Swift CAKeyframe Animation not displaying images - objective-c

Question

I have set up a very simple Single View Application (in Swift) using iOS 8.1. I have added one UIImageView to the main view controller view. I am trying to use CAKeyframeAnimation to animate a sequence of images. I was originally using the UIImageView animationImages property which worked fine but I need to be able to know precisely when the animation finishes, hence the move to CAKeyframeAnimation.
My code is as follows:
class ViewController: UIViewController {
#IBOutlet weak var imageView: UIImageView!
var animation : CAKeyframeAnimation!
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
let animationImages:[AnyObject] = [UIImage(named: "image-1")!, UIImage(named: "image-2")!, UIImage(named: "image-3")!, UIImage(named: "image-4")!]
animation = CAKeyframeAnimation(keyPath: "contents")
animation.calculationMode = kCAAnimationDiscrete
animation.duration = 25
animation.values = animationImages
animation.repeatCount = 25
animation.removedOnCompletion = false
animation.fillMode = kCAFillModeForwards
self.imageView.layer.addAnimation(animation, forKey: "contents")
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}
The problem is that the animation does not display any images and I just receive a blank screen. Is there something I am missing in the above code? How can I get the animation to display?

Answer 1

This line is never going to work:
animation.values = animationImages
Change it to:
animation.values = animationImages.map {$0.CGImage as AnyObject}
The reason is that you are trying to animate the "contents" key of this layer. But that is the contents property. But the contents property must be set to a CGImage, not a UIImage. Your animationImages, by contrast, contains UIImages, not CGImages.
Thus you need to transform your array of UIImage into an array of CGImage. Moreover, you're trying to pass this array to Objective-C, where an NSArray must contain objects only; and since CGImage is not an object in Objective-C's mind, you need to cast each of them as an AnyObject. And that is what my map call does.