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

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)
          }
        }
    }
}

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。

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)

@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
  }
}

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);
    }
}