Unreliable execution from CABasicAnimation - cocoa

Question

I'm trying to implement the animation of a custom property on a CALayer. I've set up the property as dynamic and it mostly works. In my app, I might have a dozen or more instances of this layer, all of which should animate at the same time. However, sometimes when the animation is triggered, a few of the layers don't perform the animation. They simple jump directly to the final value.
In trying to debug the problem, it looks like when the animations start, the layers with the problem get a redraw on their model layer with the final value of the animated property. The rest of the layers only get redraws on their presentation layers with the transient values of the animated property. It's usually the last of the layers that has the problem, but sometimes it's random layers.
It's so sporadic, I'm not really sure where to look next.
Here's the snippet of code that triggers the animation:
#dynamic aspectModeFraction;
- (id<CAAction>)actionForKey:(NSString *)key
{
if ([key isEqualToString:#"aspectModeFraction"])
{
CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:key];
animation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseOut];
animation.fromValue = #([[self presentationLayer] aspectModeFraction]);
animation.duration = 2.0f;
animation.delegate = self;
return animation;
}
return [super actionForKey:key];
}
+ (BOOL)needsDisplayForKey:(NSString *)key {
if ([#"fullRect" isEqualToString:key]) { return YES; }
if ([#"localRect" isEqualToString:key]) { return YES; }
if ([#"screens" isEqualToString:key]) { return YES; }
if ([#"aspectModeFraction" isEqualToString:key]) { return YES; }
return [super needsDisplayForKey:key];
}

Answer 1

It looks like you have most of the parts. There are lots of little parts in custom property animation, and this talk may still be helpful, it it kind of looks like you probably have the major things in place.
I'd first suspect the calling code. I'd make sure you're not suppressing animations for instance, or modifying any UIKit pieces off the main thread.
It's worth noting that the trick of using #dynamic to get your layer properties copied may not be officially supported. (It's one of those things that "seems to work" rather than coming out of any doc I know; and when I've spoken with some Apple engineers about it, they'd never heard of doing it that way.) If you're seeing trouble only on 10.9+, you might try replacing your use of #dynamic with an standard property that you copy over in initWithLayer:.

Answer 2

Thanks for the suggestions. I eventually tracked down the problem. I had optimized the drawing code to only do some calculations when this parameter changed. I was marking the calculations as dirty when the parameter was updated so they would be recalculated during the next draw loop. The dirty flag was set via a custom setter on the parameter. When I switched to #dynamic (which seems to be required for CAAnimations?) the custom setter had to go away, so I was updating the dirty flag by having the object do KVO on itself. It turns out that observeValueForKeyPath:... wasn't being called during the animation. It got one update for the final value, but no intermediates.
Anyway, dropping the optimization and doing the full calculations each time resolved the issue. KVO on the animated value not working as expected seemed to be the culprit.