class Autotextfields

class Autotextfields extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'ajax_example_autotextfields';
    }
    
    /**
     * {@inheritdoc}
     *
     * This form has two checkboxes which the user can check in order to then
     * reveal the first and/or last name text fields.
     *
     * We could perform this behavior with #states. We might not want to if, for
     * instance, we wanted to require a name, but let the user choose whether
     * to enter first or last or both.
     *
     * For all the requests this class gets, the buildForm() method will always be
     * called. If an AJAX request comes in, the form state will be set to the
     * state the user changed that caused the AJAX request. So if the user enabled
     * one of our checkboxes, it will be checked in $form_state.
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['description'] = [
            '#type' => 'item',
            '#markup' => $this->t('This form demonstrates changing the status of form elements through AJAX requests.'),
        ];
        $form['ask_first_name'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Ask me my first name'),
            '#ajax' => [
                'callback' => '::textfieldsCallback',
                'wrapper' => 'textfields-container',
                'effect' => 'fade',
            ],
        ];
        $form['ask_last_name'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Ask me my last name'),
            '#ajax' => [
                'callback' => '::textfieldsCallback',
                'wrapper' => 'textfields-container',
                'effect' => 'fade',
            ],
        ];
        // Wrap textfields in a container. This container will be replaced through
        // AJAX.
        $form['textfields_container'] = [
            '#type' => 'container',
            '#attributes' => [
                'id' => 'textfields-container',
            ],
        ];
        $form['textfields_container']['textfields'] = [
            '#type' => 'fieldset',
            '#title' => $this->t("Generated text fields for first and last name"),
            '#description' => $this->t('This is where we put automatically generated textfields'),
        ];
        // This form is rebuilt on all requests, so whether or not the request comes
        // from AJAX, we should rebuild everything based on the form state.
        // Checkbox values are expressed as 1 or 0, so we have to be sure to compare
        // type as well as value.
        if ($form_state->getValue('ask_first_name', NULL) === 1) {
            $form['textfields_container']['textfields']['first_name'] = [
                '#type' => 'textfield',
                '#title' => $this->t('First Name'),
                '#required' => TRUE,
            ];
        }
        if ($form_state->getValue('ask_last_name', NULL) === 1) {
            $form['textfields_container']['textfields']['last_name'] = [
                '#type' => 'textfield',
                '#title' => $this->t('Last Name'),
                '#required' => TRUE,
            ];
        }
        $form['textfields_container']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Click Me'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->messenger()
            ->addMessage($this->t('Submit handler: First name: @first_name Last name: @last_name', [
            '@first_name' => $form_state->getValue('first_name', 'n/a'),
            '@last_name' => $form_state->getValue('last_name', 'n/a'),
        ]));
    }
    
    /**
     * Callback for ajax_example_autotextfields.
     *
     * Selects the piece of the form we want to use as replacement markup and
     * returns it as a form (renderable array).
     */
    public function textfieldsCallback($form, FormStateInterface $form_state) {
        return $form['textfields_container'];
    }

}