Animating a custom property of a CALayer subclass

custom CALayer property animation

@implementation PXScrubberView

+ (Class)layerClass
{
        return [PXScrubberLayer class];
}

- (void)setFillHeight:(float)fillHeight animated:(BOOL)animated
{
        if (animated) {
		CABasicAnimation* fillHeightAnimation = [CABasicAnimation animationWithKeyPath:@"fillHeight"];
		NSParameterAssert(fillHeightAnimation);
		fillHeightAnimation.toValue = @(fillHeight);
		fillHeightAnimation.fromValue = @(self.scrubberLayer.fillHeight);
		fillHeightAnimation.duration = [[CATransaction valueForKey:kCATransactionAnimationDuration] doubleValue];
		fillHeightAnimation.fillMode = kCAFillModeForwards;
		fillHeightAnimation.removedOnCompletion = YES;
		fillHeightAnimation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseIn];
		NSLog(@"Adding animation from %@ to %@", fillHeightAnimation.fromValue, fillHeightAnimation.toValue);
		[self.layer addAnimation:fillHeightAnimation forKey:@"fillHeight"];
	}

	self.scrubberLayer.fillHeight = fillHeight;
}

... 

@end

@implementation PXScrubberLayer

- (void)setFillHeight:(float)fillHeight
{
    if (_fillHeight != fillHeight) {
        _fillHeight = fillHeight;
        	NSLog(@"Set fill height: %f", _fillHeight);
        [self setNeedsDisplay];
    }
}

- (void)setPositionValue:(float)positionValue
{
	if (MaybeSetClampedValue(&_positionValue, positionValue)) {
		[self setNeedsDisplay];
	}
}

+ (BOOL)needsDisplayForKey:(NSString *)key
{
        static NSArray* customAnimatedKeys = nil;
	if (!customAnimatedKeys) {
		customAnimatedKeys = @[NSStringFromSelector(@selector(availableColor)),
							   NSStringFromSelector(@selector(emptyColor)),
							   NSStringFromSelector(@selector(positionColor)),
							   NSStringFromSelector(@selector(availableStart)),
							   NSStringFromSelector(@selector(availableEnd)),
							   NSStringFromSelector(@selector(positionValue)),
							   NSStringFromSelector(@selector(fillHeight))];
	}
	if ([customAnimatedKeys containsObject:key]) {
		return YES;
	} else {
		return [super needsDisplayForKey:key];
	}
}

...

@end