func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        
        let rootController = ViewController.init()
        let rootController2 = ViewController.init()
        let rootController3 = ViewController.init()
        window = UIWindow.init(frame: UIScreen.main.bounds)
        let tab = UITabBarController()
        let a = UINavigationController.init(rootViewController: rootController)
        a.tabBarItem = UITabBarItem(title: "A", image: nil, selectedImage: nil)
        let b = UINavigationController.init(rootViewController: rootController2)
        b.tabBarItem = UITabBarItem(title: "B", image: nil, selectedImage: nil)
        let c = UINavigationController.init(rootViewController: rootController3)
        c.tabBarItem = UITabBarItem(title: "C", image: nil, selectedImage: nil)

        tab.viewControllers = [a,b,c]
        window?.rootViewController = tab
        window?.makeKeyAndVisible()
        
        return true
    }