Radios::valueCallback

public static Radios::valueCallback(&$element, $input, FormStateInterface $form_state)

public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
  if ($input !== FALSE) {
    // When there's user input (including NULL), return it as the value.
    // However, if NULL is submitted, FormBuilder::handleInputElement() will
    // apply the default value, and we want that validated against #options
    // unless it's empty. (An empty #default_value, such as NULL or FALSE, can
    // be used to indicate that no radio button is selected by default.)
    if (!isset($input) && !empty($element['#default_value'])) {
      $element['#needs_validation'] = TRUE;
    }
    return $input;
  }
  else {
    // For default value handling, simply return #default_value. Additionally,
    // for a NULL default value, set #has_garbage_value to prevent
    // FormBuilder::handleInputElement() converting the NULL to an empty
    // string, so that code can distinguish between nothing selected and the
    // selection of a radio button whose value is an empty string.
    $value = isset($element['#default_value']) ? $element['#default_value'] : NULL;
    if (!isset($value)) {
      $element['#has_garbage_value'] = TRUE;
    }
    return $value;
  }
}