swift：设置view的背景渐变色

//定义渐变的颜色     func returnBgview(bgview: UIView, isTopView: Bool) -> UIView {

        var alphaStart = 0.0

        var alphaEnd = 0.5

          if isTopView {

                alphaStart = 0.5

                alphaEnd = 0.0

            }                 

            let topColor = UIColor(red: 0/255, green: 0/255, blue: 0/255, alpha: CGFloat(alphaStart))

            let buttomColor = UIColor(red: 0/255, green: 0/255, blue: 0/255, alpha: CGFloat(alphaEnd))

            let gradientColors = [topColor.cgColor, buttomColor.cgColor]

            //定义每种颜色所在的位置

            let gradientLocations:[NSNumber] = [0.0, 1.0]

            //创建CAGradientLayer对象并设置参数

            let gradientLayer = CAGradientLayer()

             gradientLayer.colors = gradientColors
             gradientLayer.locations = gradientLocations

             //设置其CAGradientLayer对象的frame，并插入view的layer

            gradientLayer.frame = bgview.bounds

            bgview.layer.insertSublayer(gradientLayer, at: 0)

            return bgview

        }