如何在iPhone应用程序中获取对UITabBar的引用

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    // 创建一个UITabBarController实例
    let tabBarController = UITabBarController()
    
    // 创建并设置UITabBarController的子视图控制器
    let firstViewController = UIViewController()
    firstViewController.tabBarItem = UITabBarItem(title: "First", image: UIImage(systemName: "square.fill"), tag: 0)
    let secondViewController = UIViewController()
    secondViewController.tabBarItem = UITabBarItem(title: "Second", image: UIImage(systemName: "square.fill"), tag: 1)
    
    tabBarController.viewControllers = [firstViewController, secondViewController]
    
    // 将UITabBarController设置为窗口的根视图控制器
    window?.rootViewController = tabBarController
    window?.makeKeyAndVisible()
    
    return true
}

override func viewDidLoad() {
    super.viewDidLoad()
    
    // 获取对UITabBar的引用
    guard let tabBar = tabBarController?.tabBar else { return }
    
    // 设置代理
    tabBar.delegate = self
}

extension ViewController: UITabBarDelegate {
    func tabBar(_ tabBar: UITabBar, didSelect item: UITabBarItem) {
        guard let index = tabBar.items?.firstIndex(of: item) else { return }
        
        print("Selected tab at index: \(index)")
    }
}