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;
    }

}