Custom UIControlState

README.md

The gist demonstrates how to add a custom UIControlState and set image for that state to a custom UIButton.

MyButton.swift

class MyButton: UIButton {

    var error: Bool = false {
        didSet {
            setNeedsLayout()
        }
    }

    override var state: UIControlState {
        get {
            return error ? UIControlState(rawValue: super.state.rawValue | UIControlState.Error.rawValue) : super.state
        }
    }
}

private extension UIControlState {
    static let Error = UIControlState(rawValue: 1 << 16)
}

Usage.swift

let myButton = MyButton()
myButton.setImage(UIImage(named: "ErrorCross"), forState: .Error)
myButton.setImage(UIImage(named: "ErrorCross"), forState: [.Error, .Highlighted])
myButton.setImage(UIImage(named: "ErrorCross"), forState: [.Error, .Selected])
myButton.setImage(UIImage(named: "ErrorCross"), forState: [.Error, .Selected, .Highlighted])
myButton.error = true

and how would you define the 1 << 16 part if you wanted multiple custom states?

You could do this:

extension UIControl.State {
    static let customState = UIControl.State(rawValue: 1 << 16)
}