responder-chain.swift

// MARK: Protocols
protocol ResponderProtocol {
    var nextResponder: ResponderProtocol? { get }
}

protocol CopyProtocol {
    func copy()
}

// MARK: Concrete responder
class Responder: ResponderProtocol {
    var nextResponder: ResponderProtocol?
}

// MARK: Responder chain classes
class Button: Responder {}
class View: Responder {}
class CopyableView: Responder, CopyProtocol {
    func copy() {
        print("Copied copyable view")
    }
}
class Window: Responder, CopyProtocol {
    func copy() {
        print("Copied window")
    }
}

// MARK: Walk responder chain helper
func walkResponderChain(firstResponder: ResponderProtocol?, handleEvent: (ResponderProtocol) -> Bool) -> ResponderProtocol? {
    guard let responder = firstResponder else {
        return nil
    }
    guard handleEvent(responder) == false else {
        // Responder handled our call
        return responder
    }
    return walkResponderChain(responder.nextResponder, handleEvent: handleEvent)
}

// MARK: Copy via responder chain
func copy(firstResponder: ResponderProtocol) -> ResponderProtocol? {
    return walkResponderChain(firstResponder) { responder in
        if let validResponder = responder as? CopyProtocol {
            validResponder.copy()
            return true
        }
        return false
    }
}

// Test case
let button = Button()
let view = View()
let copyView = CopyableView()
let window = Window()

// Setup responder chain as Button -> CopyableView -> Window
button.nextResponder = copyView
view.nextResponder = window
copyView.nextResponder = window

copy(button)

// Setup responder chain as Button -> View -> Window
button.nextResponder = view
copy(button)