class ViewController: UIViewController {
    
    var userService: UserServiceInterface = UserService.shared
  
    func login(email: String, password: String, completion: ((Bool) -> Void)) {
        // ....
        userService.login(email, password, completion: { (success, error) in
            if success {
                completion(true)
            }
            completion(false)
        })
    }
}

// And make the Service implement the interface

protocol UserServiceInterface {
    func login(email: String, password: String, completion: (Bool, Error?))
}

class UserService: UserServiceInterface {
        // ....
}