class Container

class Container extends RenderElement {
  
  /**
   * {@inheritdoc}
   */
  public function getInfo() {
    $class = static::class;
    return [
      '#optional' => FALSE,
      '#process' => [
        [
          $class,
          'processGroup',
        ],
        [
          $class,
          'processContainer',
        ],
      ],
      '#pre_render' => [
        [
          $class,
          'preRenderGroup',
        ],
        [
          $class,
          'preRenderContainer',
        ],
      ],
      '#theme_wrappers' => [
        'container',
      ],
    ];
  }
  
  /**
   * Processes a container element.
   *
   * @param array $element
   *   An associative array containing the properties and children of the
   *   container.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   * @param array $complete_form
   *   The complete form structure.
   *
   * @return array
   *   The processed element.
   */
  public static function processContainer(&$element, FormStateInterface $form_state, &$complete_form) {
    // Generate the ID of the element if it's not explicitly given.
    if (!isset($element['#id'])) {
      $element['#id'] = HtmlUtility::getUniqueId(implode('-', $element['#parents']) . '-wrapper');
    }
    return $element;
  }
  
  /**
   * Prevents optional containers from rendering if they have no children.
   *
   * @param array $element
   *   An associative array containing the properties and children of the
   *   container.
   *
   * @return array
   *   The modified element.
   */
  public static function preRenderContainer($element) {
    // Do not render optional container elements if there are no children.
    if (empty($element['#printed']) && !empty($element['#optional']) && !Element::getVisibleChildren($element)) {
      $element['#printed'] = TRUE;
    }
    return $element;
  }

}