[iOS] top ViewController 얻기

extension UIApplication {
    class func topViewController(base: UIViewController? = UIApplication.shared.keyWindow?.rootViewController) -> UIViewController? {
        if let nav = base as? UINavigationController {
            return topViewController(base: nav.visibleViewController)
        }

        if let tab = base as? UITabBarController {
            if let selected = tab.selectedViewController
            { return topViewController(base: selected) }
        }

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