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

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。

其他回答

如果有人感兴趣,我创建了一个Swift 3版本的@agilityvision答案。代码:

import Foundation
import UIKit

extension UIAlertController {

    var window: UIWindow? {
        get {
            return objc_getAssociatedObject(self, "window") as? UIWindow
        }
        set {
            objc_setAssociatedObject(self, "window", newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }

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

    func show(animated: Bool = true) {
        let window = UIWindow(frame: UIScreen.main.bounds)
        window.rootViewController = UIViewController(nibName: nil, bundle: nil)

        let delegate = UIApplication.shared.delegate
        if delegate?.window != nil {
            window.tintColor = delegate!.window!!.tintColor
        }

        window.windowLevel = UIApplication.shared.windows.last!.windowLevel + 1

        window.makeKeyAndVisible()
        window.rootViewController!.present(self, animated: animated, completion: nil)

        self.window = window
    }
}

@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)
    }
}

几个月前我发过一个类似的问题,我想我终于解决了这个问题。如果你只是想看代码,请点击我文章底部的链接。

解决方案是使用一个额外的UIWindow。

当你想要显示你的UIAlertController:

使你的窗口成为键和可见窗口(window. makekeyandvisible ()) 只需使用一个普通的UIViewController实例作为新窗口的rootViewController。(窗口。rootViewController = UIViewController()) 在你窗口的rootViewController上显示你的UIAlertController

有几点需要注意:

你的UIWindow必须是强引用的。如果它没有被强引用,它将永远不会出现(因为它已经被释放了)。我建议使用属性,但我也成功地使用了关联对象。 为了确保窗口显示在其他所有内容之上(包括系统UIAlertControllers),我设置了windowLevel。(窗口。windowLevel = UIWindowLevelAlert + 1)

最后,我有一个完整的实现,如果你只是想看看。

https://github.com/dbettermann/DBAlertController

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)
          }
        }
    }
}

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。