创建helper类AlertWindow并使用as

let alertWindow = AlertWindow();
let alert = UIAlertController(title: "Hello", message: "message", preferredStyle: .alert);
let cancel = UIAlertAction(title: "Ok", style: .cancel){(action) in

    //....  action code here

    // reference to alertWindow retain it. Every action must have this at end

    alertWindow.isHidden = true;

   //  here AlertWindow.deinit{  }

}
alert.addAction(cancel);
alertWindow.present(alert, animated: true, completion: nil)


class AlertWindow:UIWindow{

    convenience init(){
        self.init(frame:UIScreen.main.bounds);
    }

    override init(frame: CGRect) {
        super.init(frame: frame);
        if let color = UIApplication.shared.delegate?.window??.tintColor {
            tintColor = color;
        }
        rootViewController = UIViewController()
        windowLevel = UIWindowLevelAlert + 1;
        makeKeyAndVisible()
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    deinit{
        //  semaphor.signal();
    }

    func present(_ ctrl:UIViewController, animated:Bool, completion: (()->Void)?){
        rootViewController!.present(ctrl, animated: animated, completion: completion);
    }
}

在WWDC上，我在一个实验室停下来，问了一个苹果工程师同样的问题:“显示UIAlertController的最佳实践是什么?”他说他们经常被问到这个问题，我们开玩笑说他们应该就此开个会。他说苹果内部创建了一个带有透明UIViewController的UIWindow，然后在上面呈现UIAlertController。基本上就是迪伦·贝特曼的答案。

但我不想使用UIAlertController的子类，因为这将需要我在整个应用程序中更改我的代码。因此，在关联对象的帮助下，我在UIAlertController上创建了一个类别，它在Objective-C中提供了一个显示方法。

以下是相关代码:

#import "UIAlertController+Window.h"
#import <objc/runtime.h>

@interface UIAlertController (Window)

- (void)show;
- (void)show:(BOOL)animated;

@end

@interface UIAlertController (Private)

@property (nonatomic, strong) UIWindow *alertWindow;

@end

@implementation UIAlertController (Private)

@dynamic alertWindow;

- (void)setAlertWindow:(UIWindow *)alertWindow {
    objc_setAssociatedObject(self, @selector(alertWindow), alertWindow, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (UIWindow *)alertWindow {
    return objc_getAssociatedObject(self, @selector(alertWindow));
}

@end

@implementation UIAlertController (Window)

- (void)show {
    [self show:YES];
}

- (void)show:(BOOL)animated {
    self.alertWindow = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    self.alertWindow.rootViewController = [[UIViewController alloc] init];

    id<UIApplicationDelegate> delegate = [UIApplication sharedApplication].delegate;
    // Applications that does not load with UIMainStoryboardFile might not have a window property:
    if ([delegate respondsToSelector:@selector(window)]) {
        // we inherit the main window's tintColor
        self.alertWindow.tintColor = delegate.window.tintColor;
    }

    // window level is above the top window (this makes the alert, if it's a sheet, show over the keyboard)
    UIWindow *topWindow = [UIApplication sharedApplication].windows.lastObject;
    self.alertWindow.windowLevel = topWindow.windowLevel + 1;

    [self.alertWindow makeKeyAndVisible];
    [self.alertWindow.rootViewController presentViewController:self animated:animated completion:nil];
}

- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];
    
    // precaution to ensure window gets destroyed
    self.alertWindow.hidden = YES;
    self.alertWindow = nil;
}

@end

下面是一个用法示例:

// need local variable for TextField to prevent retain cycle of Alert otherwise UIWindow
// would not disappear after the Alert was dismissed
__block UITextField *localTextField;
UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"Global Alert" message:@"Enter some text" preferredStyle:UIAlertControllerStyleAlert];
[alert addAction:[UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:^(UIAlertAction *action) {
    NSLog(@"do something with text:%@", localTextField.text);
// do NOT use alert.textfields or otherwise reference the alert in the block. Will cause retain cycle
}]];
[alert addTextFieldWithConfigurationHandler:^(UITextField *textField) {
    localTextField = textField;
}];
[alert show];

