Swift - 如何创建带有裁剪形状的视图

我正在尝试使用 swift 1.2 和 xcode 6 来实现图像中显示的结果。

基本上我想创建一个带有切入形状的视图，以便能够看到下面的视图，为我的应用程序制作教程。 我知道如何创建圆形形状，但不知道如何在视图中将其剪掉。 我需要一个完整的例子来说明如何做到这一点。 提前致谢

尽管有答案，我还是想分享我的方法：

// Let's say that you have an outlet to the image view called imageView
// Create the white view 
let whiteView = UIView(frame: imageView.bounds) 
let maskLayer = CAShapeLayer() //create the mask layer

// Set the radius to 1/3 of the screen width
let radius : CGFloat = imageView.bounds.width/3 

// Create a path with the rectangle in it.
let path = UIBezierPath(rect: imageView.bounds)
// Put a circle path in the middle
path.addArcWithCenter(imageView.center, radius: radius, startAngle: 0.0, endAngle: CGFloat(2*M_PI), clockwise: true)

// Give the mask layer the path you just draw
maskLayer.path = path.CGPath
// Fill rule set to exclude intersected paths
maskLayer.fillRule = kCAFillRuleEvenOdd

// By now the mask is a rectangle with a circle cut out of it. Set the mask to the view and clip.
whiteView.layer.mask = maskLayer
whiteView.clipsToBounds = true

whiteView.alpha = 0.8
whiteView.backgroundColor = UIColor.whiteColor()

//If you are in a VC add to the VC's view (over the image)
view.addSubview(whiteView)    
// Annnnnd you're done.