如何在Swift中使用另一个UIBezierPath剪辑UIBezierPath？

Question

我正在基于Swift中的这篇博客文章http://labs.byhook.com/2011/09/26/vector-metaballs/中的第三个示例构建一个元球系统。当我拖动中间元球时，它可能会重叠右边或左边的元球（它们是静止的）。我想用“目标”剪辑固定的元球。元球（被拖的那个）。这是我简化的Metaball类

class Metaball: UIView {
    let circlePathLayer = CAShapeLayer()

    override init(frame: CGRect) {
        super.init(frame: frame)
        configure()
    }

    func configure() {
        circlePathLayer.frame = bounds
        circlePathLayer.lineWidth = 10
        circlePathLayer.strokeColor = UIColor.blueColor().CGColor
        layer.addSublayer(circlePathLayer)
    }

    func circleFrame() -> CGRect {
        let circleFrame = CGRect(x: 0, y: 0, width: 2*circleRadius, height: 2*circleRadius)
        return circleFrame
    }

    func circlePath() -> UIBezierPath {
        return UIBezierPath(ovalInRect: circleFrame())
    }

    override func layoutSubviews() {
        super.layoutSubviews()
        circlePathLayer.frame = bounds
        circlePathLayer.path = circlePath().CGPath
    }
}

我试图在touchesEnded中的GameScene类中进行剪切方法。这是touchesEnded方法：

override func touchesEnded(touches: Set<UITouch>, withEvent event: UIEvent?) {
    touching = false
    let touch = touches.first
    let touchLocation = touch!.locationInView(self)
    target.center = touchLocation
    var distPoints = dist(leftMetaball.center, pos2: touchLocation)
    // if the distance between touchPoint and the center of the leftmetaball is less than the radius of a metaball x2 (200) they must be overlapping
    if distPoints < 200 {
        // clip leftMetaball path here
    }
}

这就是现在它们相交时的样子：

我希望两个相交的圆形成一个奇数形状的圆，其中一个贝塞尔曲线路夹住另一个以移除相交线。