除了给出的很好的答案(agilityvision, adib, malhal)。为了达到像以前的UIAlertViews那样的排队行为(避免警报窗口重叠)，使用这个块来观察窗口级别的可用性:

@interface UIWindow (WLWindowLevel)

+ (void)notifyWindowLevelIsAvailable:(UIWindowLevel)level withBlock:(void (^)())block;

@end

@implementation UIWindow (WLWindowLevel)

+ (void)notifyWindowLevelIsAvailable:(UIWindowLevel)level withBlock:(void (^)())block {
    UIWindow *keyWindow = [UIApplication sharedApplication].keyWindow;
    if (keyWindow.windowLevel == level) {
        // window level is occupied, listen for windows to hide
        id observer;
        observer = [[NSNotificationCenter defaultCenter] addObserverForName:UIWindowDidBecomeHiddenNotification object:keyWindow queue:nil usingBlock:^(NSNotification *note) {
            [[NSNotificationCenter defaultCenter] removeObserver:observer];
            [self notifyWindowLevelIsAvailable:level withBlock:block]; // recursive retry
        }];

    } else {
        block(); // window level is available
    }
}

@end

完整的例子:

[UIWindow notifyWindowLevelIsAvailable:UIWindowLevelAlert withBlock:^{
    UIWindow *alertWindow = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    alertWindow.windowLevel = UIWindowLevelAlert;
    alertWindow.rootViewController = [UIViewController new];
    [alertWindow makeKeyAndVisible];

    UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"Alert" message:nil preferredStyle:UIAlertControllerStyleAlert];
    [alertController addAction:[UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleCancel handler:^(UIAlertAction *action) {
        alertWindow.hidden = YES;
    }]];

    [alertWindow.rootViewController presentViewController:alertController animated:YES completion:nil];
}];

这将允许您避免警报窗口重叠。同样的方法可以用于为任意数量的窗口层分离和放入队列视图控制器。

这在Swift中适用于普通的视图控制器，即使屏幕上有一个导航控制器:

let alert = UIAlertController(...)

let alertWindow = UIWindow(frame: UIScreen.main.bounds)
alertWindow.rootViewController = UIViewController()
alertWindow.windowLevel = UIWindowLevelAlert + 1;
alertWindow.makeKeyAndVisible()
alertWindow.rootViewController?.presentViewController(alert, animated: true, completion: nil)

更新到与iOS 13场景，打破了新的UIWindow方法。斯威夫特5.1。

fileprivate var alertWindows = [UIAlertController:UIWindow]()

extension UIAlertController {

    func presentInNewWindow(animated: Bool, completion: (() -> Void)?) {
        let foregroundActiveScene = UIApplication.shared.connectedScenes.filter { $0.activationState == .foregroundActive }.first
        guard let foregroundWindowScene = foregroundActiveScene as? UIWindowScene else { return }

        let window = UIWindow(windowScene: foregroundWindowScene)
        alertWindows[self] = window

        window.rootViewController = UIViewController()
        window.windowLevel = .alert + 1
        window.makeKeyAndVisible()
        window.rootViewController!.present( self, animated: animated, completion: completion)
    }

    open override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        alertWindows[self] = nil
    }

}

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)
          }
        }
    }
}

在Objective-C中显示警报的简单方法:

[[[[UIApplication sharedApplication] keyWindow] rootViewController] presentViewController:alertController animated:YES completion:nil];

alertController是你的UIAlertController对象。

注意:你还需要确保你的助手类扩展了UIViewController

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。