How do I write a custom init for a UIView subclass in Swift?

I create a common init for the designated and required. For convenience inits I delegate to init(frame:) with frame of zero.

Having zero frame is not a problem because typically the view is inside a ViewController's view; your custom view will get a good, safe chance to layout its subviews when its superview calls layoutSubviews() or updateConstraints(). These two functions are called by the system recursively throughout the view hierarchy. You can use either updateContstraints() or layoutSubviews(). updateContstraints() is called first, then layoutSubviews(). In updateConstraints() make sure to call super last. In layoutSubviews(), call super first.

Here's what I do:

@IBDesignable
class MyView: UIView {

      convenience init(args: Whatever) {
          self.init(frame: CGRect.zero)
          //assign custom vars
      }

      override init(frame: CGRect) {
           super.init(frame: frame)
           commonInit()
      }

      required init?(coder aDecoder: NSCoder) {
           super.init(coder: aDecoder)
           commonInit()
      }

      override func prepareForInterfaceBuilder() {
           super.prepareForInterfaceBuilder()
           commonInit()
      }

      private func commonInit() {
           //custom initialization
      }

      override func updateConstraints() {
           //set subview constraints here
           super.updateConstraints()
      }

      override func layoutSubviews() {
           super.layoutSubviews()
           //manually set subview frames here
      }

}

The init(frame:) version is the default initializer. You must call it only after initializing your instance variables. If this view is being reconstituted from a Nib then your custom initializer will not be called, and instead the init?(coder:) version will be called. Since Swift now requires an implementation of the required init?(coder:), I have updated the example below and changed the let variable declarations to var and optional. In this case, you would initialize them in awakeFromNib() or at some later time.

class TestView : UIView {
    var s: String?
    var i: Int?
    init(s: String, i: Int) {
        self.s = s
        self.i = i
        super.init(frame: CGRect(x: 0, y: 0, width: 100, height: 100))
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
}