Statically typed alternatives to delegates, using the full power of Swift

Button.swift

struct Touch { }
struct Image { }

public final class Button {
    public enum State { case Enabled, Disabled, Pressed }
    public struct DynamicButtonConfiguration {
        let buttonWasPressed: (button: Button, touch: Touch) -> ()
        let imageForButtonState: (buttonState: State) -> (Image)
    }
    
    let configuration: DynamicButtonConfiguration
    private var currentImage: Image // TODO: add property observers to redraw
    required public init(configuration: DynamicButtonConfiguration) {
        self.configuration = configuration
        self.currentImage = configuration.imageForButtonState(buttonState: .Enabled)
    }
    
    private func hitTest(touch: Touch) -> () {
        // TODO: hit testing
        if true {
            configuration.buttonWasPressed(button: self, touch: touch)
            currentImage = configuration.imageForButtonState(buttonState: .Pressed)
        }
    }
}

let buttonConfiguration = Button.DynamicButtonConfiguration(
    buttonWasPressed: { (button, touch) -> () in
        return
    },
    imageForButtonState: { (state) -> (Image) in
        return Image()
    }
)

let myButton = Button(configuration: buttonConfiguration)