class AjaxAddMore

class AjaxAddMore extends DemoBase {
    
    /**
     * Form with 'add more' and 'remove' buttons.
     *
     * This example shows a button to "add more" - add another textfield, and
     * the corresponding "remove" button.
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['description'] = [
            '#type' => 'item',
            '#markup' => $this->t('This example shows an add-more and a remove-last button.'),
        ];
        // Gather the number of names in the form already.
        $num_names = $form_state->get('num_names');
        // We have to ensure that there is at least one name field.
        if ($num_names === NULL) {
            $name_field = $form_state->set('num_names', 1);
            $num_names = 1;
        }
        $form['#tree'] = TRUE;
        $form['names_fieldset'] = [
            '#type' => 'fieldset',
            '#title' => $this->t('People coming to picnic'),
            '#prefix' => '<div id="names-fieldset-wrapper">',
            '#suffix' => '</div>',
        ];
        for ($i = 0; $i < $num_names; $i++) {
            $form['names_fieldset']['name'][$i] = [
                '#type' => 'textfield',
                '#title' => $this->t('Name'),
            ];
        }
        $form['names_fieldset']['actions'] = [
            '#type' => 'actions',
        ];
        $form['names_fieldset']['actions']['add_name'] = [
            '#type' => 'submit',
            '#value' => $this->t('Add one more'),
            '#submit' => [
                '::addOne',
            ],
            '#ajax' => [
                'callback' => '::addmoreCallback',
                'wrapper' => 'names-fieldset-wrapper',
            ],
        ];
        // If there is more than one name, add the remove button.
        if ($num_names > 1) {
            $form['names_fieldset']['actions']['remove_name'] = [
                '#type' => 'submit',
                '#value' => $this->t('Remove one'),
                '#submit' => [
                    '::removeCallback',
                ],
                '#ajax' => [
                    'callback' => '::addmoreCallback',
                    'wrapper' => 'names-fieldset-wrapper',
                ],
            ];
        }
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Submit'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_api_example_ajax_addmore';
    }
    
    /**
     * Callback for both ajax-enabled buttons.
     *
     * Selects and returns the fieldset with the names in it.
     */
    public function addmoreCallback(array &$form, FormStateInterface $form_state) {
        return $form['names_fieldset'];
    }
    
    /**
     * Submit handler for the "add-one-more" button.
     *
     * Increments the max counter and causes a rebuild.
     */
    public function addOne(array &$form, FormStateInterface $form_state) {
        $name_field = $form_state->get('num_names');
        $add_button = $name_field + 1;
        $form_state->set('num_names', $add_button);
        // Since our buildForm() method relies on the value of 'num_names' to
        // generate 'name' form elements, we have to tell the form to rebuild. If we
        // don't do this, the form builder will not call buildForm().
        $form_state->setRebuild();
    }
    
    /**
     * Submit handler for the "remove one" button.
     *
     * Decrements the max counter and causes a form rebuild.
     */
    public function removeCallback(array &$form, FormStateInterface $form_state) {
        $name_field = $form_state->get('num_names');
        if ($name_field > 1) {
            $remove_button = $name_field - 1;
            $form_state->set('num_names', $remove_button);
        }
        // Since our buildForm() method relies on the value of 'num_names' to
        // generate 'name' form elements, we have to tell the form to rebuild. If we
        // don't do this, the form builder will not call buildForm().
        $form_state->setRebuild();
    }
    
    /**
     * Final submit handler.
     *
     * Reports what values were finally set.
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $values = $form_state->getValue([
            'names_fieldset',
            'name',
        ]);
        $output = $this->t('These people are coming to the picnic: @names', [
            '@names' => implode(', ', $values),
        ]);
        $this->messenger()
            ->addMessage($output);
    }

}