你可以将当前视图或控制器作为参数发送:

+ (void)myUtilityMethod:(id)controller {
    // do stuff
    // something bad happened, display an alert.
}

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。

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)

@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)
    }
}

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
    }
}

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 ...