在一个非常罕见的情况下，使用自定义segue，最顶层的视图控制器不在导航堆栈或选项卡栏控制器中或被呈现，但它的视图被插入到关键窗口子视图的顶部。

在这种情况下，有必要检查UIApplication.shared.keyWindow.subviews.last == self。视图来确定当前视图控制器是否是最顶端的。

你可以在AppDelegate中定义一个UIViewController变量，并在每个viewWillAppear中设置这个变量为self。(然而，dianz的答案是最好的答案。)

override func viewWillAppear(animated: Bool) {
    super.viewWillAppear(animated)
    let appDel = UIApplication.sharedApplication().delegate as! AppDelegate
    appDel.currentVC = self
}

extension UIViewController {
    func topMostViewController() -> UIViewController {
        if self.presentedViewController == nil {
            return self
        }
        if let navigation = self.presentedViewController as? UINavigationController {
            return navigation.visibleViewController.topMostViewController()
        }
        if let tab = self.presentedViewController as? UITabBarController {
            if let selectedTab = tab.selectedViewController {
                return selectedTab.topMostViewController()
            }
            return tab.topMostViewController()
        }
        return self.presentedViewController!.topMostViewController()
    }
}

extension UIApplication {
    func topMostViewController() -> UIViewController? {
        return self.keyWindow?.rootViewController?.topMostViewController()
    }
}

presentViewController显示一个视图控制器。它不会返回一个视图控制器。如果你不使用UINavigationController，你可能正在寻找presenttedviewcontroller，你需要从根开始，向下迭代所呈现的视图。

if var topController = UIApplication.sharedApplication().keyWindow?.rootViewController {
    while let presentedViewController = topController.presentedViewController {
        topController = presentedViewController
    }

    // topController should now be your topmost view controller
}

对于Swift 3+:

if var topController = UIApplication.shared.keyWindow?.rootViewController {
    while let presentedViewController = topController.presentedViewController {
        topController = presentedViewController
    }

    // topController should now be your topmost view controller
}

适用于iOS 13+

let keyWindow = UIApplication.shared.windows.filter {$0.isKeyWindow}.first

if var topController = keyWindow?.rootViewController {
    while let presentedViewController = topController.presentedViewController {
        topController = presentedViewController
    }

// topController should now be your topmost view controller
}

在Swift 3中找到可见的viewController

if let viewControllers = window?.rootViewController?.childViewControllers {

     let prefs = UserDefaults.standard

     if viewControllers[viewControllers.count - 1] is ABCController{
        print("[ABCController] is visible")

     }
}

这段代码找到最后添加的或最后可见的活动控制器。

我已经在AppDelegate中使用它来查找活动视图控制器

import UIKit

extension UIApplication {

    // MARK: Choose keyWindow as per your choice
    var currentWindow: UIWindow? {
        connectedScenes
        .filter({$0.activationState == .foregroundActive})
        .map({$0 as? UIWindowScene})
        .compactMap({$0})
        .first?.windows
        .filter({$0.isKeyWindow}).first
    }

    // MARK: Choose keyWindow as per your choice
    var keyWindow: UIWindow? {
        UIApplication.shared.windows.first { $0.isKeyWindow }
    }

    class func topMostViewController(base: UIViewController? = UIApplication.shared.currentWindow?.rootViewController) -> UIViewController? {

        if let nav = base as? UINavigationController {
            return topMostViewController(base: nav.visibleViewController)
        }

        if let tab = base as? UITabBarController {
            let moreNavigationController = tab.moreNavigationController

            if let top = moreNavigationController.topViewController, top.view.window != nil {
                return topMostViewController(base: top)
            } else if let selected = tab.selectedViewController {
                return topMostViewController(base: selected)
            }
        }
        if let presented = base?.presentedViewController {
            return topMostViewController(base: presented)
        }
        return base
    }
}