泽夫·艾森伯格的答案简单而直接，但它并不总是有效，它可能会因为以下警告信息而失败:

Warning: Attempt to present <UIAlertController: 0x7fe6fd951e10>  
 on <ThisViewController: 0x7fe6fb409480> which is already presenting 
 <AnotherViewController: 0x7fe6fd109c00>

这是因为windows rootViewController不在所呈现视图的顶部。为了纠正这个问题，我们需要沿着表示链向上走，正如我用Swift 3编写的UIAlertController扩展代码所示:

   /// show the alert in a view controller if specified; otherwise show from window's root pree
func show(inViewController: UIViewController?) {
    if let vc = inViewController {
        vc.present(self, animated: true, completion: nil)
    } else {
        // find the root, then walk up the chain
        var viewController = UIApplication.shared.keyWindow?.rootViewController
        var presentedVC = viewController?.presentedViewController
        while presentedVC != nil {
            viewController = presentedVC
            presentedVC = viewController?.presentedViewController
        }
        // now we present
        viewController?.present(self, animated: true, completion: nil)
    }
}

func show() {
    show(inViewController: nil)
}

2017年9月15日更新:

经过测试并确认，上述逻辑在新推出的iOS 11转基因种子中仍然有效。然而，敏捷视觉投票最多的方法并没有:在新创建的UIWindow中显示的警报视图位于键盘下方，可能会阻止用户点击按钮。这是因为在iOS 11中，所有高于键盘窗口的窗口级别都会降低到低于键盘窗口的级别。

从keyWindow中呈现的一个工件是，当警报出现时，键盘向下滑动，当警报被取消时，键盘再次向上滑动。如果你想让键盘在显示过程中保持在那里，你可以尝试从顶部窗口本身显示，如下所示代码:

func show(inViewController: UIViewController?) {
    if let vc = inViewController {
        vc.present(self, animated: true, completion: nil)
    } else {
        // get a "solid" window with the highest level
        let alertWindow = UIApplication.shared.windows.filter { $0.tintColor != nil || $0.className() == "UIRemoteKeyboardWindow" }.sorted(by: { (w1, w2) -> Bool in
            return w1.windowLevel < w2.windowLevel
        }).last
        // save the top window's tint color
        let savedTintColor = alertWindow?.tintColor
        alertWindow?.tintColor = UIApplication.shared.keyWindow?.tintColor

        // walk up the presentation tree
        var viewController = alertWindow?.rootViewController
        while viewController?.presentedViewController != nil {
            viewController = viewController?.presentedViewController
        }

        viewController?.present(self, animated: true, completion: nil)
        // restore the top window's tint color
        if let tintColor = savedTintColor {
            alertWindow?.tintColor = tintColor
        }
    }
}

上述代码唯一不太好的部分是它检查类名UIRemoteKeyboardWindow，以确保我们也可以包括它。尽管如此，上面的代码在iOS 9、10和11 GM种子中工作得很好，有正确的色调颜色，没有键盘滑动工件。

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 ...

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)

iOS13场景支持(当使用UIWindowScene时)

import UIKit

private var windows: [String:UIWindow] = [:]

extension UIWindowScene {
    static var focused: UIWindowScene? {
        return UIApplication.shared.connectedScenes
            .first { $0.activationState == .foregroundActive && $0 is UIWindowScene } as? UIWindowScene
    }
}

class StyledAlertController: UIAlertController {

    var wid: String?

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

        //let window = UIWindow(frame: UIScreen.main.bounds)
        guard let window = UIWindowScene.focused.map(UIWindow.init(windowScene:)) else {
            return
        }
        window.rootViewController = UIViewController()
        window.windowLevel = .alert + 1
        window.makeKeyAndVisible()
        window.rootViewController!.present(self, animated: animated, completion: completion)

        wid = UUID().uuidString
        windows[wid!] = window
    }

    open override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        if let wid = wid {
            windows[wid] = nil
        }

    }

}