class DetailViewController: UIViewController,DetailShowable {
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .yellowColor()
    }
    func showDetail() {
        view.backgroundColor = .blackColor()   
    }
}

class ViewController: UIViewController, DetailShowable {
    override func loadView() {
        view = View()
        vc = DetailViewController()
        let button = UIButton(type: .System)
        button.setTitle("show", forState: .Normal)
        button.addTarget(nil,
                         action: #selector(pushVC),
                         forControlEvents: .TouchUpInside)
        button.frame = CGRect(x: 100, y: 100, width: 100, height: 40)
        view.addSubview(button)
    }
    var vc: DetailViewController!
    func showDetail() {
        view.backgroundColor = .purpleColor()
    }
    func pushVC() {
        navigationController?.pushViewController(vc, animated: true)
    }
}