使用 CAShapeLayer 作为 CAShapeLayer 的蒙版时的细边框

Question

在 Swift 中，我有两个半透明的圆圈，都是CAShapeLayer。由于它们是半透明的，它们之间的任何重叠都会像这样可见：

相反，我希望它们在视觉上“合并”在一起。我尝试过的解决方案是使用圆 2 作为圆 1 的蒙版，从而消除重叠。

此解决方案通常有效，但我在圈 2 的外侧得到一条细线：

我的问题：我怎样才能摆脱右边圆圈上的细外线？为什么它还在那里？

---

代码如下（Xcode playground can be found here）：

    private let yPosition: CGFloat = 200
    private let circle1Position: CGFloat = 30
    private let circle2Position: CGFloat = 150
    private let circleDiameter: CGFloat = 200
    private var circleRadius: CGFloat { return self.circleDiameter/2.0 }

    override func loadView() {
        let view = UIView()
        view.backgroundColor = .black

        self.view = view

        let circle1Path = UIBezierPath(
            roundedRect: CGRect(
                x: circle1Position,
                y: yPosition,
                width: circleDiameter,
                height: circleDiameter),
            cornerRadius: self.circleDiameter)

        let circle2Path = UIBezierPath(
            roundedRect: CGRect(
                x: circle2Position,
                y: yPosition,
                width: circleDiameter,
                height: circleDiameter),
            cornerRadius: self.circleDiameter)

        let circle1Layer = CAShapeLayer()
        circle1Layer.path = circle1Path.cgPath
        circle1Layer.fillColor = UIColor.white.withAlphaComponent(0.6).cgColor

        let circle2Layer = CAShapeLayer()
        circle2Layer.path = circle2Path.cgPath
        circle2Layer.fillColor = UIColor.white.withAlphaComponent(0.6).cgColor

        self.view.layer.addSublayer(circle1Layer)
        self.view.layer.addSublayer(circle2Layer)

        //Create a mask from the surrounding rectangle of circle1, and
        //then cut out where it overlaps circle2
        let maskPath = UIBezierPath(rect: CGRect(x: circle1Position, y: yPosition, width: circleDiameter, height: circleDiameter))
        maskPath.append(circle2Path)
        maskPath.usesEvenOddFillRule = true
        maskPath.lineWidth = 0

        let maskLayer = CAShapeLayer()
        maskLayer.path = maskPath.cgPath
        maskLayer.fillColor = UIColor.black.cgColor
        maskLayer.fillRule = kCAFillRuleEvenOdd

        circle1Layer.mask = maskLayer
    }

Answer 1

如果两个 CAShapeLayer 具有相同的 alpha 值，您可以将它们放在新的父 CALayer 中，然后设置父级的 alpha。