class OptionsDefaultFormatter

class OptionsDefaultFormatter extends FormatterBase {
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    // Only collect allowed options if there are actually items to display.
    if ($items->count()) {
      $provider = $items->getFieldDefinition()
        ->getFieldStorageDefinition()
        ->getOptionsProvider('value', $items->getEntity());
      // Flatten the possible options, to support opt groups.
      $options = OptGroup::flattenOptions($provider->getPossibleOptions());
      foreach ($items as $delta => $item) {
        $value = $item->value;
        // If the stored value is in the current set of allowed values, display
        // the associated label, otherwise just display the raw value.
        $output = $options[$value] ?? $value;
        $elements[$delta] = [
          '#markup' => $output,
          '#allowed_tags' => FieldFilteredMarkup::allowedTags(),
        ];
      }
    }
    return $elements;
  }

}