Swift:如何在没有topBorder的情况下将阴影添加到UITabBarController

override func viewDidLoad() {
    super.viewDidLoad()
    
    // 创建一个阴影视图
    let shadowView = UIView(frame: CGRect(x: 0, y: 0, width: view.frame.width, height: 5))
    shadowView.backgroundColor = .clear
    
    // 添加阴影效果
    shadowView.layer.shadowColor = UIColor.black.cgColor
    shadowView.layer.shadowOffset = CGSize(width: 0, height: 2)
    shadowView.layer.shadowOpacity = 0.5
    shadowView.layer.shadowRadius = 4
    
    // 将阴影视图添加到UITabBarController的视图中
    view.addSubview(shadowView)
    
    // 调整UITabBar的位置，使其覆盖在阴影视图上方
    tabBar.frame.origin.y = shadowView.frame.height - tabBar.frame.height
}

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    // 创建自定义的UITabBarController子类
    let tabBarController = CustomTabBarController()
    
    // 设置为根视图控制器
    window?.rootViewController = tabBarController
    window?.makeKeyAndVisible()
    
    return true
}