无法在AppDelegate中的rootViewController上显示alertViewController

Question

我希望在app启动时在AppDelegate中的rootViewController上显示alertViewController。这里是一段代码：

 func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        let alert: UIAlertController = UIAlertController(title: "This is title", message: "This is message.", preferredStyle: UIAlertControllerStyle.alert)
        alert.addAction(UIAlertAction(title: "Ok", style: .default))
        self.window?.rootViewController?.present(alert, animated: true, completion: nil)
        return true
 }

警报未出现在rootViewController上。请帮帮我。

Answer 1

原因是视图尚未加载，如果您查看控制台，您可能会发现这样的日志：

  

警告：尝试出现'swiftHere.ViewController：0x7ff289007740'，其视图不在窗口层次结构中！

您可以在rootViewController加载之前调度它：

DispatchQueue.main.asyncAfter(deadline: .now() + 2) {

       let alert: UIAlertController = UIAlertController(title: "This is title", message: "This is message.", preferredStyle: UIAlertControllerStyle.alert)
       alert.addAction(UIAlertAction(title: "Ok", style: .default))
      self.window?.rootViewController?.present(alert, animated: true, completion: nil)

    }