Swift: rainbow colour circle

Hi i am trying to write colour picker in swift that looks like this.

But so far I managed this.

Draw circle was easy, heres code...

 fileprivate func setupScene(){

    let circlePath: UIBezierPath = UIBezierPath(arcCenter: CGPoint(x: self.wheelView.frame.width/2, y: self.wheelView.frame.height/2), radius: CGFloat(self.wheelView.frame.height/2), startAngle: CGFloat(0), endAngle:CGFloat(Double.pi * 2), clockwise: true)

    let shapeLayer = CAShapeLayer()
    shapeLayer.path = circlePath.cgPath

    //color inside circle
    shapeLayer.fillColor = UIColor.clear.cgColor
    //colored border of circle
    shapeLayer.strokeColor = UIColor.purple.cgColor
    //width size of border
    shapeLayer.lineWidth = 10

    wheelView.layer.addSublayer(shapeLayer)
}

@IBOutlet var wheelView: UIView!

But now I don't know how to insert rainbow colours ... I tried CAGradientLayer but it was not visible. Any good advice?