ios 渐变色怎么写？

项目有个需求，就给给一个进度圈添加渐变色，小白完全不知道怎么实现，请教下，下图是视觉效果

实现上面的那个圈圈外侧的渐变色进度圈，如何取出渐变色彩（比如红黄渐变 、黄白渐变这样）呢？

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自定义View如下

private let kProgressWidth: CGFloat = 80
private let kProgressLineWidth: CGFloat = 10
class ColorRingView: UIView {

    var progressLayer: CAShapeLayer
    override init(frame: CGRect) {
        self.progressLayer = CAShapeLayer.init()
        super.init(frame: frame)

        let path = UIBezierPath.init(arcCenter: CGPointMake(frame.size.width / 2, frame.size.height / 2), radius: kProgressWidth, startAngle: degreesToRadians(-90), endAngle: degreesToRadians(90), clockwise: true)

        progressLayer.frame = self.bounds
        progressLayer.fillColor = UIColor.clearColor().CGColor
        progressLayer.strokeColor = UIColor.whiteColor().CGColor
        progressLayer.lineCap = kCALineCapRound
        progressLayer.lineWidth = kProgressLineWidth
        progressLayer.path = path.CGPath
        progressLayer.strokeEnd = 1.0

        let gradientLayer = CALayer.init()
        let mainGradientLayer = CAGradientLayer.init()

        mainGradientLayer.frame = CGRectMake(0, 0, self.bounds.size.width, self.bounds.size.height);
        mainGradientLayer.colors = [UIColor.init(netHex: 0xFF9B55).CGColor, UIColor.whiteColor().CGColor]
        mainGradientLayer.startPoint = CGPointMake(0.5, 0)
        mainGradientLayer.endPoint = CGPointMake(0.5, 1)
        gradientLayer.addSublayer(mainGradientLayer)

        gradientLayer.mask = progressLayer

        self.layer.addSublayer(gradientLayer)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    func degreesToRadians(degress: Double) -> CGFloat {
        return CGFloat(M_PI * (degress) / 180.0)
    }

    func setPercent(percent: CGFloat) {
        CATransaction.begin()
        CATransaction.setDisableActions(true)
        let camtf = CAMediaTimingFunction.init(name: kCAMediaTimingFunctionEaseIn)
        CATransaction.setAnimationTimingFunction(camtf)
        CATransaction.setAnimationDuration(5)
        progressLayer.strokeEnd = percent / 100.0

        CATransaction.commit()
    }

}

extension UIColor {
    convenience init(red: Int, green: Int, blue: Int) {
        assert(red >= 0 && red <= 255, "Invalid red component")
        assert(green >= 0 && green <= 255, "Invalid green component")
        assert(blue >= 0 && blue <= 255, "Invalid blue component")

        self.init(red: CGFloat(red) / 255.0, green: CGFloat(green) / 255.0, blue: CGFloat(blue) / 255.0, alpha: 1.0)
    }

    convenience init(netHex: Int) {
        self.init(red: (netHex >> 16) & 0xff, green: (netHex >> 8) & 0xff, blue: netHex & 0xff)
    }
}

然后在Controller里面使用

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        let ring = ColorRingView.init(frame: CGRectMake(100, 100, 240, 240))
        self.view.addSubview(ring)
    }
}

效果图：

主要知识点就是CAGradientLayer，稍微理解下startPoint、endPoint和locations这几个属性就能掌握使用了。

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用CAGradientLayer显示一个渐变矩形
然后用CAShapeLayer绘制一个圆形mask就好了