ViewController-version1

ViewController.swift

class ViewController: UIViewController {
    
    // ....
    
    @IBAction func loginButtonPressed() {
        login(emailTextField.text ?? "", passwordTextField.text ?? "", completion: { success in
            if !success {
                showLoginError()
                return
            }
            goToApp()
        })
    }
    
    func login(email: String, password: String, completion: ((Bool) -> Void)) {
        if email.isEmpty {
            completion(false)
        } else if !isValidEmail(email) {
            completion(false)
        }
        
        if password.isEmpty {
            completion(false)
        }
        UserService.shared.login(email, password, completion: { (success, error) in
            if success {
                completion(true)
            }
            completion(false)
        })
    }
    
    // ....
    
}