Sorry I'm still struggling to wrap my head around this. Would you possibly be able to give a code example? I'm also using autolayout to position my views so I don't know if that adds any complexity. Here is the entire function, essentially I have an empty and a filled image and I want the mask to only show the left-half of the filled image.    private func setupViews() {     let image = UIImageView()     let filledImage = UIImageView()     image.image = UIImage(named: "empty")     filledImage.image = UIImage(named: "filled")     view.addSubview(image)     view.addSubview(filledImage)     image.translatesAutoresizingMaskIntoConstraints = false     filledImage.translatesAutoresizingMaskIntoConstraints = false     image.widthAnchor.constraint(equalToConstant: 90).isActive = true     image.heightAnchor.constraint(equalToConstant: 90).isActive = true     image.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true     image.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor).isActive = true     filledImage.widthAnchor.constraint(equalToConstant: 90).isActive = true     filledImage.heightAnchor.constraint(equalToConstant: 90).isActive = true     filledImage.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true     filledImage.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor).isActive = true     let maskView = UIView(frame: CGRect(x: 0, y: 0, width: 45, height: 90))     maskView.backgroundColor = .black     filledImage.mask = maskView   } Many thanks, I don't know how much of an optimisation this makes but its probably not insignificant given that a Run Time warning was created for it.