AJGetTopViewControllerTool

如何获取当前屏幕中显示的viewController？
AJGetTopViewControllerTool.swift 文件定义：

import UIKit

class AJGetTopViewControllerTool: NSObject {

    @objc class func topViewController() -> UIViewController? {
        return self.topViewControllerWithRootViewController(viewController: self.getCurrentWindow()?.rootViewController)
    }
    
    @objc class func topViewControllerWithRootViewController(viewController :UIViewController?) -> UIViewController? {
        
        if viewController == nil {
            return nil
        }
        
        if viewController?.presentedViewController != nil {
            /* 递归找最上面的viewController */
            return self.topViewControllerWithRootViewController(viewController: viewController?.presentedViewController!)
        }
        else if viewController?.isKind(of: UITabBarController.self) == true {
            return self.topViewControllerWithRootViewController(viewController: (viewController as! UITabBarController).selectedViewController)
        }
        else if viewController?.isKind(of: UINavigationController.self) == true {
            return self.topViewControllerWithRootViewController(viewController: (viewController as! UINavigationController).visibleViewController)
        }
        else {
            return viewController
        }
        
    }
    
    // 找到当前显示的window
    class func getCurrentWindow() -> UIWindow? {
        // 找到当前显示的UIWindow
        var window: UIWindow? = UIApplication.shared.keyWindow
        /**
         window有一个属性：windowLevel
         当 windowLevel == UIWindowLevelNormal 的时候，表示这个window是当前屏幕正在显示的window
         */
        if window?.windowLevel != UIWindowLevelNormal {
            for tempWindow in UIApplication.shared.windows {
                if tempWindow.windowLevel == UIWindowLevelNormal {
                    window = tempWindow
                    break
                }
            }
        }
        return window
    }

}