用三种颜色的渐变绘制3/4的圆

Question

我想用渐变边框绘制3/4的圆圈，如下所示：

我可以用这段代码绘制这个圆圈：

func createProgressLayer() {
     let startAngle = CGFloat(M_PI_2)
     let endAngle = CGFloat(M_PI * 2 )
     let centerPoint = CGPointMake(CGRectGetWidth(frame)/2 , CGRectGetHeight(frame)/2)

     let gradientMaskLayer = gradientMask()
     progressLayer.path = UIBezierPath(arcCenter:centerPoint, radius: CGRectGetWidth(frame)/2 - 30.0, startAngle:startAngle, endAngle:endAngle, clockwise: true).CGPath
     progressLayer.backgroundColor = UIColor.clearColor().CGColor
     progressLayer.fillColor = nil
     progressLayer.strokeColor = UIColor.blackColor().CGColor
     progressLayer.lineWidth = 4.0
     progressLayer.strokeStart = 0.0
     progressLayer.strokeEnd = 0.0    
}

但我想添加渐变效果，请帮助。感谢。