Swift 渐变遮罩不显示

我试图画一个渐变，在不透明度中淡出

我看不出绘画有什么效果。我试着换颜色。镜框看起来不错。为什么不显示这种不透明度渐变

func createGradient() {
        let h = UIScreen.mainScreen().bounds.size.height
        let y = collectionView.frame.origin.y + (cellSize() * 3)
        let v = UIView(frame: CGRect(x: 0, y: y, width: collectionView.frame.size.width, height: h - y))

        let l = CAGradientLayer()
        l.frame = v.bounds
        l.colors = [UIColor.greenColor().CGColor, UIColor.blueColor().CGColor]

        l.startPoint = CGPoint(x: CGFloat(0.5), y: CGFloat(0.0));
        l.endPoint = CGPoint(x: CGFloat(0.5), y: CGFloat(1.0));
        v.layer.mask = l;
        view.addSubview(v)
    }

我的日志：

(lldb) po l
<CAGradientLayer:0x618000228b00; position = CGPoint (207 101); bounds = CGRect (0 0; 414 202); allowsGroupOpacity = YES; colors = (
    "<CGColor 0x6180000b2c00> [<CGColorSpace 0x6100000376c0> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; sRGB IEC61966-2.1; extended range)] ( 0 1 0 1 )",
    "<CGColor 0x6180000b2c60> [<CGColorSpace 0x6100000376c0> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; sRGB IEC61966-2.1; extended range)] ( 0 0 1 1 )"
)>

(lldb) po v
<UIView: 0x7fe1b3d1d3b0; frame = (0 534; 414 202); layer = <CALayer: 0x618000228880>>

(lldb) 

（lldb）采购订单
（lldb）pov
（lldb）

将

v.layer.mask=l

替换为

v.layer.addSublayer（l）

func createGradient() {
        let h = UIScreen.mainScreen().bounds.size.height
        let y = collectionView.frame.origin.y + (cellSize() * 3)
        let v = UIView(frame: CGRect(x: 0, y: y, width: collectionView.frame.size.width, height: h - y))

        let l = CAGradientLayer()
        l.frame = v.bounds
        l.colors = [UIColor.greenColor().CGColor, UIColor.blueColor().CGColor]

        l.startPoint = CGPoint(x: CGFloat(0.5), y: CGFloat(0.0));
        l.endPoint = CGPoint(x: CGFloat(0.5), y: CGFloat(1.0));
        v.layer.addSublayer(l)
        view.addSubview(v)
    }