对于UINavigationController和/或UITabBarController的所有情况，非常通用的UIAlertController扩展。如果屏幕上有一个模态VC，也可以工作。

用法:

//option 1:
myAlertController.show()
//option 2:
myAlertController.present(animated: true) {
    //completion code...
}

这是扩展:

//Uses Swift1.2 syntax with the new if-let
// so it won't compile on a lower version.
extension UIAlertController {

    func show() {
        present(animated: true, completion: nil)
    }

    func present(#animated: Bool, completion: (() -> Void)?) {
        if let rootVC = UIApplication.sharedApplication().keyWindow?.rootViewController {
            presentFromController(rootVC, animated: animated, completion: completion)
        }
    }

    private func presentFromController(controller: UIViewController, animated: Bool, completion: (() -> Void)?) {
        if  let navVC = controller as? UINavigationController,
            let visibleVC = navVC.visibleViewController {
                presentFromController(visibleVC, animated: animated, completion: completion)
        } else {
          if  let tabVC = controller as? UITabBarController,
              let selectedVC = tabVC.selectedViewController {
                presentFromController(selectedVC, animated: animated, completion: completion)
          } else {
              controller.presentViewController(self, animated: animated, completion: completion)
          }
        }
    }
}

@agilityvision的回答非常好。我有在swift项目中使用的感觉，所以我想我将分享我使用swift 3.0的答案

fileprivate class MyUIAlertController: UIAlertController {

  typealias Handler = () -> Void

  struct AssociatedKeys {
    static var alertWindowKey = "alertWindowKey"
  }

  dynamic var _alertWindow: UIWindow?

  var alertWindow: UIWindow? {
    return objc_getAssociatedObject(self, &AssociatedKeys.alertWindowKey) as? UIWindow
  }


  func setAlert(inWindow window: UIWindow) {
    objc_setAssociatedObject(self, &AssociatedKeys.alertWindowKey, _alertWindow, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
  }

  func show(completion: Handler? = nil) {
    show(animated: true, completion: completion)
  }

  func show(animated: Bool, completion: Handler? =  nil) {
    _alertWindow = UIWindow(frame: UIScreen.main.bounds)
    _alertWindow?.rootViewController = UIViewController()

    if let delegate: UIApplicationDelegate = UIApplication.shared.delegate, let window = delegate.window {
      _alertWindow?.tintColor = window?.tintColor

    }

    let topWindow = UIApplication.shared.windows.last
    _alertWindow?.windowLevel = topWindow?.windowLevel ?? 0 + 1
    _alertWindow?.makeKeyAndVisible()
    _alertWindow?.rootViewController?.present(self, animated: animated, completion: completion)
  }

  fileprivate override func viewDidDisappear(_ animated: Bool) {
    super.viewDidDisappear(animated)
    _alertWindow?.isHidden = true
    _alertWindow = nil
  }
}

交叉张贴我的答案，因为这两个线程没有被标记为欺骗…

现在UIViewController是responder chain的一部分，你可以这样做:

if let vc = self.nextResponder()?.targetForAction(#selector(UIViewController.presentViewController(_:animated:completion:)), withSender: self) as? UIViewController {

    let alert = UIAlertController(title: "A snappy title", message: "Something bad happened", preferredStyle: .Alert)
    alert.addAction(UIAlertAction(title: "OK", style: .Default, handler: nil))

    vc.presentViewController(alert, animated: true, completion: nil)
}

extension UIApplication {
    /// The top most view controller
    static var topMostViewController: UIViewController? {
        return UIApplication.shared.keyWindow?.rootViewController?.visibleViewController
    }
}

extension UIViewController {
    /// The visible view controller from a given view controller
    var visibleViewController: UIViewController? {
        if let navigationController = self as? UINavigationController {
            return navigationController.topViewController?.visibleViewController
        } else if let tabBarController = self as? UITabBarController {
            return tabBarController.selectedViewController?.visibleViewController
        } else if let presentedViewController = presentedViewController {
            return presentedViewController.visibleViewController
        } else {
            return self
        }
    }
}

有了这个，你可以很容易地呈现你的警告，就像这样

UIApplication.topMostViewController?.present(viewController, animated: true, completion: nil)

需要注意的一件事是，如果有一个UIAlertController当前正在显示，UIApplication。topMostViewController将返回一个UIAlertController。在UIAlertController的顶部呈现有奇怪的行为，应该避免。因此，你应该手动检查!topMostViewController is UIAlertController)，或者如果self是UIAlertController，添加一个else if case返回nil

extension UIViewController {
    /// The visible view controller from a given view controller
    var visibleViewController: UIViewController? {
        if let navigationController = self as? UINavigationController {
            return navigationController.topViewController?.visibleViewController
        } else if let tabBarController = self as? UITabBarController {
            return tabBarController.selectedViewController?.visibleViewController
        } else if let presentedViewController = presentedViewController {
            return presentedViewController.visibleViewController
        } else if self is UIAlertController {
            return nil
        } else {
            return self
        }
    }
}

在调用类方法之前注册通知。

斯威夫特代码:

NSNotificationCenter.defaultCenter().addObserver(self, selector: "displayAlert", name: "ErrorOccured", object: nil)

在displayAlert实例方法中，您可以显示警报。

几个月前我发过一个类似的问题，我想我终于解决了这个问题。如果你只是想看代码，请点击我文章底部的链接。

解决方案是使用一个额外的UIWindow。

当你想要显示你的UIAlertController:

使你的窗口成为键和可见窗口(window. makekeyandvisible ()) 只需使用一个普通的UIViewController实例作为新窗口的rootViewController。(窗口。rootViewController = UIViewController()) 在你窗口的rootViewController上显示你的UIAlertController

有几点需要注意:

你的UIWindow必须是强引用的。如果它没有被强引用，它将永远不会出现(因为它已经被释放了)。我建议使用属性，但我也成功地使用了关联对象。 为了确保窗口显示在其他所有内容之上(包括系统UIAlertControllers)，我设置了windowLevel。(窗口。windowLevel = UIWindowLevelAlert + 1)

最后，我有一个完整的实现，如果你只是想看看。

https://github.com/dbettermann/DBAlertController