Injectable.swift

protocol Injectable {
    associatedtype T
    func inject(_ parameters: T)
}

LoginFlowInjecter.swift

struct LoginFlowInjecter: Injecter {
    func prepare(for segue: UIStoryboardSegue, parameters: Injectee...) {
        switch segue.destination {
        case let vc as SignUpViewController:
            let parameter: InjectableUserManager = self.parameter(from: parameters)
            vc.inject(parameter)
        default:
            break
        }
    }
}

LoginViewController.swift

class LoginViewController: UIViewController {
    @IBOutlet weak var emailTextfield: UITextField!
    @IBOutlet weak var passwordTextfield: UITextField!
    
    private var userManager: InjectableUserManager?
    private let router = LoginFlowInjecter()
    
    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.
    }
    
    @IBAction func login(_ sender: Any) {
        guard let email = emailTextfield.text, let password = passwordTextfield.text else { return }
        userManager?.signIn(withEmail: email, password: password, completion: { success in
            // Hooray!
        })
    }
    
    @IBAction func signup(_ sender: Any) {
        
    }
    
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        // TODO: prepare is a variadic func
        guard let userManager = self.userManager else { return }
        router.prepare(for: segue, parameters: userManager)
    }
}

Routable.swift

protocol Routable {
    func prepare(for segue: UIStoryboardSegue, parameters: Injectee...)
}

extension Routable {
    func parameter<T>(from parameters: [Injectee]) -> T {
        for someParameter in parameters {
            if let parameter = someParameter as? T {
                return parameter
            }
        }
        fatalError("Can not find injectable parameter")
    }
}

UserManager.swift

protocol InjectableUserManager: Injectee {
    func signIn(withEmail email: String, password: String, completion: @escaping (Result<Bool, UserError>) -> Void)
    func signUp(withName name: String, email: String, password: String, completion: @escaping (Result<User, UserError>) -> Void)
}

class UserManager: InjectableUserManager {
  
}