var window: UIWindow?
    
    //MARK: Set Tabbar and view controllers
    let tabBar                = UITabBarController()
    let firstViewController   = UIViewController()
    let secondViewController  = UIViewController()
    let thirdViewController   = UIViewController()
    
    func addingViewControllerToTabBarController() {
        //MARK: Adding ViewControllers to navigation controllers,
        let firstNavigationController = UINavigationController(rootViewController: firstViewController)
        let secondNavigationController = UINavigationController(rootViewController: secondViewController)
        let thirdNavigationController = UINavigationController(rootViewController: thirdViewController)
        
        //MARK: set view controllers to tabbar according to the order
        tabBar.setViewControllers([firstNavigationController, secondNavigationController, thirdNavigationController], animated: true)
    }
    
    //MARK: Scene delegate function.
    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        
        //MARK: Programmatically adding tabbar to the SceneDelegate
        guard let windowScene = (scene as? UIWindowScene) else { return }
        window = UIWindow(windowScene: windowScene)
        window?.rootViewController = tabBar
        window?.backgroundColor = .systemBackground
        window?.makeKeyAndVisible()
        
        //MARK: Set titles to related controllers
        firstViewController.title = "firstController"
        secondViewController.title = "secondController"
        thirdViewController.title = "thirdController"
        
        //MARK: UITabbar appearances in selected and unselected conditions
        UITabBarItem.appearance().setTitleTextAttributes([NSAttributedString.Key.foregroundColor: UIColor.label], for: UIControl.State.selected)
        UITabBarItem.appearance().setTitleTextAttributes([NSAttributedString.Key.foregroundColor: UIColor.systemGray], for: UIControl.State.normal)
        
        //MARK: call function
        addingViewControllerToTabBarController()
    }