Uitabbarcontroller 如何正确显示选项卡栏？tabbarcontroltest.ViewController开始/结束外观转换的调用不平衡：

我在显示tabbarVC时遇到问题

代码如下：

类ViewController:UIViewController{

override func viewDidLoad() {
    super.viewDidLoad()

}

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    let tabBarVC = UITabBarController()
    guard let vc1 = storyboard?.instantiateViewController(identifier: "FirstController") as?  FirstController else {
        print("failed to get vc1 from Storyboard")
        return
    }
    guard let vc2 = storyboard?.instantiateViewController(identifier: "SecondController") as? SecondController else {
        print("failed to get vc2 from Storyboard")
        return
    }
    guard let vc3 = storyboard?.instantiateViewController(identifier: "ThirdController") as? ThirdController else {
        print("failed to get vc3 from Storyboard")
        return
    }
    let vc4 = UINavigationController(rootViewController: vc1)
    let vc5 = UINavigationController(rootViewController: vc2)
    let vc6 = UINavigationController(rootViewController: vc3)
    
    vc4.title = "XXX"
    vc5.title = "YYY"
    vc6.title = "ZZZ"
    
    tabBarVC.setViewControllers([vc4,vc5,vc6], animated: false)
    tabBarVC.modalPresentationStyle = .fullScreen
    self.present(tabBarVC, animated: true)
}

}

---

选项卡栏显示正确，但我收到了一条警告：“对的开始/结束外观转换的调用不平衡，我自己找到了解决方案。正确的方法如下：

    tabBarVC.modalPresentationStyle = .fullScreen
    tabBarVC.view.frame = self.view.bounds
    addChild(tabBarVC)
    view.addSubview(tabBarVC.view)
    tabBarVC.willMove(toParent: self)