How can I animate the contents property of a CALayer using an array of UIImages?

I'd like to animate a mask I placed on a UIImageView using the contents property of the CALayer. Unfortunately my current implementation only seems to animate between the first and last images of the array without passing between the transitions. Is there any way of achieving this, or should I be looking at other things like CAReplicatorLayer?

Here's what I currently have:

        let mask = CALayer()
        mask.frame = self.frame
        self.layer.mask = mask
        mask.contents = transitions.first?.cgImage

        let maskAnimation = CABasicAnimation(keyPath: "contents")
        maskAnimation.fromValue = transitions.first?.cgImage
        maskAnimation.toValue = transitions.last?.cgImage
        maskAnimation.duration = duration
        mask.contents = transitions.last
        mask.add(maskAnimation, forKey: "ContentsAnimation")

Solution

The animation system doesn't magically know what it means to interpolate between the first and last images; you have to tell it, explicitly. That is exactly what a keyframe animation is for.

Here's an example of forming such an animation, from a project of mine:

    let anim = CAKeyframeAnimation(keyPath:#keyPath(CALayer.contents))
    anim.values = self.images.map {$0.cgImage!}
    anim.keyTimes = [0.0, 0.25, 0.5, 0.75, 1.0]
    anim.calculationMode = kCAAnimationDiscrete
    anim.duration = 1.5

As you can see, we have an array of images; they are UIImages, so I have to derive their CGImage backing (I assume your transitions is similar). The keyTimes array is used to divide up the duration into frames; I'm doing this division a little oddly because of the nature of my animation, so feel free to fix the times to divide the duration evenly.