How to mask one UIView using another UIView

All views have a mask property that allows you to cut out parts depending on what you need. This mask can be any other kind of UIView, so you could for example use a label to cut out an image view.
To try it out, first create a view with some obvious content such as a background color:

let redView = UIView(frame: CGRect(x: 50, y: 50, width: 128, height: 128))
redView.backgroundColor = .red
view.addSubview(redView)

Now create your mask as a separate UIView. Although it won’t be directly visible you should give this either a background color or some other content because the alpha channel of this mask determines what shows through in the original view.
To demonstrate this, here’s a mask view that’s the same size as the original view, but it’s offset 64 pixels to the right and has a 64-point corner radius. When used as a mask for the previous view it will have the effect of turning it into a semi-circle:

let maskView = UIView(frame: CGRect(x: 64, y: 0, width: 128, height: 128))
maskView.backgroundColor = .blue
maskView.layer.cornerRadius = 64
redView.mask = maskView

The blue background color won’t be visible – that’s just there to make sure all pixels in the mask are opaque.