以下是mythicalcoder的答案，作为一个扩展，在Swift 4中测试和工作:

extension UIAlertController {

    func presentInOwnWindow(animated: Bool, completion: (() -> Void)?) {
        let alertWindow = UIWindow(frame: UIScreen.main.bounds)
        alertWindow.rootViewController = UIViewController()
        alertWindow.windowLevel = UIWindowLevelAlert + 1
        alertWindow.makeKeyAndVisible()
        alertWindow.rootViewController?.present(self, animated: animated, completion: completion)
    }

}

使用示例:

let alertController = UIAlertController(title: "<Alert Title>", message: "<Alert Message>", preferredStyle: .alert)
alertController.addAction(UIAlertAction(title: "Close", style: .cancel, handler: nil))
alertController.presentInOwnWindow(animated: true, completion: {
    print("completed")
})

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)
          }
        }
    }
}

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。

如果有人感兴趣，我创建了一个Swift 3版本的@agilityvision答案。代码:

import Foundation
import UIKit

extension UIAlertController {

    var window: UIWindow? {
        get {
            return objc_getAssociatedObject(self, "window") as? UIWindow
        }
        set {
            objc_setAssociatedObject(self, "window", newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }

    open override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        self.window?.isHidden = true
        self.window = nil
    }

    func show(animated: Bool = true) {
        let window = UIWindow(frame: UIScreen.main.bounds)
        window.rootViewController = UIViewController(nibName: nil, bundle: nil)

        let delegate = UIApplication.shared.delegate
        if delegate?.window != nil {
            window.tintColor = delegate!.window!!.tintColor
        }

        window.windowLevel = UIApplication.shared.windows.last!.windowLevel + 1

        window.makeKeyAndVisible()
        window.rootViewController!.present(self, animated: animated, completion: nil)

        self.window = window
    }
}

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!

似乎有效:

static UIViewController *viewControllerForView(UIView *view) {
    UIResponder *responder = view;
    do {
        responder = [responder nextResponder];
    }
    while (responder && ![responder isKindOfClass:[UIViewController class]]);
    return (UIViewController *)responder;
}

-(void)showActionSheet {
    UIAlertController *alertController = [UIAlertController alertControllerWithTitle:nil message:nil preferredStyle:UIAlertControllerStyleActionSheet];
    [alertController addAction:[UIAlertAction actionWithTitle:@"Cancel" style:UIAlertActionStyleCancel handler:nil]];
    [alertController addAction:[UIAlertAction actionWithTitle:@"Do it" style:UIAlertActionStyleDefault handler:nil]];
    [viewControllerForView(self) presentViewController:alertController animated:YES completion:nil];
}