GroupedEncapsulation.swift

extension UINavigationController {

    // - MARK: Extracted reusable methods so that implementation changes reflect wherever used. 

    func viewController() {
        navigationController.pushViewController(ViewController.instantiate(), animated: false)
    }
    
    func buySubscription() {
        navigationController.pushViewController(BuyViewController.instantiate(), animated: true)
    }

    func createAccount() {
        navigationController.pushViewController(CreateAccountViewController.instantiate(), animated: true)
    }
}

// ---- 
// call site: 

class ViewController: UIViewController {
    
    @IBAction func buyTapped(_ sender: Any) {
        navigationController?.buySubscription()
    }

    @IBAction func createAccount(_ sender: Any) {
        navigationController?.createAccount()
    }
}