如何在没有视图控制器的情况下在swift上创建uitabbar项目

import UIKit

class CustomTabBarController: UITabBarController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 设置TabBar的外观样式
        tabBar.barTintColor = UIColor.white
        tabBar.tintColor = UIColor.blue
        
        // 添加子视图控制器
        let viewController1 = UIViewController()
        viewController1.view.backgroundColor = UIColor.red
        viewController1.tabBarItem = UITabBarItem(title: "Tab 1", image: UIImage(named: "tab1"), tag: 0)
        
        let viewController2 = UIViewController()
        viewController2.view.backgroundColor = UIColor.green
        viewController2.tabBarItem = UITabBarItem(title: "Tab 2", image: UIImage(named: "tab2"), tag: 1)
        
        let viewController3 = UIViewController()
        viewController3.view.backgroundColor = UIColor.yellow
        viewController3.tabBarItem = UITabBarItem(title: "Tab 3", image: UIImage(named: "tab3"), tag: 2)
        
        viewControllers = [viewController1, viewController2, viewController3]
    }
}

// 在AppDelegate.swift文件中设置根视图控制器为自定义的TabBar视图控制器
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    window = UIWindow(frame: UIScreen.main.bounds)
    window?.rootViewController = CustomTabBarController()
    window?.makeKeyAndVisible()
    return true
}