swift 3 --- 获取顶层视图控制器

import UIKit

extension UIViewController {

static func topViewController(_ viewController: UIViewController? = nil) -> UIViewController? {
    let viewController = viewController ?? UIApplication.shared.keyWindow?.rootViewController
    
    if let navigationController = viewController as? UINavigationController,
        !navigationController.viewControllers.isEmpty
    {
        return self.topViewController(navigationController.viewControllers.last)
        
    } else if let tabBarController = viewController as? UITabBarController,
        let selectedController = tabBarController.selectedViewController
    {
        return self.topViewController(selectedController)
        
    } else if let presentedController = viewController?.presentedViewController {
        return self.topViewController(presentedController)
        
    }
    
    return viewController
}
}