Topmost view controller

UIApplication.swift

import UIKit

public extension UIApplication {
    /// Returns the topmost view controller in the hierarchy of the given view controller.
    ///
    /// If no parameter passed, by default the function takes the root view controller set for the key window in the application.
    ///
    /// ```swift
    /// UIApplication.shared.keyWindow?.rootViewController
    /// ```
    ///
    /// - Parameter rootViewController: The root view controller
    /// - Returns: The topmost view controller
    func topViewController(
        for rootViewController: UIViewController? = UIApplication.shared.keyWindow?.rootViewController
        ) -> UIViewController? {
        guard let rootViewController = rootViewController else {
            return .none
        }

        if let navigationController = rootViewController as? UINavigationController {
            return topViewController(for: navigationController.viewControllers.last)
        }

        if let tabbarController = rootViewController as? UITabBarController {
            return topViewController(for: tabbarController.selectedViewController)
        }

        if let presentedViewController = rootViewController.presentedViewController {
            return topViewController(for: presentedViewController)
        }

        return rootViewController
    }
}