didFinishLaunchingWithOptions

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        var vc : UIViewController = storyboard.instantiateViewController(withIdentifier: "mainScreen") as UIViewController
        window = UIWindow(frame: UIScreen.main.bounds)
        self.window?.rootViewController = vc
        self.window?.makeKeyAndVisible()
      
        return true
 }