Swift 3 / Xcode 8.2 - viewDidLoad 中的切换视图不起作用

我想在用户已经登录时切换加载时的 ViewController 。 问题：当用户等于“true”时，视图没有改变......

先感谢您！！

override func viewDidLoad() {
    super.viewDidLoad()

    var user: String?
    user = UserDefaults.standard.value(forKey: "loginSuccessfull") as? String

    if user == "true" {
        let vc = self.storyboard?.instantiateViewController(withIdentifier: "SecondView") as UIViewController!
        self.show(vc!, sender: vc)
    }

    // Do any additional setup after loading the view, typically from a nib.
}

从中删除代码viewDidLoad方法并使用以下方法。

 override func viewDidAppear() {
     guard let user = UserDefaults.standard.string(forKey: "loginSuccessfull"), user == "true" else {
      return
     }
     let vc = self.storyboard?.instantiateViewController(withIdentifier: "SecondView")!
     self.present(vc, animated: true)
 }