Myujth

Yes this question has been asked before, the solutions did not work or had different applications.

It is the most basic setup. I have two rectangular UIViews, red and blue.
I would like the blue square to cut into the red square, so the red square looks like an "L"

import Foundation
import UIKit

class TestController: UIViewController 
 override func viewDidLoad() 
 view.backgroundColor = .gray
 view.addSubview(viewA)
 view.addSubview(maskView)

 viewA.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
 viewA.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
 viewA.widthAnchor.constraint(equalToConstant: 100).isActive = true
 viewA.heightAnchor.constraint(equalToConstant: 100).isActive = true
 viewA.translatesAutoresizingMaskIntoConstraints = false

 maskView.centerXAnchor.constraint(equalTo: view.centerXAnchor, constant: 50).isActive = true
 maskView.centerYAnchor.constraint(equalTo: view.centerYAnchor, constant: -50).isActive = true
 maskView.widthAnchor.constraint(equalToConstant: 100).isActive = true
 maskView.heightAnchor.constraint(equalToConstant: 100).isActive = true
 maskView.translatesAutoresizingMaskIntoConstraints = false

 // Things which don't work
 //viewA.mask = maskView // both views disappear
 //viewA.layer.mask = maskView.layer // both views disappear
 //viewA.layer.addSublayer(maskView.layer) // hides mask view
 

 var viewA: UIView = 
 let view = UIView()
 view.backgroundColor = .red
 view.layer.masksToBounds = true
 return view
 ()

 var maskView: UIView = 
 let view = UIView()
 view.backgroundColor = .blue
 return view
 ()

This is the result I am expecting: (done in Photoshop)

As there is no magic way to mask the way in iOS, I present here a simple way to achieve this.

Don't forget to pan the clear area, If leaving the red square, it will become a blue square.

It's not hard to modify the subclass of UIViews for your own purpose, especially views.

 import UIKit

 class TestController: UIViewController 

 override func viewDidLoad() 
 view.backgroundColor = .gray
 view.addSubview(viewA)
 view.addSubview(maskView)
 maskView.maskedView = viewA
 viewA.activeMask = maskView
 viewA.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
 viewA.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
 viewA.widthAnchor.constraint(equalToConstant: 100).isActive = true
 viewA.heightAnchor.constraint(equalToConstant: 100).isActive = true
 viewA.translatesAutoresizingMaskIntoConstraints = false
 maskView.centerXAnchor.constraint(equalTo: view.centerXAnchor, constant: 50).isActive = true
 maskView.centerYAnchor.constraint(equalTo: view.centerYAnchor, constant: -50).isActive = true
 maskView.widthAnchor.constraint(equalToConstant: 100).isActive = true
 maskView.heightAnchor.constraint(equalToConstant: 100).isActive = true
 maskView.translatesAutoresizingMaskIntoConstraints = false
 

 var viewA: MyUIView = 
 let view = MyUIView()
 view.backgroundColor = .clear
 view.layer.masksToBounds = true
 return view
 ()


 var maskView: ActiveMaskView = 
 let view = ActiveMaskView()
 view.backgroundColor = .clear
 return view
 ()


 

 class ActiveMaskView: UIView
 override func didMoveToSuperview() 
 super.didMoveToSuperview()
 let panGesture = UIPanGestureRecognizer.init(target: self, action: #selector(moveAround(_:)))
 self.addGestureRecognizer(panGesture)
 
 weak var maskedView : UIView?
 private var frameOrigin : CGPoint = CGPoint.zero

 @objc func moveAround(_ panGesture: UIPanGestureRecognizer)
 guard let superview = superview else return
 switch panGesture.state 
 case .began:
 frameOrigin = frame.origin
 self.backgroundColor = UIColor.blue
 case .changed:
 let translation = panGesture.translation(in: superview)
 frame = CGRect.init(origin: CGPoint.init(x: frameOrigin.x + translation.x, y: frameOrigin.y + translation.y), size: frame.size)
 maskedView?.setNeedsDisplay()
 break
 case .ended:
 self.backgroundColor =
 frame.intersects(maskedView!.frame) ?
 UIColor.clear : UIColor.blue
 maskedView?.setNeedsDisplay()
 case .cancelled:
 frame = CGRect.init(origin: frameOrigin , size: frame.size)
 self.backgroundColor =
 frame.intersects(maskedView!.frame) ?
 UIColor.clear : UIColor.blue
 maskedView?.setNeedsDisplay()
 default:
 break;
 
 
 

 class MyUIView: UIView

 weak var activeMask: ActiveMaskView?

 override func draw(_ rect: CGRect) 
 super.draw(rect)
 let ctx = UIGraphicsGetCurrentContext()
 ctx?.setFillColor(UIColor.red.cgColor)
 ctx?.fill(self.layer.bounds)
 ctx?.setBlendMode(.sourceOut)
 guard let activeMask = activeMask , let superview = superview else 
 return
 
 let sc = frame.intersection(activeMask.frame)
 let interSection = superview.convert(sc, to: self)
 ctx?.fill(interSection )