补充Zev的回答(并切换回Objective-C)，你可能会遇到这样的情况，你的根视图控制器通过segue或其他东西呈现其他VC。在根VC上调用presenttedviewcontroller会处理这个:

[[UIApplication sharedApplication].keyWindow.rootViewController.presentedViewController presentViewController:alertController animated:YES completion:^{}];

这解决了一个问题，我有根VC已经segue到另一个VC，而不是显示警报控制器，像上面报告的警告发出:

Warning: Attempt to present <UIAlertController: 0x145bfa30> on <UINavigationController: 0x1458e450> whose view is not in the window hierarchy!

我还没有测试它，但如果你的根VC恰好是一个导航控制器，这可能也是必要的。

我知道这是针对iOS的，因为搜索引擎中几乎所有的链接都能找到iOS的帖子，我想我应该把这个提供给macOS的开发者。

macOS上的Swift 5.5

根据Darkngs的回答，我在另一个类中添加了这个方法:

let alert = NSAlert()
let viewController = NSApplication.shared.keyWindow?.contentViewController
alert.messageText = "An Alert Message."
alert.addButton(withTitle: "Ok")
alert.beginSheetModal(for: (viewController?.view.window)!) {
    (returnCode: NSApplication.ModalResponse) -> Void in
}

对于iOS 13，基于mythicalcoder和bobbyrehm的回答:

在iOS 13中，如果你正在创建自己的窗口来显示警报，你需要保持对该窗口的强引用，否则你的警报将不会显示，因为当它的引用退出作用域时，窗口将立即被释放。

此外，在警报解除后，您需要再次将引用设置为nil，以便删除窗口，继续允许用户在它下面的主窗口上进行交互。

你可以创建一个UIViewController子类来封装窗口内存管理逻辑:

class WindowAlertPresentationController: UIViewController {

    // MARK: - Properties

    private lazy var window: UIWindow? = UIWindow(frame: UIScreen.main.bounds)
    private let alert: UIAlertController

    // MARK: - Initialization

    init(alert: UIAlertController) {

        self.alert = alert
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) {

        fatalError("This initializer is not supported")
    }

    // MARK: - Presentation

    func present(animated: Bool, completion: (() -> Void)?) {

        window?.rootViewController = self
        window?.windowLevel = UIWindow.Level.alert + 1
        window?.makeKeyAndVisible()
        present(alert, animated: animated, completion: completion)
    }

    // MARK: - Overrides

    override func dismiss(animated flag: Bool, completion: (() -> Void)? = nil) {

        super.dismiss(animated: flag) {
            self.window = nil
            completion?()
        }
    }
}

你可以这样使用它，或者如果你想在你的UIAlertController上使用一个方便的方法，你可以把它扔到一个扩展中:

extension UIAlertController {

    func presentInOwnWindow(animated: Bool, completion: (() -> Void)?) {

        let windowAlertPresentationController = WindowAlertPresentationController(alert: self)
        windowAlertPresentationController.present(animated: animated, completion: completion)
    }
}

斯威夫特 4+

解决方案我使用多年没有任何问题。首先，我扩展UIWindow来找到它的visibleViewController。注意:如果您使用自定义集合*类(如侧菜单)，您应该在以下扩展中为这种情况添加处理程序。在获得top most视图控制器后，很容易呈现UIAlertController，就像UIAlertView一样。

extension UIAlertController {

  func show(animated: Bool = true, completion: (() -> Void)? = nil) {
    if let visibleViewController = UIApplication.shared.keyWindow?.visibleViewController {
      visibleViewController.present(self, animated: animated, completion: completion)
    }
  }

}

extension UIWindow {

  var visibleViewController: UIViewController? {
    guard let rootViewController = rootViewController else {
      return nil
    }
    return visibleViewController(for: rootViewController)
  }

  private func visibleViewController(for controller: UIViewController) -> UIViewController {
    var nextOnStackViewController: UIViewController? = nil
    if let presented = controller.presentedViewController {
      nextOnStackViewController = presented
    } else if let navigationController = controller as? UINavigationController,
      let visible = navigationController.visibleViewController {
      nextOnStackViewController = visible
    } else if let tabBarController = controller as? UITabBarController,
      let visible = (tabBarController.selectedViewController ??
        tabBarController.presentedViewController) {
      nextOnStackViewController = visible
    }

    if let nextOnStackViewController = nextOnStackViewController {
      return visibleViewController(for: nextOnStackViewController)
    } else {
      return controller
    }
  }

}

斯威夫特5

在显示消息后隐藏窗口是很重要的。

func showErrorMessage(_ message: String) {
    let alertWindow = UIWindow(frame: UIScreen.main.bounds)
    alertWindow.rootViewController = UIViewController()

    let alertController = UIAlertController(title: "Error", message: message, preferredStyle: UIAlertController.Style.alert)
    alertController.addAction(UIAlertAction(title: "Close", style: UIAlertAction.Style.cancel, handler: { _ in
        alertWindow.isHidden = true
    }))
    
    alertWindow.windowLevel = UIWindow.Level.alert + 1;
    alertWindow.makeKeyAndVisible()
    alertWindow.rootViewController?.present(alertController, animated: true, completion: nil)
}

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)
          }
        }
    }
}