1. 懒加载渐变涂层

private lazy var subLayer: CAGradientLayer = {
        let layer = CAGradientLayer()
        // 设置从什么颜色开始
        let fromColor = UIColor.red
        // 设置以什么颜色结束
        let toColor = UIColor.black
        let colors = [fromColor, toColor].map { $0.cgColor }
        //设置开始和结束位置(设置渐变的方向)
        layer.startPoint = CGPoint(x: 0, y: 0);
        layer.endPoint = CGPoint(x: 0, y: 1);
        layer.colors = colors;
        return layer
    }()

2. layer添加渐变涂层

self.layer.addSublayer(subLayer)

3. 设置渐变涂层frame

override func layoutSubviews() {
        super.layoutSubviews()
        subLayer.frame = CGRect(x: 0, y: 0, width: self.width, height: self.height);
        
}