FormTestGroupVerticalTabsForm.php

<?php

namespace Drupal\form_test\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Builds a simple form to test the #group property on #type 'vertical_tabs'.
 *
 * @internal
 */
class FormTestGroupVerticalTabsForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_group_vertical_tabs';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['vertical_tabs'] = [
            '#type' => 'vertical_tabs',
        ];
        $form['meta'] = [
            '#type' => 'details',
            '#title' => 'First group element',
            '#group' => 'vertical_tabs',
        ];
        $form['meta']['element'] = [
            '#type' => 'textfield',
            '#title' => 'First nested element in details element',
        ];
        $form['meta_2'] = [
            '#type' => 'details',
            '#title' => 'Second group element',
            '#group' => 'vertical_tabs',
        ];
        $form['meta_2']['element_2'] = [
            '#type' => 'textfield',
            '#title' => 'Second nested element in details element',
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => 'Save',
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        if ($form_state->getValue('element_2') === 'bad') {
            $form_state->setErrorByName('element_2', $this->t('there was an error'));
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
    }

}