iOS invert mask in drawRect

Question

With the code below, I am successfully masking part of my drawing, but it\'s the inverse of what I want masked. This masks the inner portion of the drawing, where I would li

Answer 1

For Swift 4.2

func mask(viewToMask: UIView, maskRect: CGRect, invert: Bool = false) {
    let maskLayer = CAShapeLayer()
    let path = CGMutablePath()
    if (invert) {
        path.addRect(viewToMask.bounds)
    }
    path.addRect(maskRect)

    maskLayer.path = path
    if (invert) {
        maskLayer.fillRule = .evenOdd
    }

    // Set the mask of the view.
    viewToMask.layer.mask = maskLayer;
}

Answer 2

With even odd filling on the shape layer (maskLayer.fillRule = kCAFillRuleEvenOdd;) you can add a big rectangle that covers the entire frame and then add the shape you are masking out. This will in effect invert the mask.

CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
CGMutablePathRef maskPath = CGPathCreateMutable();
CGPathAddRect(maskPath, NULL, someBigRectangle); // this line is new
CGPathAddPath(maskPath, nil, myPath.CGPath);
[maskLayer setPath:maskPath];
maskLayer.fillRule = kCAFillRuleEvenOdd;         // this line is new
CGPathRelease(maskPath);
self.layer.mask = maskLayer;

Answer 3

Swift 5

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        let red = UIView(frame: view.bounds)
        view.addSubview(red)
        view.backgroundColor = UIColor.cyan
        red.backgroundColor = UIColor.red
        red.mask(CGRect(x: 50, y: 50, width: 50, height: 50), invert: true)
    }
}

extension UIView{

    func mask(_ rect: CGRect, invert: Bool = false) {
        let maskLayer = CAShapeLayer()
        let path = CGMutablePath()
        if (invert) {
            path.addRect(bounds)
        }
        path.addRect(rect)
        maskLayer.path = path
        if (invert) {
            maskLayer.fillRule = CAShapeLayerFillRule.evenOdd
        }
        // Set the mask of the view.
        layer.mask = maskLayer
    }
}

Thanks @arvidurs

Answer 4

In order to invert mask you can to something like this

1. Here I have mask of crossed rectancles like

    let crossPath =  UIBezierPath(rect: cutout.insetBy(dx: 30, dy: -5))
             crossPath.append(UIBezierPath(rect: cutout.insetBy(dx: -5, dy: 30)))
               let crossMask = CAShapeLayer()
               crossMask.path = crossPath.cgPath
               crossMask.backgroundColor = UIColor.clear.cgColor
               crossMask.fillRule = .evenOdd
   

2. And here I add 3rd rectancle around my crossed rectancles so using .evenOdd it takes area that is equal to (New Rectancle - Old Crossed Rectangle) so in other words outside area of crossed rectancles

     let crossPath = UIBezierPath(rect: cutout.insetBy(dx: -5, dy: -5) )
       crossPath.append(UIBezierPath(rect: cutout.insetBy(dx: 30, dy: -5)))
       crossPath.append(UIBezierPath(rect: cutout.insetBy(dx: -5, dy: 30)))
       let crossMask = CAShapeLayer()
       crossMask.path = crossPath.cgPath
       crossMask.backgroundColor = UIColor.clear.cgColor
       crossMask.fillRule = .evenOdd
   

Answer 5

For Swift 3.0

func mask(viewToMask: UIView, maskRect: CGRect, invert: Bool = false) {
    let maskLayer = CAShapeLayer()
    let path = CGMutablePath()
    if (invert) {
        path.addRect(viewToMask.bounds)
    }
    path.addRect(maskRect)

    maskLayer.path = path
    if (invert) {
        maskLayer.fillRule = kCAFillRuleEvenOdd
    }

    // Set the mask of the view.
    viewToMask.layer.mask = maskLayer;
}

Answer 6

Based on the accepted answer, here's another mashup in Swift. I've made it into a function and made the invert optional

class func mask(viewToMask: UIView, maskRect: CGRect, invert: Bool = false) {
    let maskLayer = CAShapeLayer()
    let path = CGPathCreateMutable()
    if (invert) {
        CGPathAddRect(path, nil, viewToMask.bounds)
    }
    CGPathAddRect(path, nil, maskRect)

    maskLayer.path = path
    if (invert) {
        maskLayer.fillRule = kCAFillRuleEvenOdd
    }

    // Set the mask of the view.
    viewToMask.layer.mask = maskLayer;
}