import UIKit

protocol GiverType {
    var message: String { get }
}

protocol ReceiverType: class {
    var message: String? { get set }
}

protocol TransitionMediator {
    var giver: GiverType { get }
    var receiver: ReceiverType { get }

    func mediate()
}

extension TransitionMediator {
    func mediate() {
        receiver.message = giver.message
    }
}

extension TransitionMediator where Self: UIStoryboardSegue {
    var giver: GiverType {
        return sourceViewController as! GiverType
    }

    var receiver: ReceiverType {
        return destinationViewController as! ReceiverType
    }
}

class MediateSegue: UIStoryboardSegue, TransitionMediator {
    override func perform() {
        mediate()
        super.perform()
    }
}

class ViewController: UIViewController, GiverType {
    var message: String {
        return "test"
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

class SecondViewController: UIViewController, ReceiverType {
    @IBOutlet weak var label: UILabel!

    var message: String?

    override func viewDidLoad() {
        super.viewDidLoad()

        label.text = message
    }
}