let username = usernameField.text ?? ""
        let password = passwordField.text ?? ""

        PFUser.logInWithUsername(inBackground: username, password: password) { (user: PFUser?, error: Error?) in
            if let error = error {
                print("User log in failed: \(error.localizedDescription)")
            } else {
                print("User logged in successfully")
                // display view controller that needs to shown after successful login
                self.performSegue(withIdentifier: "loginSegue", sender: nil)
            }
        }