基于Dianz的答案，Objective-C版本

- (UIViewController *) topViewController {
   UIViewController *baseVC = UIApplication.sharedApplication.keyWindow.rootViewController;
   if ([baseVC isKindOfClass:[UINavigationController class]]) {
       return ((UINavigationController *)baseVC).visibleViewController;
   }

   if ([baseVC isKindOfClass:[UITabBarController class]]) {
       UIViewController *selectedTVC = ((UITabBarController*)baseVC).selectedViewController;
       if (selectedTVC) {
           return selectedTVC;
       }
   }

   if (baseVC.presentedViewController) {
       return baseVC.presentedViewController;
   }
   return baseVC;
}

我使用下面的代码来获得topViewController-

它向后兼容旧的iOS版本，并照顾UIScene太

extension UIApplication {
    func topViewController() -> UIViewController? {
        var topViewController: UIViewController? = nil
        if #available(iOS 13, *) {
            for scene in connectedScenes {
                if let windowScene = scene as? UIWindowScene {
                    for window in windowScene.windows {
                        if window.isKeyWindow {
                            topViewController = window.rootViewController
                        }
                    }
                }
            }
        } else {
            topViewController = keyWindow?.rootViewController
        }
        while true {
            if let presented = topViewController?.presentedViewController {
                topViewController = presented
            } else if let navController = topViewController as? UINavigationController {
                topViewController = navController.topViewController
            } else if let tabBarController = topViewController as? UITabBarController {
                topViewController = tabBarController.selectedViewController
            } else {
                // Handle any other third party container in `else if` if required
                break
            }
        }
        return topViewController
    }
}

它可以这样使用:

let topController = UIApplication.shared.topViewController()
topController?.present(controllerToPresent, animated: true, completion: nil)

    extension UIApplication {
        
       public var mainKeyWindow: UIWindow? {
           if #available(iOS 13, *) {
             return UIApplication.shared.connectedScenes
            .filter { $0.activationState == .foregroundActive }
            .first(where: { $0 is UIWindowScene })
            .flatMap { $0 as? UIWindowScene }?.windows
            .first(where: \.isKeyWindow)
           } else {
              return UIApplication.shared.windows.first { $0.isKeyWindow }
           }
        }
    

    public var rootViewController: UIViewController? {
        guard let keyWindow = UIApplication.shared.mainKeyWindow, let rootViewController = keyWindow.rootViewController else {
            return nil
        }
        return rootViewController
    }

    public func topViewController(controller: UIViewController? = UIApplication.shared.rootViewController) -> UIViewController? {

        if controller == nil {
            return topViewController(controller: rootViewController)
        }
        
        if let navigationController = controller as? UINavigationController {
            return topViewController(controller: navigationController.visibleViewController)
        }

        if let tabController = controller as? UITabBarController {
            if let selectedViewController = tabController.selectedViewController {
                return topViewController(controller: selectedViewController)
            }
        }

        if let presentedViewController = controller?.presentedViewController {
            return topViewController(controller: presentedViewController)
        }

        return controller
    }
}

Swift 5.2及以上版本

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 UIWindow {

    func visibleViewController() -> UIViewController? {
        if let rootViewController: UIViewController = self.rootViewController {
            return UIWindow.getVisibleViewControllerFrom(vc: rootViewController)
        }
        return nil
    }

    static func getVisibleViewControllerFrom(vc:UIViewController) -> UIViewController {
        if let navigationController = vc as? UINavigationController,
            let visibleController = navigationController.visibleViewController  {
            return UIWindow.getVisibleViewControllerFrom( vc: visibleController )
        } else if let tabBarController = vc as? UITabBarController,
            let selectedTabController = tabBarController.selectedViewController {
            return UIWindow.getVisibleViewControllerFrom(vc: selectedTabController )
        } else {
            if let presentedViewController = vc.presentedViewController {
                return UIWindow.getVisibleViewControllerFrom(vc: presentedViewController)
            } else {
                return vc
            }
        }
    }
}

用法:

if let topController = window.visibleViewController() {
    println(topController)
}

  var topViewController: UIViewController? {
        guard var topViewController = UIApplication.shared.keyWindow?.rootViewController else { return nil }
        while let presentedViewController = topViewController.presentedViewController {
            topViewController = presentedViewController
        }
        return topViewController
    }