MultipleCompletionHandlers.swift

        fetchJSON(fromURL: "someURL") { (response, results, error) in
            switch response {
            case .success:
                fetchUserData(for: results) { (response, results, error) in
                    switch response {
                    case .success:
                        loginUser(user: results) { (response, results, error) in
                            switch response {
                            case .success:
                                // finialy all of the callbacks are finished
                                print("Handle logic here")
                            case .fail:
                                print(error)
                            }
                        }
                    case .fail:
                        print(error)
                    }
                }
            case .fail:
                print(error)
            }
        }