class MessageViewBuilder

class MessageViewBuilder extends EntityViewBuilder {
    
    /**
     * {@inheritdoc}
     */
    protected function getBuildDefaults(EntityInterface $entity, $view_mode) {
        $build = parent::getBuildDefaults($entity, $view_mode);
        // The message fields are individually rendered into email templates, so
        // the entity has no template itself.
        unset($build['#theme']);
        return $build;
    }
    
    /**
     * {@inheritdoc}
     */
    public function view(EntityInterface $entity, $view_mode = 'full', $langcode = NULL) {
        $build = parent::view($entity, $view_mode, $langcode);
        if ($view_mode == 'mail') {
            // Convert field labels into headings.
            // @todo Improve \Drupal\Core\Mail\MailFormatHelper::htmlToText() to
            // convert DIVs correctly.
            foreach (Element::children($build) as $key) {
                if (isset($build[$key]['#label_display']) && $build[$key]['#label_display'] == 'above') {
                    $build[$key] += [
                        '#prefix' => '',
                    ];
                    $build[$key]['#prefix'] = $build[$key]['#title'] . ":\n";
                    $build[$key]['#label_display'] = 'hidden';
                }
            }
        }
        return $build;
    }

}