iOS Animate Mask Over UIImage

I am using Swift 1.2 and my goal is to animate an image mask over a static UIImage. What I have implemented is a swift version of masking an image that I originally found in Objective-C.

func maskImage(image: UIImage, mask: UIImage) -> UIImage! {

    let maskRef = mask.CGImage;

    let mask = CGImageMaskCreate(
        CGImageGetWidth(maskRef),
        CGImageGetHeight(maskRef),
        CGImageGetBitsPerComponent(maskRef),
        CGImageGetBitsPerPixel(maskRef),
        CGImageGetBytesPerRow(maskRef),
        CGImageGetDataProvider(maskRef), nil, false);

    let masked = CGImageCreateWithMask(image.CGImage, mask);
    let retImage = UIImage(CGImage: masked);
    return retImage;
}

It works great! However, putting it in motion is my challenge.

Is there a way to either iteratively apply the mask with a different horizontal offset or a better way to approach this problem entirely - perhaps with a CALayer implementation?

Thanks for your help!

EDIT: Based on what was posted as an answer, I added this:

    let image = UIImage(named: "clouds");

    let imageView = UIImageView(image: image);
    let layer = CALayer();
    layer.contents = UIImage(named: "alpha-mask")?.CGImage;
    layer.bounds = CGRectMake(0, 0, image.size.width, image.size.height);

    let animation = CABasicAnimation(keyPath: "bounds.origin.x");
    animation.duration = 2;
    animation.delegate = self;
    animation.fillMode = kCAFillModeForwards;
    animation.repeatCount = 0;
    animation.fromValue = 0.0;
    animation.toValue = image.size.width;
    animation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionLinear);
    animation.removedOnCompletion = false;

    layer.addAnimation(animation, forKey: "transform");

    imageView.layer.mask = layer;

    self.addSubview(imageView);

I am able to see the alpha mask properly, but the animation does not work. Any ideas?