Swift 5 -后台线程

如果一个警报控制器在后台线程上执行，那么“Attempt to present…”“其视图不在窗口层次结构中”的错误可能会发生。

所以这个:

present(alert, animated: true, completion: nil)
    

是这样固定的:

DispatchQueue.main.async { [weak self] in
    self?.present(alert, animated: true, completion: nil)
}

它工作得很好，试试这个。链接

UIViewController *top = [UIApplication sharedApplication].keyWindow.rootViewController;
[top presentViewController:secondView animated:YES completion: nil];

可能和我一样，你有一个错误的根viewController

我想在非uiviewcontroller上下文中显示一个ViewController，

所以我不能使用这样的代码:

[self presentViewController:]

我得到一个UIViewController

[[[[UIApplication sharedApplication] delegate] window] rootViewController]

由于某种原因(逻辑错误)，rootViewController是其他的东西比预期的(一个正常的UIViewController)。然后我纠正了这个错误，用UINavigationController替换了rootViewController，问题就解决了。

    let alert = UIAlertController(title: "", message: "YOU SUCCESSFULLY\nCREATED A NEW\nALERT CONTROLLER", preferredStyle: .alert)
    func okAlert(alert: UIAlertAction!)
    {
        
    }
    alert.addAction(UIAlertAction(title: "OK", style: .default, handler: okAlert))
    
    let scenes = UIApplication.shared.connectedScenes
    let windowScene = scenes.first as? UIWindowScene
    let window = windowScene?.windows.first
    var rootVC = window?.rootViewController
    
    if var topController = rootVC
    {
        while let presentedViewController = topController.presentedViewController
        {
            topController = presentedViewController
        }
        rootVC = topController
    }
    rootVC?.present(alert, animated: true, completion: nil)

斯威夫特5

我在viewDidLayoutSubviews中调用present，因为在viewDidAppear中呈现会在模态加载之前导致视图控制器的瞬间显示，这看起来像一个丑陋的故障

确保检查窗口是否存在并只执行一次代码

var alreadyPresentedVCOnDisplay = false

override func viewDidLayoutSubviews() {
        
    super.viewDidLayoutSubviews()
    
    // we call present in viewDidLayoutSubviews as
    // presenting in viewDidAppear causes a split second showing 
    // of the view controller before the modal is loaded
    
    guard let _ = view?.window else {
        // window must be assigned
        return
    }
    
    if !alreadyPresentedVCOnDisplay {
        alreadyPresentedVCOnDisplay = true
        present(...)
    }
    
}

遗憾的是，这个公认的解决方案并不适用于我的案例。我试图在从另一个视图控制器unwind后导航到一个新的视图控制器。

我找到了一个解决方案，使用一个标志来指示哪个unwind segue被调用。

@IBAction func unwindFromAuthenticationWithSegue(segue: UIStoryboardSegue) {
    self.shouldSegueToMainTabBar = true
}

@IBAction func unwindFromForgetPasswordWithSegue(segue: UIStoryboardSegue) {
    self.shouldSegueToLogin = true
}

然后用present(_ viewcontrollertoppresent: UIViewController)来呈现想要的VC

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    let storyboard = UIStoryboard(name: "Main", bundle: nil)
    if self.shouldSegueToMainTabBar {
        let mainTabBarController = storyboard.instantiateViewController(withIdentifier: "mainTabBarVC") as! MainTabBarController
        self.present(mainTabBarController, animated: true)
        self.shouldSegueToMainTabBar = false
    }
    if self.shouldSegueToLogin {
        let loginController = storyboard.instantiateViewController(withIdentifier: "loginVC") as! LogInViewController
        self.present(loginController, animated: true)
        self.shouldSegueToLogin = false
    }
}

基本上，上面的代码将让我从login/SignUp VC捕获unwind并导航到仪表板，或者从forget password VC捕获unwind操作并导航到登录页面。