属性

1. startPoint和endPoint
   决定渐变方向，以单位坐标系定义。左上角{0,0}，右下角{1,1}
2. colors
   渐变的颜色，是一个CGColorRef的数组。
3. locations
   定义了渲染的每个颜色的位置。取值在0到1之间。长度一定和colors相同。

例子

1. 多重渐变

       var gradientLayer = CAGradientLayer.init()
   
       gradientLayer.colors = [UIColor.init(red: 0, green: 0, blue: 0, alpha: 0.6).CGColor,UIColor.init(red: 0, green: 0, blue: 0, alpha: 0.9).CGColor,UIColor.init(white: 0.4, alpha: 0.5).CGColor]
   
       gradientLayer.colors = [UIColor.blackColor().CGColor,UIColor.yellowColor().CGColor,UIColor.blueColor().CGColor]
   
       gradientLayer.locations = [0.1,0.3,0.5]
   
       gradientLayer.frame = textLayer.bounds
   
       gradientLayer.startPoint = CGPointMake(1, 0)
   
       gradientLayer.endPoint = CGPointMake(1, 1)
   
   //        textLayer.mask = gradientLayer
   
       self.contentView.layer.addSublayer(gradientLayer)
   

   

2. 作为蒙版，实现文字渐变

       var gradientLayer = CAGradientLayer.init()
   
       gradientLayer.colors = [UIColor.init(red: 0, green: 0, blue: 0, alpha: 0.6).CGColor,UIColor.init(red: 0, green: 0, blue: 0, alpha: 0.9).CGColor,UIColor.init(white: 0.4, alpha: 0.5).CGColor]
   
   //        gradientLayer.colors = [UIColor.blackColor().CGColor,UIColor.yellowColor().CGColor,UIColor.blueColor().CGColor]
   
       gradientLayer.locations = [0.1,0.3,0.5]
   
       gradientLayer.frame = textLayer.bounds
   
       gradientLayer.startPoint = CGPointMake(1, 0)
   
       gradientLayer.endPoint = CGPointMake(1, 1)
   
       textLayer.mask = gradientLayer