How to inverse Core Graphics Context drawing in Swift? - ios

Question

I'm using the below code to draw and remove drawing in an UIView. Now, I need to find a way to inverse the drawing. By clicking a button I have to paint only the non-painted part of my view. Can I use the existing lines array to calculate the path using the touch event and invert it? Or How can I redraw to inverse the drawing with a single click?
struct Line {
var points: [CGPoint]
var strokeColor: UIColor
var strokeWidth: CGFloat
}
class CanvasView: UIView {
var vm: viewModel = viewModel.instance ---> var lines: [Line] = []
private var strokeWidth: CGFloat = 25.0
private var strokeColor: UIColor = .white
override func draw(_ rect: CGRect) {
super.draw(rect)
guard let context = UIGraphicsGetCurrentContext() else { return }
vm.lines.forEach { line in
context.setStrokeColor(line.strokeColor.cgColor)
context.setLineWidth(line.strokeWidth)
context.setLineCap(.round)
context.setLineJoin(.round)
for (index, point) in line.points.enumerated() {
if index == 0 {
context.move(to: point)
} else {
context.addLine(to: point)
}
}
context.strokePath()
}
}
func clear() {
vm.lines.removeAll()
setNeedsDisplay()
}
func inverse() {
// Inverse the drawing
setNeedsDisplay()
}
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
guard let point = touches.first?.location(in: self) else { return }
let newLine = Line(points: [point], strokeColor: strokeColor, strokeWidth: strokeWidth)
vm.lines.append(newLine)
// setNeedsDisplay()
}
override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
guard let point = touches.first?.location(in: self) else { return }
guard var lastLine = vm.lines.popLast() else { return }
lastLine.points.append(point)
vm.lines.append(lastLine)
setNeedsDisplay()
}
}
Now, I want to invert the drawing in my UIView. I can see people fill them using Even Odd (EO) fill-in for CAShapeLayers. Is it possible to do a similar method? Any help would be highly appreciated.