Crop a CAShapeLayer retrieving the external path

Question

I am trying to crop a layer with other, but, instead of creating a mask (B) and crop the layer (A) getting a cropped layer A with the shape of B, I want to get a layer with

Answer 1

You have to create a mask that covers the area that you want to keep. This can be done by using an even-odd fill rule and create a path for a shape layer with both rectangles. You can create the shape like this (where the two rectangles would be your two frames). Then you set this as the mask to get the result that you are after.

CAShapeLayer *maskWithHole = [CAShapeLayer layer];

// Both frames are defined in the same coordinate system
CGRect biggerRect = CGRectMake(30, 50, 120, 200);
CGRect smallerRect = CGRectMake(80, 100, 50, 80);

UIBezierPath *maskPath = [UIBezierPath bezierPath];
[maskPath moveToPoint:CGPointMake(CGRectGetMinX(biggerRect), CGRectGetMinY(biggerRect))];
[maskPath addLineToPoint:CGPointMake(CGRectGetMinX(biggerRect), CGRectGetMaxY(biggerRect))];
[maskPath addLineToPoint:CGPointMake(CGRectGetMaxX(biggerRect), CGRectGetMaxY(biggerRect))];
[maskPath addLineToPoint:CGPointMake(CGRectGetMaxX(biggerRect), CGRectGetMinY(biggerRect))];
[maskPath addLineToPoint:CGPointMake(CGRectGetMinX(biggerRect), CGRectGetMinY(biggerRect))];

[maskPath moveToPoint:CGPointMake(CGRectGetMinX(smallerRect), CGRectGetMinY(smallerRect))];
[maskPath addLineToPoint:CGPointMake(CGRectGetMinX(smallerRect), CGRectGetMaxY(smallerRect))];
[maskPath addLineToPoint:CGPointMake(CGRectGetMaxX(smallerRect), CGRectGetMaxY(smallerRect))];
[maskPath addLineToPoint:CGPointMake(CGRectGetMaxX(smallerRect), CGRectGetMinY(smallerRect))];
[maskPath addLineToPoint:CGPointMake(CGRectGetMinX(smallerRect), CGRectGetMinY(smallerRect))];

[maskWithHole setPath:[maskPath CGPath]];
[maskWithHole setFillRule:kCAFillRuleEvenOdd];
[maskWithHole setFillColor:[[UIColor orangeColor] CGColor]];

Answer 2

Swift 3.0 solution:

class MakeTransparentHoleOnOverlayView: UIView {

    @IBOutlet weak var transparentHoleView: UIView!

    // MARK: - Drawing

    override func draw(_ rect: CGRect) {
        super.draw(rect)

        if self.transparentHoleView != nil {
            // Ensures to use the current background color to set the filling color
            self.backgroundColor?.setFill()
            UIRectFill(rect)

            let layer = CAShapeLayer()
            let path = CGMutablePath()

            // Make hole in view's overlay
            // NOTE: Here, instead of using the transparentHoleView UIView we could use a specific CFRect location instead...
            path.addRect(transparentHoleView.frame)
            path.addRect(bounds)

            layer.path = path
            layer.fillRule = kCAFillRuleEvenOdd
            self.layer.mask = layer
        }
    }

    override func layoutSubviews () {
        super.layoutSubviews()
    }

    // MARK: - Initialization

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

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