效果如图：

实现效果如图

1、UI布局

1.1进入LaunchScreen.storyboard中设置背景色为蓝色
1.2拖入一个ImageView，设置大小100 81，并且垂直水平居中

效果如图

2、在AppDelegate.swift中实现效果

思路解析：

从效果上看是有一张背景图片，还有一张前景图片（遮罩效果），然后前景图片不断扩大，然后在取消遮罩效果。
所以需要一个背景图，一个前景图，一个动画扩大的效果
OK代码实现

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2.1创建背景图

//懒加载图片
    private lazy var imageView : UIImageView = {
       let iv = UIImageView(frame: UIScreen.mainScreen().bounds)
        iv.image = UIImage(named: "7501334")
        return iv
    }()

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2.2创建遮罩

//懒加载Layer
    private lazy var imageViewMask : CALayer = {
        let layer = CALayer()
        layer.contents = UIImage(named: "twitter logo mask")?.CGImage
        layer.bounds = CGRect(x: 0, y: 0, width: 100, height: 81)
        layer.anchorPoint = CGPoint(x: 0.5, y: 0.5)
        layer.contentsGravity = kCAGravityResizeAspect
        return layer
    }()

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2.3在didFinishLaunchingWithOptions方法中，创建Window，并设置好属性，以及开始动画

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
        
        //创建window
        window = UIWindow(frame: UIScreen.mainScreen().bounds)
        //设置window根视图
        window?.rootViewController = UIViewController()
        //添加图片
        window?.addSubview(imageView)
        //设置遮罩
        imageView.layer.mask = imageViewMask
        //设置遮罩居中
        imageViewMask.position = (window?.center)!
        //开始动画
        startAnimationMask()
        //设置背景颜色
        window?.backgroundColor = UIColor(red:0.117, green:0.631, blue:0.949, alpha:1)
        //设置为可见
        window?.makeKeyAndVisible()
        
        return true
    }

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2.4实现动画代码，使用关键帧动画CAKeyframeAnimation

private func startAnimationMask(){
        //关键帧动画
        //1、创建keyFrameAnimation
        let keyFrameAnimation = CAKeyframeAnimation(keyPath: "bounds")
        //2、设置时长
        keyFrameAnimation.duration = 0.6
        //3、设置代理
        keyFrameAnimation.delegate = self
        //3.1设置开始时间
        keyFrameAnimation.beginTime = CACurrentMediaTime() + 0.5
        //3.2设置动画模式
        keyFrameAnimation.timingFunctions = [CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut), CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)]
        //4、设置帧
        let key1 = NSValue(CGRect: imageViewMask.bounds)
        let key2 = NSValue(CGRect: CGRect(x: 0, y: 0, width: 80, height: 65))
        let key3 = NSValue(CGRect: CGRect(x: 0, y: 0, width: 1600, height:  1300))
        keyFrameAnimation.values = [key1,key2,key3]
        //4.1、设置帧动画时长
        keyFrameAnimation.keyTimes = [0,0.3,1]
        //5、添加到mask上
        imageViewMask.addAnimation(keyFrameAnimation, forKey: "bounds")
    }

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2.5在动画结束后，我们要把遮罩层移除

//动画结束后，把遮罩删除
    override func animationDidStop(anim: CAAnimation, finished flag: Bool) {
        imageView.layer.mask = nil
    }

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到这里就完成效果了，你们可以修改动画方式来完成更多的效果
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Demo - ImageViewMask