当应用程序进入前台时显示视图控制器，但不知道在 Swift 4 中关闭此视图控制器

Question

我想在用户使用密码身份验证后转到当前打开的视图控制器。

这是我在AppDelegate中的(EnterForeground(上的代码：

     func applicationWillEnterForeground(_ application: UIApplication) {
    // Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background.
    let stb = UIStoryboard(name: "Main", bundle: nil)
    let ps = stb.instantiateViewController(withIdentifier: "psID") as! PassC_VC
    let appdelegate = UIApplication.shared.delegate as! AppDelegate
    self.window?.makeKeyAndVisible()
    DispatchQueue.global().async {
        DispatchQueue.main.async {
            ps.modalPresentationStyle = UIModalPresentationStyle.currentContext
            appdelegate.window?.rootViewController = ps
            print("in forground")
        }
    }
}

此代码成功地工作并显示此窗口，但是解散此控制器并显示当前活动视图控制器的问题，该视图控制器是在显示Passcode View Controller之前打开的。

这是我的passc_vc.swift代码：加法信息：我不使用任何Uinavigation Controller或其他东西

输入密码后，按钮单击

self.dismiss(animated: true, completion: nil)

Answer 1

您只能解雇控制器您 pranister 。您没有出现" PS"，而是设置为window rootviewController。这意味着这是屏幕上的第一个控制器，因此解散它是没有意义的。

要回到屏幕上的控制器，您可以做以下操作之一：

选项1：查看控制器参考

当您的应用程序进入前面时，请在AppDelegate中对该控制器进行引用：

func applicationWillEnterForeground(_ application: UIApplication) {
    // Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background.
    let stb = UIStoryboard(name: "Main", bundle: nil)
    let ps = stb.instantiateViewController(withIdentifier: "psID") as! PassC_VC
    let appdelegate = UIApplication.shared.delegate as! AppDelegate
    self.window?.makeKeyAndVisible()
    DispatchQueue.global().async {
        DispatchQueue.main.async {
            ps.modalPresentationStyle = UIModalPresentationStyle.currentContext
            if let delegate = UIApplication.shared.delegate as? AppDelegate {
                //Storing the reference
                delegate.yourReferenceToPreviousController = delegate.window?.rootViewController
                //Assigning the new controller
                delegate.window?.rootViewController = ps
                print("in forground")
            }
        }
    }
}

和，在输入密码后单击"确定按钮"执行：

if let delegate = UIApplication.shared.delegate as? AppDelegate {
    //You can now use your stored reference to reassign the root view controller
    delegate.window?.rootViewController = delegate.yourReferenceToPreviousController
}

选项2：重新呈现先前的视图控制器

，或者您可以避免对上一个控制器进行引用，并通过执行"输入密码后的确定按钮单击"来重新确定它：

let previousController = PreviousController() //This is in code but you can initialize it via storyboard
self.present(previousController, animated: true)

选项3：正确呈现/关闭视图控制器

另一种方法是通过实际获取VisibleController并使他像这样的密码控制器来改变您显示控制器的方式：

您可以使用此扩展名来获取可见的控制器：

extension UIWindow {
    func visibleViewController() -> UIViewController? {
        if let rootViewController: UIViewController = self.rootViewController {
            return UIWindow.getVisibleViewControllerFrom(vc: rootViewController)
        }
        return nil
    }
    private class func getVisibleViewControllerFrom(vc:UIViewController) -> UIViewController {
        if vc.isKind(of: UINavigationController.self) {
            let navigationController = vc as? UINavigationController
            return UIWindow.getVisibleViewControllerFrom(vc: navigationController!.visibleViewController!)
        } else if vc.isKind(of: UITabBarController.self) {
            let tabBarController = vc as? UITabBarController
            return UIWindow.getVisibleViewControllerFrom(vc: tabBarController!.selectedViewController!)
        } else {
            if let presentedViewController = vc.presentedViewController {
                return UIWindow.getVisibleViewControllerFrom(vc: presentedViewController)
            } else {
                return vc
            }
        }
    }
}

然后在应用程序意义前绑定：

func applicationWillEnterForeground(_ application: UIApplication) {
    // Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background.
    let stb = UIStoryboard(name: "Main", bundle: nil)
    let ps = stb.instantiateViewController(withIdentifier: "psID") as! PassC_VC
    let appdelegate = UIApplication.shared.delegate as! AppDelegate
    self.window?.makeKeyAndVisible()
    DispatchQueue.global().async {
        DispatchQueue.main.async {
            ps.modalPresentationStyle = UIModalPresentationStyle.currentContext
            //You get the controller that is on screen
            let visibleViewController = UIWindow.getVisibleViewControllerFrom(vc: window?.rootViewController!)
            //Then make it present the controller you want
            visibleViewController.present(ps, animated: true)
            print("in forground")
        }
    }
}

这样，您实际上是在显示控制器，因此您可以：

self.dismiss(animated: true, completion: nil)

就像您想要的一样。

希望它有帮助。