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

Question

我想在用户登录时切换加载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.
}

Answer 1

从viewDidLoad方法中删除代码并使用以下方法。

 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)
 }

Answer 2

您应该在viewWillAppear()或viewDidAppear()显示您显示第二个View Controller的代码。

override func viewDidAppear() {
    super.viewDidAppear()

    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.
}

原因是在viewDidLoad()期间，您的ViewController的某些属性尚未定义，并且尚未准备好呈现另一个ViewController。

Answer 3

你需要使它成为异步，不需要任何延迟，就像这样：

 override func viewDidLoad() {
    super.viewDidLoad()

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

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

}

Answer 4

而不是在viewDidLoad中编写逻辑，在呈现当前viewcontroller之前编写它

例如，如果要在应用程序启动时检查它，请检查用户是否已登录didFinishLaunching方法并相应地设置rootViewController。

如果想要从其他viewController中呈现它，首先检查用户登录状态并相应地显示viewcontroller，而不是检查viewDidLoad。

此外，您可以通过在一些延迟后执行操作来呈现视图控制器。

DispatchQueue.main.asyncAfter(deadline: .now() + 1.0) {
        // present view contoller
    }