UITabBarController加载之后不显示sub view controller

原代码：

    fileprivate func createSubiewControllers() {
        let newsVC = NewsViewController()
        let newsItem = UITabBarItem(title: "动态", image: UIImage(named: "TabNewsNormal"), selectedImage: UIImage(named: "TabNewsPressed"));
        newsVC.tabBarItem = newsItem
        
        let discoverVC = DiscoverViewController()
        let discoverItem = UITabBarItem(title: "发现", image: UIImage(named: "TabDiscoverNormal"), selectedImage: UIImage(named: "TabDiscoverPressed"));
        discoverVC.tabBarItem = discoverItem
        
        let nullVC = UIViewController()
        let nullItem = UITabBarItem()
        nullVC.tabBarItem = nullItem
        
        let messageVC = MessageViewController()
        let messageItem = UITabBarItem(title: "消息", image: UIImage(named: "TabMessageNormal"), selectedImage: UIImage(named: "TabMessagePressed"));
        messageVC.tabBarItem = messageItem
        
        let meVC = MeViewController()
        let meItem = UITabBarItem(title: "我", image: UIImage(named: "TabMeNormal"), selectedImage: UIImage(named: "TabMePressed"));
        meVC.tabBarItem = meItem
       
        self.viewControllers = [newsVC, discoverVC, nullVC, messageVC, meVC]
        
        let tabBarHeight = self.tabBar.frame.size.height
        let addButton = UIButton(type: .custom)
        if let addImage = UIImage(named: "TabAddButton") {
            addButton.frame = CGRect(x: 0, y: 0,  addImage.size.width * (tabBarHeight - 12) / addImage.size.height, height: tabBarHeight - 12)
            addButton.setImage(addImage, for: .normal)
        }
        addButton.layer.masksToBounds = true
        addButton.layer.cornerRadius = 5
        var center = self.tabBar.center
        center.y = self.tabBar.frame.size.height / 2
        addButton.center = center
        self.tabBar.addSubview(addButton)
    }

效果：

没有显示第一个“动态”viewController。

原因：

self.viewControllers中要放UINavigationController所以将

self.viewControllers = [newsVC, discoverVC, nullVC, messageVC, meVC]

改为

        let navNewsVC = UINavigationController(rootViewController: newsVC)
        let navDiscoverVC = UINavigationController(rootViewController: discoverVC)
        let navMessageVC = UINavigationController(rootViewController: messageVC)
        let navMeVC = UINavigationController(rootViewController: meVC)
        self.viewControllers = [navNewsVC, navDiscoverVC, nullVC, navMessageVC, navMeVC]

即可正确显示。