当UIAlertController被释放时，创建的UIWindow将被销毁，因为它是唯一保留UIWindow的对象。但是如果你将UIAlertController分配给一个属性，或者通过在其中一个动作块中访问警报而导致其保留计数增加，UIWindow将停留在屏幕上，锁定你的UI。请参阅上面的示例使用代码，以避免在需要访问UITextField的情况下。

我用一个测试项目FFGlobalAlertController做了一个GitHub回购

Kevin Sliech提供了一个很好的解决方案。

我现在在我的主UIViewController子类中使用下面的代码。

我做的一个小改动是检查最好的表示控制器是不是一个普通的UIViewController。如果不是，那就一定是某个VC表示一个普通VC。因此，我们返回正在呈现的VC。

- (UIViewController *)bestPresentationController
{
    UIViewController *bestPresentationController = [UIApplication sharedApplication].keyWindow.rootViewController;

    if (![bestPresentationController isMemberOfClass:[UIViewController class]])
    {
        bestPresentationController = bestPresentationController.presentedViewController;
    }    

    return bestPresentationController;
}

在我的测试中似乎一切正常。

谢谢你，凯文!

extension UIApplication {
    /// The top most view controller
    static var topMostViewController: UIViewController? {
        return UIApplication.shared.keyWindow?.rootViewController?.visibleViewController
    }
}

extension UIViewController {
    /// The visible view controller from a given view controller
    var visibleViewController: UIViewController? {
        if let navigationController = self as? UINavigationController {
            return navigationController.topViewController?.visibleViewController
        } else if let tabBarController = self as? UITabBarController {
            return tabBarController.selectedViewController?.visibleViewController
        } else if let presentedViewController = presentedViewController {
            return presentedViewController.visibleViewController
        } else {
            return self
        }
    }
}

有了这个，你可以很容易地呈现你的警告，就像这样

UIApplication.topMostViewController?.present(viewController, animated: true, completion: nil)

需要注意的一件事是，如果有一个UIAlertController当前正在显示，UIApplication。topMostViewController将返回一个UIAlertController。在UIAlertController的顶部呈现有奇怪的行为，应该避免。因此，你应该手动检查!topMostViewController is UIAlertController)，或者如果self是UIAlertController，添加一个else if case返回nil

extension UIViewController {
    /// The visible view controller from a given view controller
    var visibleViewController: UIViewController? {
        if let navigationController = self as? UINavigationController {
            return navigationController.topViewController?.visibleViewController
        } else if let tabBarController = self as? UITabBarController {
            return tabBarController.selectedViewController?.visibleViewController
        } else if let presentedViewController = presentedViewController {
            return presentedViewController.visibleViewController
        } else if self is UIAlertController {
            return nil
        } else {
            return self
        }
    }
}

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。

为了改进敏捷视觉的答案，你需要创建一个带有透明根视图控制器的窗口，并从那里显示警报视图。

然而，只要在警报控制器中有一个动作，就不需要保持对窗口的引用。作为动作处理程序块的最后一步，您只需要将窗口隐藏为清理任务的一部分。通过在处理程序块中有一个对窗口的引用，这将创建一个临时的循环引用，一旦警报控制器被解除，该引用将被打破。

UIWindow* window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
window.rootViewController = [UIViewController new];
window.windowLevel = UIWindowLevelAlert + 1;

UIAlertController* alertCtrl = [UIAlertController alertControllerWithTitle:... message:... preferredStyle:UIAlertControllerStyleAlert];

[alertCtrl addAction:[UIAlertAction actionWithTitle:NSLocalizedString(@"OK",@"Generic confirm") style:UIAlertActionStyleCancel handler:^(UIAlertAction * _Nonnull action) {
    ... // do your stuff

    // very important to hide the window afterwards.
    // this also keeps a reference to the window until the action is invoked.
    window.hidden = YES;
}]];

[window makeKeyAndVisible];
[window.rootViewController presentViewController:alertCtrl animated:YES completion:nil];