Swift 4+ / iOS 13:

基于https://stackoverflow.com/a/47797463/7493938，它使用了iOS 13已弃用的。keywindow方法。

extension UIAlertController {
  func show(animated: Bool = true, completion: (() -> Void)? = nil) {
    if let visibleViewController = UIApplication.shared.windows.filter({$0.isKeyWindow}).first?.visibleViewController {
      visibleViewController.present(self, animated: animated, completion: completion)
    }
  }
}

(我无法编辑原始答案，因为编辑队列已满。

你可以尝试在UIViewController上用一个方法来实现一个类别 - (void)presentErrorMessage;，在这个方法中你实现了UIAlertController，然后在self上呈现它。然后在你的客户端代码中，你会有这样的东西:

[myViewController presentErrorMessage];

这样可以避免不必要的参数和关于视图不在窗口层次结构中的警告。

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")
})

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。

我在我的AppDelegate类中使用了这段代码和一些个人变化

-(UIViewController*)presentingRootViewController
{
    UIViewController *vc = self.window.rootViewController;
    if ([vc isKindOfClass:[UINavigationController class]] ||
        [vc isKindOfClass:[UITabBarController class]])
    {
        // filter nav controller
        vc = [AppDelegate findChildThatIsNotNavController:vc];
        // filter tab controller
        if ([vc isKindOfClass:[UITabBarController class]]) {
            UITabBarController *tbc = ((UITabBarController*)vc);
            if ([tbc viewControllers].count > 0) {
                vc = [tbc viewControllers][tbc.selectedIndex];
                // filter nav controller again
                vc = [AppDelegate findChildThatIsNotNavController:vc];
            }
        }
    }
    return vc;
}
/**
 *   Private helper
 */
+(UIViewController*)findChildThatIsNotNavController:(UIViewController*)vc
{
    if ([vc isKindOfClass:[UINavigationController class]]) {
        if (((UINavigationController *)vc).viewControllers.count > 0) {
            vc = [((UINavigationController *)vc).viewControllers objectAtIndex:0];
        }
    }
    return vc;
}

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
        }

    }

}