ios - UITabBarController 中的 UINavigationController 未完全包装 UIViewController

Question

我有一个 UITabBarController 作为我的 rootViewcontroller，每个选项卡有 3 个 UINavigationController。每个UINavigationController都有一个初始的UIViewController，它只有红色背景色。 我的问题是 UINavigationController 在第一次启动时没有完全覆盖 UIViewController 。切换选项卡后，它会覆盖 UIViewController。那么我在这里做错了什么？

提前致谢。

应用程序委托(delegate):

window = UIWindow(frame: UIScreen.main.bounds)
window?.rootViewController = TabBarViewController()
window?.makeKeyAndVisible()

UITabBarController:

class TabBarViewController: UITabBarController {

    override func viewDidLoad() {
        super.viewDidLoad()

        // Navigation Tab
        let navVC = NavigationViewController()

        // Departure Tab
        let depVC = DeparturesViewController()

        // Settings Tab
        let setVC = SettingsViewController()

        self.viewControllers = [
            createNavigationController(title: "Navigation", rootViewController: navVC, imageName: "map"),
            createNavigationController(title: "Abfahrten", rootViewController: depVC, imageName: "station"),
            createNavigationController(title: "Einstellungen", rootViewController: setVC, imageName: "user"),
        ]        
    }

    private func createNavigationController(title: String, rootViewController: UIViewController, imageName: String) -> UINavigationController {

        rootViewController.title = title
        let nc = UINavigationController(rootViewController: rootViewController)
        nc.title = title
        nc.view.backgroundColor = .white
        nc.navigationBar.prefersLargeTitles = true
        nc.navigationController?.navigationItem.largeTitleDisplayMode = .always
        nc.tabBarItem.image = UIImage(named: imageName)?.withRenderingMode(UIImage.RenderingMode.alwaysTemplate)
        return nc
    }

    override func viewWillAppear(_ animated: Bool) {
        self.selectedIndex = 0
    }

}

非常简单的 UIViewController:

class NavigationViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.backgroundColor = .red
    }

}

Answer 1

从 TabBarController 中删除以下代码后，它完全包装了 Viewcontroller

override func viewWillAppear(_ animated: Bool) {
    self.selectedIndex = 0
}

但我无法解释为什么