FormTestOptionalContainerForm.php

<?php

namespace Drupal\form_test\Form;

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

/**
 * Builds a simple form to test the #optional property on #type 'container'.
 *
 * @internal
 */
class FormTestOptionalContainerForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_optional_container';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        // Empty containers.
        $form['empty_optional'] = [
            '#type' => 'container',
            '#attributes' => [
                'class' => [
                    'empty_optional',
                ],
            ],
            '#optional' => TRUE,
        ];
        $form['empty_non_optional'] = [
            '#type' => 'container',
            '#attributes' => [
                'class' => [
                    'empty_non_optional',
                ],
            ],
            '#optional' => FALSE,
        ];
        // Non-empty containers
        $form['nonempty_optional'] = [
            '#type' => 'container',
            '#attributes' => [
                'class' => [
                    'nonempty_optional',
                ],
            ],
            '#optional' => TRUE,
        ];
        $form['nonempty_optional']['child_1'] = [];
        $form['nonempty_non_optional'] = [
            '#type' => 'container',
            '#attributes' => [
                'class' => [
                    'nonempty_non_optional',
                ],
            ],
            '#optional' => FALSE,
        ];
        $form['nonempty_non_optional']['child_2'] = [];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
    }

}