Mask view created with the negative space within a stroked path

gistfile1.txt

//: A UIKit based Playground for presenting user interface
  
import UIKit
import PlaygroundSupport

class StrokedPathFillMaskView: UIView {
    var path: UIBezierPath? {
        didSet {
            setNeedsDisplay()
        }
    }

    override init(frame: CGRect) {
        super.init(frame: frame)
        backgroundColor = .clear
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

extension StrokedPathFillMaskView {
    override func draw(_ rect: CGRect) {
        guard let ctx = UIGraphicsGetCurrentContext(),
              let path else {
            return
        }
        guard let maskImage = ImageCreator.strokedInvertedFillMask(for: path, rect: ctx.boundingBoxOfClipPath),
              let fillColorImage = ctx.image(with: UIColor.black.cgColor)else {
            print("Failed to create mask image")
            return
        }

        // https://stackoverflow.com/a/634146/2383003
        ctx.clip(to: ctx.boundingBoxOfClipPath, mask: maskImage)
        ctx.draw(fillColorImage, in: ctx.boundingBoxOfClipPath, byTiling: false)
    }
}

enum ImageCreator {
    static func strokedInvertedFillMask(for path: UIBezierPath, rect: CGRect) -> CGImage? {
        // Masking with an image requires that the image have no alpha channel
        // https://developer.apple.com/documentation/coregraphics/cgimage/1456337-masking
        // Even though we're using `CGContext.clip(to:mask:)` instead, this is still needed
        let colorSpace = CGColorSpaceCreateDeviceRGB()
        guard let ctx = CGContext(
            data: nil,
            width: Int(ceil(rect.width)),
            height: Int(ceil(rect.height)),
            bitsPerComponent: 8,
            bytesPerRow: 0,
            space: colorSpace,
            bitmapInfo: CGImageAlphaInfo.noneSkipLast.rawValue
        ) else {
            return nil
        }

        ctx.saveGState()
        defer {
            ctx.clear(ctx.boundingBoxOfClipPath)
            ctx.restoreGState()
        }
        
        let maskPath = UIBezierPath(rect: ctx.boundingBoxOfClipPath)
        maskPath.append(path.reversing())

        // Paint background white to allow for mask differential
        let backgroundColor = UIColor.white
        ctx.setFillColor(backgroundColor.cgColor)
        ctx.fill(ctx.boundingBoxOfClipPath)
        
        let maskColor = UIColor.black
        ctx.setFillColor(maskColor.cgColor)
        ctx.setStrokeColor(maskColor.cgColor)
        
        // Fill path
        ctx.addPath(maskPath.cgPath)
        ctx.fillPath()
        
        // Stroke path
        ctx.setLineWidth(path.lineWidth)
        ctx.addPath(maskPath.cgPath)
        ctx.strokePath()
        
        return ctx.makeImage()
    }
}

extension CGContext {
    func image(with color: CGColor) -> CGImage? {
        saveGState()
        defer {
            clear(boundingBoxOfClipPath)
            restoreGState()
        }

        setFillColor(color)
        fill(boundingBoxOfClipPath)
        return makeImage()
    }
}

// Present the view controller in the Live View window
//PlaygroundPage.current.liveView = MyViewController()
let view = StrokedPathFillMaskView(frame: CGRect(x: 0, y: 0, width: 300, height: 300))
//let path = UIBezierPath(ovalIn: view.bounds)
let path = UIBezierPath()
path.addLine(to: view.center)
path.addArc(withCenter: CGPoint(x: view.frame.maxX, y: view.frame.midY),
            radius: view.frame.width / 2,
            startAngle: .pi,
            endAngle: .pi / 2,
            clockwise: true)
path.addLine(to: CGPoint(x: view.frame.maxX, y: view.frame.maxY))
path.addLine(to: CGPoint(x: view.frame.minX, y: view.frame.maxY))
path.close()
path.lineWidth = 50
view.path = path
//view.backgroundColor = .systemCyan
//print(view.layer)
//print((view.layer as! InvertedPathLayer).path)
let container = UIView(frame: view.bounds)
container.backgroundColor = .systemCyan
//container.addSubview(view)
container.mask = view
let outerContainer = UIView(frame: container.bounds)
outerContainer.backgroundColor = .systemIndigo
outerContainer.addSubview(container)
PlaygroundPage.current.liveView = outerContainer