class Hidden

class Hidden extends FormElement {

  /**
   * {@inheritdoc}
   */
  public function getInfo() {
    $class = get_class($this);
    return [
      '#input' => TRUE,
      '#process' => [
        [
          $class,
          'processAjaxForm',
        ],
      ],
      '#pre_render' => [
        [
          $class,
          'preRenderHidden',
        ],
      ],
      '#theme' => 'input__hidden',
    ];
  }

  /**
   * Prepares a #type 'hidden' render element for input.html.twig.
   *
   * @param array $element
   *   An associative array containing the properties of the element.
   *   Properties used: #name, #value, #attributes.
   *
   * @return array
   *   The $element with prepared variables ready for input.html.twig.
   */
  public static function preRenderHidden($element) {
    $element['#attributes']['type'] = 'hidden';
    Element::setAttributes($element, [
      'name',
      'value',
    ]);
    return $element;
  }

}