Kevin Sliech提供了一个很好的解决方案。

我现在在我的主UIViewController子类中使用下面的代码。

我做的一个小改动是检查最好的表示控制器是不是一个普通的UIViewController。如果不是，那就一定是某个VC表示一个普通VC。因此，我们返回正在呈现的VC。

- (UIViewController *)bestPresentationController
{
    UIViewController *bestPresentationController = [UIApplication sharedApplication].keyWindow.rootViewController;

    if (![bestPresentationController isMemberOfClass:[UIViewController class]])
    {
        bestPresentationController = bestPresentationController.presentedViewController;
    }    

    return bestPresentationController;
}

在我的测试中似乎一切正常。

谢谢你，凯文!

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
        }
    }
}

斯威夫特

let alertController = UIAlertController(title: "title", message: "message", preferredStyle: .alert)
//...
var rootViewController = UIApplication.shared.keyWindow?.rootViewController
if let navigationController = rootViewController as? UINavigationController {
    rootViewController = navigationController.viewControllers.first
}
if let tabBarController = rootViewController as? UITabBarController {
    rootViewController = tabBarController.selectedViewController
}
//...
rootViewController?.present(alertController, animated: true, completion: nil)

objective - c

UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"Title" message:@"message" preferredStyle:UIAlertControllerStyleAlert];
//...
id rootViewController = [UIApplication sharedApplication].delegate.window.rootViewController;
if([rootViewController isKindOfClass:[UINavigationController class]])
{
    rootViewController = ((UINavigationController *)rootViewController).viewControllers.firstObject;
}
if([rootViewController isKindOfClass:[UITabBarController class]])
{
    rootViewController = ((UITabBarController *)rootViewController).selectedViewController;
}
//...
[rootViewController presentViewController:alertController animated:YES completion:nil];

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)
    }
}

其中一些答案只对我起了部分作用，将它们组合在AppDelegate中的以下类方法中是我的解决方案。它在iPad上工作，在UITabBarController视图中，在UINavigationController中，在呈现情态时。在iOS 10和13上进行测试。

+ (UIViewController *)rootViewController {
    UIViewController *rootViewController = [UIApplication sharedApplication].delegate.window.rootViewController;
    if([rootViewController isKindOfClass:[UINavigationController class]])
        rootViewController = ((UINavigationController *)rootViewController).viewControllers.firstObject;
    if([rootViewController isKindOfClass:[UITabBarController class]])
        rootViewController = ((UITabBarController *)rootViewController).selectedViewController;
    if (rootViewController.presentedViewController != nil)
        rootViewController = rootViewController.presentedViewController;
    return rootViewController;
}

用法:

[[AppDelegate rootViewController] presentViewController ...

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)
          }
        }
    }
}