场景:用户点击视图控制器上的按钮。视图控制器是导航堆栈中最顶层的(很明显)。tap调用在另一个类上调用的实用程序类方法。这里发生了不好的事情我想在控件返回到视图控制器之前在那里显示一个警告。

+ (void)myUtilityMethod {
    // do stuff
    // something bad happened, display an alert.
}

这是可能的UIAlertView(但可能不太合适)。

在这种情况下,你如何在myUtilityMethod中呈现UIAlertController ?


当前回答

@agilityvision的答案翻译为Swift4/iOS11。我没有使用本地化字符串,但你可以很容易地改变:

import UIKit

/** An alert controller that can be called without a view controller.
 Creates a blank view controller and presents itself over that
 **/
class AlertPlusViewController: UIAlertController {

    private var alertWindow: UIWindow?

    override func viewDidLoad() {
        super.viewDidLoad()
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        self.alertWindow?.isHidden = true
        alertWindow = nil
    }

    func show() {
        self.showAnimated(animated: true)
    }

    func showAnimated(animated _: Bool) {

        let blankViewController = UIViewController()
        blankViewController.view.backgroundColor = UIColor.clear

        let window = UIWindow(frame: UIScreen.main.bounds)
        window.rootViewController = blankViewController
        window.backgroundColor = UIColor.clear
        window.windowLevel = UIWindowLevelAlert + 1
        window.makeKeyAndVisible()
        self.alertWindow = window

        blankViewController.present(self, animated: true, completion: nil)
    }

    func presentOkayAlertWithTitle(title: String?, message: String?) {

        let alertController = AlertPlusViewController(title: title, message: message, preferredStyle: .alert)
        let okayAction = UIAlertAction(title: "Ok", style: .default, handler: nil)
        alertController.addAction(okayAction)
        alertController.show()
    }

    func presentOkayAlertWithError(error: NSError?) {
        let title = "Error"
        let message = error?.localizedDescription
        presentOkayAlertWithTitle(title: title, message: message)
    }
}

其他回答

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 ...

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];

补充Zev的回答(并切换回Objective-C),你可能会遇到这样的情况,你的根视图控制器通过segue或其他东西呈现其他VC。在根VC上调用presenttedviewcontroller会处理这个:

[[UIApplication sharedApplication].keyWindow.rootViewController.presentedViewController presentViewController:alertController animated:YES completion:^{}];

这解决了一个问题,我有根VC已经segue到另一个VC,而不是显示警报控制器,像上面报告的警告发出:

Warning: Attempt to present <UIAlertController: 0x145bfa30> on <UINavigationController: 0x1458e450> whose view is not in the window hierarchy!

我还没有测试它,但如果你的根VC恰好是一个导航控制器,这可能也是必要的。

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)

@agilityvision的回答非常好。我有在swift项目中使用的感觉,所以我想我将分享我使用swift 3.0的答案

fileprivate class MyUIAlertController: UIAlertController {

  typealias Handler = () -> Void

  struct AssociatedKeys {
    static var alertWindowKey = "alertWindowKey"
  }

  dynamic var _alertWindow: UIWindow?

  var alertWindow: UIWindow? {
    return objc_getAssociatedObject(self, &AssociatedKeys.alertWindowKey) as? UIWindow
  }


  func setAlert(inWindow window: UIWindow) {
    objc_setAssociatedObject(self, &AssociatedKeys.alertWindowKey, _alertWindow, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
  }

  func show(completion: Handler? = nil) {
    show(animated: true, completion: completion)
  }

  func show(animated: Bool, completion: Handler? =  nil) {
    _alertWindow = UIWindow(frame: UIScreen.main.bounds)
    _alertWindow?.rootViewController = UIViewController()

    if let delegate: UIApplicationDelegate = UIApplication.shared.delegate, let window = delegate.window {
      _alertWindow?.tintColor = window?.tintColor

    }

    let topWindow = UIApplication.shared.windows.last
    _alertWindow?.windowLevel = topWindow?.windowLevel ?? 0 + 1
    _alertWindow?.makeKeyAndVisible()
    _alertWindow?.rootViewController?.present(self, animated: animated, completion: completion)
  }

  fileprivate override func viewDidDisappear(_ animated: Bool) {
    super.viewDidDisappear(animated)
    _alertWindow?.isHidden = true
    _alertWindow = nil
  }
}