@agilityvision的答案翻译为Swift4/iOS11。我没有使用本地化字符串，但你可以很容易地改变:

import UIKit

/** An alert controller that can be called without a view controller.
 Creates a blank view controller and presents itself over that
 **/
class AlertPlusViewController: UIAlertController {

    private var alertWindow: UIWindow?

    override func viewDidLoad() {
        super.viewDidLoad()
    }

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

    func show() {
        self.showAnimated(animated: true)
    }

    func showAnimated(animated _: Bool) {

        let blankViewController = UIViewController()
        blankViewController.view.backgroundColor = UIColor.clear

        let window = UIWindow(frame: UIScreen.main.bounds)
        window.rootViewController = blankViewController
        window.backgroundColor = UIColor.clear
        window.windowLevel = UIWindowLevelAlert + 1
        window.makeKeyAndVisible()
        self.alertWindow = window

        blankViewController.present(self, animated: true, completion: nil)
    }

    func presentOkayAlertWithTitle(title: String?, message: String?) {

        let alertController = AlertPlusViewController(title: title, message: message, preferredStyle: .alert)
        let okayAction = UIAlertAction(title: "Ok", style: .default, handler: nil)
        alertController.addAction(okayAction)
        alertController.show()
    }

    func presentOkayAlertWithError(error: NSError?) {
        let title = "Error"
        let message = error?.localizedDescription
        presentOkayAlertWithTitle(title: title, message: message)
    }
}

你可以用Swift 2.2做以下事情:

let alertController: UIAlertController = ...
UIApplication.sharedApplication().keyWindow?.rootViewController?.presentViewController(alertController, animated: true, completion: nil)

Swift 3.0:

let alertController: UIAlertController = ...
UIApplication.shared.keyWindow?.rootViewController?.present(alertController, animated: true, completion: nil)

另一个选择:

    var topController:UIViewController = UIApplication.shared.keyWindow!.rootViewController!
    while ((topController.presentedViewController) != nil) {
        topController = topController.presentedViewController!
    }
    topController.present(alert, animated:true, completion: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)
}

你可以使用两种方法:

-使用UIAlertView或'UIActionSheet'代替(不推荐，因为它在iOS 8中已弃用，但现在可以使用了)

-记得上次显示的视图控制器。举个例子。

@interface UIViewController (TopController)
+ (UIViewController *)topViewController;
@end

// implementation

#import "UIViewController+TopController.h"
#import <objc/runtime.h>

static __weak UIViewController *_topViewController = nil;

@implementation UIViewController (TopController)

+ (UIViewController *)topViewController {
    UIViewController *vc = _topViewController;
    while (vc.parentViewController) {
        vc = vc.parentViewController;
    }
    return vc;
}

+ (void)load {
    [super load];
    [self swizzleSelector:@selector(viewDidAppear:) withSelector:@selector(myViewDidAppear:)];
    [self swizzleSelector:@selector(viewWillDisappear:) withSelector:@selector(myViewWillDisappear:)];
}

- (void)myViewDidAppear:(BOOL)animated {
    if (_topViewController == nil) {
        _topViewController = self;
    }

    [self myViewDidAppear:animated];
}

- (void)myViewWillDisappear:(BOOL)animated {
    if (_topViewController == self) {
        _topViewController = nil;
    }

    [self myViewWillDisappear:animated];
}

+ (void)swizzleSelector:(SEL)sel1 withSelector:(SEL)sel2
{
    Class class = [self class];

    Method originalMethod = class_getInstanceMethod(class, sel1);
    Method swizzledMethod = class_getInstanceMethod(class, sel2);

    BOOL didAddMethod = class_addMethod(class,
                                        sel1,
                                        method_getImplementation(swizzledMethod),
                                        method_getTypeEncoding(swizzledMethod));

    if (didAddMethod) {
        class_replaceMethod(class,
                            sel2,
                            method_getImplementation(originalMethod),
                            method_getTypeEncoding(originalMethod));
    } else {
        method_exchangeImplementations(originalMethod, swizzledMethod);
    }
}

@end 

用法:

[[UIViewController topViewController] presentViewController:alertController ...];

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
}