Coordinator

BuyCoordinator.swift

class BuyCoordinator: Coordinator {
    
    var childCoordinators = [Coordinator]()
    weak var parentCoordinator: MainCoordinator?
    var navigationController: UINavigationController
    
    init(navigationController: UINavigationController) {
        self.navigationController = navigationController
    }
    
    func start(){
        let vc = BuyViewController.instantiate()
        vc.coordinator = self
        navigationController.pushViewController(vc, animated: true)
    }
    
    func didFinishBuying(){
        parentCoordinator?.childDidFinish(self)
    }
}

BuyViewController.swift

class BuyViewController : UIViewController, Storyboarded {
    
    weak var coordinator: BuyCoordinator?
     
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .cyan
    }
    
    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        coordinator?.didFinishBuying()
    }
    
}

Coordinator.swift

import UIKit

protocol Coordinator: AnyObject {
    var childCoordinators: [Coordinator] { get set }
    var navigationController: UINavigationController { get set }
    
    func start()
}

MainCoordinator.swift

import UIKit

class MainCoordinator: Coordinator {
    
    var childCoordinators = [Coordinator]()
    var navigationController: UINavigationController
    
    init(navigationController: UINavigationController) {
        self.navigationController = navigationController
    }
    
    func start(){
        let vc = ViewController.instantiate()
        vc.coordinator = self
        navigationController.pushViewController(vc, animated: false)
    }
    
    func buySubscription(){
        let child = BuyCoordinator(navigationController: navigationController)
        child.parentCoordinator = self
        childCoordinators.append(child)
        child.start()
    }
    
    func childDidFinish(_ child: Coordinator?){
        for (index, coordinator) in childCoordinators.enumerated() {
            if coordinator === child {
                childCoordinators.remove(at: index)
                break
            }
        }
    }
}

SceneDelegate.swift

import UIKit

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?
    var coordinator: MainCoordinator?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {

        guard let windowScene = (scene as? UIWindowScene) else { return }
        
        let navController = UINavigationController()
        coordinator = MainCoordinator(navigationController: navController)
        coordinator?.start()
        
        let appWindow = UIWindow(frame: windowScene.coordinateSpace.bounds)
        appWindow.windowScene = windowScene
        appWindow.rootViewController = navController
        appWindow.makeKeyAndVisible()
        
        window = appWindow
        
    }

}

Storyboarded.swift

import UIKit

protocol Storyboarded {
    static func instantiate() -> Self
}

extension Storyboarded where Self: UIViewController {
    static func instantiate() -> Self {
        let fullName = NSStringFromClass(self)
        let className = fullName.components(separatedBy: ".")[1]
        let storyboard = UIStoryboard(name: "Main", bundle: Bundle.main)
        return storyboard.instantiateViewController(withIdentifier: className) as! Self
    }
}

ViewController.swift

import UIKit

class ViewController: UIViewController, Storyboarded {

    weak var coordinator: MainCoordinator?
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }

    @IBAction func didTapNextButton(_ sender: Any) {
        coordinator?.buySubscription()
    }
   
}