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;
  }

}