DelegateLoginExample.swift

// MARK: LoginViewController

class LoginViewController: UIViewController, AuthenticationSessionDelegate {

    func login(with username: String,
               password: String) {
        let authenticationSession = AuthenticationSession()
        authenticationSession.delegate = self
        authenticationSession.performLogin(with: username,
                                           password: password)
    }

    // MARK: AuthenticationSessionDelegate Methods
    
    func authenticationSession(_ authenticationSession: AuthenticationSession, didLoginWith userIdentifier: String) {
        // Handle the login with userIdentifier
    }
}

// MARK: AuthenticationSessionDelegate

protocol AuthenticationSessionDelegate: class {
    func authenticationSession(_ authenticationSession: AuthenticationSession, didLoginWith userIdentifier: String)
}

// MARK: AuthenticationSession

class AuthenticationSession {

    var delegate: AuthenticationSessionDelegate?

    func performLogin(with username: String,
                      password: String) {
        let userIdentifier = "12345" //Service.login(username, password)
        delegate?.authenticationSession(self, didLoginWith: userIdentifier)
    }
}