有没有一种方法可以在Swift中使用CAShapeLayer.compositingFilters合成遮罩？

Question

我正在为iPad应用实现一个绘图功能。我使用UIBezierPaths在CAShapeLayers上进行绘制。通过为每个TouchesBegan事件创建一个新的CAShapeLayer，我正在构建一个堆叠的CAShapeLayer数组，以便通过弹出和推送图层到数组中轻松实现撤消和重做。我还使用一些CAShapeLayer.compositingFilters进行一些有趣的图层混合技术。这一切都非常顺利。我的挑战是擦除。

我试图创建第二个CAShapeLayers数组并使用它们来遮罩第一组。当使用不透明颜色进行绘制时，我能够将其添加到遮罩组中，但我无法从遮罩组中删除不透明区域。

我认为我应该能够从一个不透明的基础层开始使用遮罩技术（黑色、白色或其他颜色）。然后，我希望使用UIColor.clear.cgColor绘制UIBezierPaths，并将我的绘制清晰路径与底层的不透明基础遮罩组合或合成。这实际上应该“擦除”遮罩的那个区域，并隐藏我绘制的堆叠的CAShapeLayers。我不想将遮罩层合并成图像，因为这样我将失去通过弹出和推送遮罩数组轻松撤消和重做的能力。

我在下面包含了一些伪代码。任何指针、帮助或解决方案策略将不胜感激！我已经为此工作了几个星期，但还是束手无策。我找不到任何关于我正在努力实现的策略的信息。另外，如果我从一开始就错误地进行绘图功能，并且有一种更容易的方法来绘图，同时保持简单的撤消/重做，并添加擦除，请告诉我。我完全可以调整我的方法！提前感谢您的任何帮助。

// setup the layer hierarchy for drawing    
private func setupView() {
        self.mainDrawLayer = CAShapeLayer()
        self.mainDrawLayer.backgroundColor = UIColor.clear.cgColor
        self.layer.addSublayer(self.mainDrawLayer)

        // set up the mask. add an opaque background so everything shows through to start
        self.maskLayer = CALayer()
        let p = UIBezierPath.init(rect: self.bounds)
        self.maskShapeLayer = CAShapeLayer()
        self.maskShapeLayer?.fillColor = UIColor.black.cgColor
        self.maskShapeLayer?.path = p.cgPath
        self.maskLayer?.addSublayer(self.maskShapeLayer!)

        // apply the mask
        self.layer.mask = self.maskLayer
    }

override public func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        super.touchesBegan(touches, with: event)

        guard let touch = touches.first else {
            return
        }

        var erasing = false


        // setup the currentDrawLayer which captures the bezier path
        self.currentDrawLayer = CAShapeLayer()
        self.currentDrawLayer?.lineCap = CAShapeLayerLineCap.round
        self.currentDrawLayer?.fillColor = nil

        // set the ink color to use for drawing
        if let ink = UserDefaults.standard.string(forKey: "ink") {
            self.currentDrawLayer?.strokeColor = UIColor(hex: ink)?.cgColor
        } else {
            self.currentDrawLayer?.strokeColor = UIColor(hex: Constants.inkBlack3)?.cgColor
        }

        if UserDefaults.standard.string(forKey: "ink") == Constants.inkBlack5 {
            // removing the filter makes white overlay other colors
            // this is essentially erasing with a white background
            self.currentDrawLayer?.compositingFilter = nil
        } else {
            // this blend mode ads a whole different feeling to the drawing!
            self.currentDrawLayer?.compositingFilter = "darkenBlendMode"
        }

        // THIS IS THE ERASER COLOR!
        if UserDefaults.standard.string(forKey: "ink") == Constants.inkBlack4 {
            // trying erasing via drawing with clear
            self.currentDrawLayer?.strokeColor = UIColor.clear.cgColor
            // is there a compositingFilter available to 'combine' my clear color with the black opaque mask layer created above?
            self.currentDrawLayer?.compositingFilter = "iDontHaveADamnClueIfTheresOneThatWillWorkIveTriedThemAllItSeems:)"

            erasing = true

        }

        self.currentDrawLayer?.path = self.mainDrawPath.cgPath

        if erasing {
            // add the layer to the masks
            self.maskLayer!.addSublayer(self.currentDrawLayer!)
        } else {
            // add the layer to the drawings
            self.layer.addSublayer(self.currentDrawLayer!)
        }

        let location = touch.location(in: self)

        self.ctr = 0
        self.pts[0] = location
    }

- eloehfelm

Answer 1

我只能通过将路径附加到遮罩的形状图层来实现类似这样的操作。

        CALayer *penStroke = ... the layer that has all the pen strokes ...;

        UIBezierPath *eraserStroke = [[UIBezierPath bezierPathWithRect:[self bounds]] bezierPathByReversingPath];
        [eraserStroke appendPath:[UIBezierPath bezierPathWithRect:CGRectMake(110, 110, 200, 30)]];

        CAShapeLayer *maskFill = [CAShapeLayer layer];
        maskFill.path = [eraserStroke CGPath];
        maskFill.fillRule = kCAFillRuleEvenOdd;
        maskFill.fillColor = [[UIColor blackColor] CGColor];
        maskFill.backgroundColor = [[UIColor clearColor] CGColor];

        penStroke.mask = maskFill;

以上代码将掩盖所有橡皮擦笔画。但是绘图应用程序希望能够在之前的橡皮擦笔画上进行绘制。我认为这需要通过不断地将现有的penStroke层包装在另一个新的penStroke层中来处理，并始终将penStrokes添加到最外层。

这将得到如下结果：

 - Layer C: most recent pen strokes
   - mask: no mask, so that all strokes are pen strokes
   - Layer B: sublayer of layer C, contains some previous pen strokes
     - Layer B's mask: most recent eraser strokes
     - Layer A: sublayer on Layer B, contains pen strokes before the most recent eraser strokes
       - Layer A's mask: eraser strokes before layer B's pen strokes

希望这样说得通。基本上，它将嵌套层/蒙版。每当用户从笔切换到橡皮擦或橡皮擦切换到笔时，一个新的图层将包裹所有现有的笔/橡皮擦笔画层。

- adam.wulf