objective-c – Swift CAKeyframe动画不显示图像

我使用iOS 8.1设置了一个非常简单的单视图应用程序(在 Swift中).我在主视图控制器视图中添加了一个UIImageView.我正在尝试使用CAKeyframeAnimation来动画一系列图像.我最初使用UIImageView animationImages属性工作正常,但我需要能够准确知道动画何时完成,因此转移到CAKeyframeAnimation.

我的代码如下：

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.
}

}

问题是动画不显示任何图像,我只是收到一个空白屏幕.上面的代码中是否有我遗漏的东西？如何显示动画？

animation.values = animationImages

animation.values = animationImages.map {$0.CGImage as AnyObject}