class Radio

class Radio extends FormElementBase {

  /**
   * {@inheritdoc}
   */
  public function getInfo() {
    $class = static::class;
    return [
      '#input' => TRUE,
      '#default_value' => NULL,
      '#process' => [
        [
          $class,
          'processAjaxForm',
        ],
      ],
      '#pre_render' => [
        [
          $class,
          'preRenderRadio',
        ],
      ],
      '#theme' => 'input__radio',
      '#theme_wrappers' => [
        'form_element',
      ],
      '#title_display' => 'after',
    ];
  }

  /**
   * Prepares a #type 'radio' render element for input.html.twig.
   *
   * @param array $element
   *   An associative array containing the properties of the element.
   *   Properties used: #required, #return_value, #value, #attributes, #title,
   *   #description. The #name property will be sanitized before output. This is
   *   currently done by initializing Drupal\Core\Template\Attribute with all
   *   the attributes.
   *
   * @return array
   *   The $element with prepared variables ready for input.html.twig.
   */
  public static function preRenderRadio($element) {
    $element['#attributes']['type'] = 'radio';
    Element::setAttributes($element, [
      'id',
      'name',
      '#return_value' => 'value',
    ]);

    // To avoid auto-casting during '==' we convert $element['#value'] and
    // $element['#return_value'] to strings. It will prevent wrong true-checking
    // for both cases: 0 == 'string' and 'string' == 0, this will occur because
    // all numeric array values will be integers and all submitted values will
    // be strings. Array values are never valid for radios and are skipped. To
    // account for FALSE and empty string values in the #return_value, we will
    // consider any #value that evaluates to empty to be the same as any
    // #return_value that evaluates to empty.
    if (isset($element['#return_value']) && $element['#value'] !== FALSE && !is_array($element['#value']) && (empty($element['#value']) && empty($element['#return_value']) || (string) $element['#value'] === (string) $element['#return_value'])) {
      $element['#attributes']['checked'] = 'checked';
    }
    static::setAttributes($element, [
      'form-radio',
    ]);
    return $element;
  }

}