a bug or a feature?