问求助一个手机Xcode助手教程里UITabBarControllerProject的问题？

import UIKit

class FirstSubViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        self.title = "Item #1"
        self.tabBarItem.image = UIImage(named: "home")
        self.tabBarItem.selectedImage = UIImage(named: "home")
        self.view.backgroundColor = UIColor.red

        // Do any additional setup after loading the view.
    }
    override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()

    /*
    // MARK: - Navigation

    // In a storyboard-based application, you will often want to do a little preparation before navigation
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        // Get the new view controller using segue.destination.
        // Pass the selected object to the new view controller.
    }
    */

}
}

import UIKit

class SecondSubViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        self.title = "Item #2"
        self.tabBarItem.image = UIImage(named: "settings")
        self.view.backgroundColor = UIColor.purple
        // Do any additional setup after loading the view.
    }
    override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
        self.tabBarItem.badgeValue = "8"
        
    }
    
    override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    }

    /*
    // MARK: - Navigation

    // In a storyboard-based application, you will often want to do a little preparation before navigation
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        // Get the new view controller using segue.destination.
        // Pass the selected object to the new view controller.
    }
    */


}

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        let viewController1 = FirstSubViewController()
        let viewController2 = SecondSubViewController()
        
        let tabViewController = UITabBarController()
        tabViewController.viewControllers = [viewController1, viewController2]
        tabViewController.selectedIndex = 0
        tabViewController.view.backgroundColor = UIColor.red
        self.window?.rootViewController = tabViewController
        
        let tabBar = tabViewController.tabBar
        let item = tabBar.items![0]
        item.image = UIImage(named: "home")
        item.title = "home"
        
        let item2 = tabBar.items![1]
        item2.image = UIImage(named: "settings")
        item2.title = "settings"
        item2.badgeValue = "9"
        return true
    }
   
    // MARK: UISceneSession Lifecycle

    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        // Called when a new scene session is being created.
        // Use this method to select a configuration to create the new scene with.
        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
    }

    func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
        // Called when the user discards a scene session.
        // If any sessions were discarded while the application was not running, this will be called shortly after application:didFinishLaunchingWithOptions.
        // Use this method to release any resources that were specific to the discarded scenes, as they will not return.
    }


}