mask的用法

• mask是遮罩，它会挡住父视图的渲染
• 父视图渲染部分是mask不透明的像素点; mask透明部分，父视图部分就会不显示

      let frame = CGRect(x: 0, y: 0, width: 300, height: 500)
        let blueView = UIView(frame: frame)
        blueView.backgroundColor = .blue

        let redView = UIImageView(frame: frame)
        redView.image = UIImage(named: "girl")

        let mask = UIImageView(frame: frame)
        mask.contentMode = .scaleAspectFit
        mask.image =  UIImage(named: "light")
        redView.mask = mask

        blueView.addSubview(redView)
        view.backgroundColor = .white
        view.addSubview(blueView)