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

交叉张贴我的答案，因为这两个线程没有被标记为欺骗…

现在UIViewController是responder chain的一部分，你可以这样做:

if let vc = self.nextResponder()?.targetForAction(#selector(UIViewController.presentViewController(_:animated:completion:)), withSender: self) as? UIViewController {

    let alert = UIAlertController(title: "A snappy title", message: "Something bad happened", preferredStyle: .Alert)
    alert.addAction(UIAlertAction(title: "OK", style: .Default, handler: nil))

    vc.presentViewController(alert, animated: true, completion: nil)
}

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。

斯威夫特5

在显示消息后隐藏窗口是很重要的。

func showErrorMessage(_ message: String) {
    let alertWindow = UIWindow(frame: UIScreen.main.bounds)
    alertWindow.rootViewController = UIViewController()

    let alertController = UIAlertController(title: "Error", message: message, preferredStyle: UIAlertController.Style.alert)
    alertController.addAction(UIAlertAction(title: "Close", style: UIAlertAction.Style.cancel, handler: { _ in
        alertWindow.isHidden = true
    }))
    
    alertWindow.windowLevel = UIWindow.Level.alert + 1;
    alertWindow.makeKeyAndVisible()
    alertWindow.rootViewController?.present(alertController, animated: true, completion: nil)
}

你可以用Swift 2.2做以下事情:

let alertController: UIAlertController = ...
UIApplication.sharedApplication().keyWindow?.rootViewController?.presentViewController(alertController, animated: true, completion: nil)

Swift 3.0:

let alertController: UIAlertController = ...
UIApplication.shared.keyWindow?.rootViewController?.present(alertController, animated: true, completion: nil)

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