trait RevisionControllerTrait in Entity API 8

trait RevisionControllerTrait {

  /**
   * Returns the entity type manager.
   *
   * @return \Drupal\Core\Entity\EntityTypeManagerInterface
   */
  protected abstract function entityTypeManager();

  /**
   * Returns the langauge manager.
   *
   * @return \Drupal\Core\Language\LanguageManagerInterface
   */
  public abstract function languageManager();

  /**
   * Determines if the user has permission to revert revisions.
   *
   * @param \Drupal\Core\Entity\EntityInterface $entity
   *   The entity to check revert access for.
   *
   * @return bool
   *   TRUE if the user has revert access.
   */
  protected abstract function hasRevertRevisionAccess(EntityInterface $entity);

  /**
   * Determines if the user has permission to delete revisions.
   *
   * @param \Drupal\Core\Entity\EntityInterface $entity
   *   The entity to check delete revision access for.
   *
   * @return bool
   *   TRUE if the user has delete revision access.
   */
  protected abstract function hasDeleteRevisionAccess(EntityInterface $entity);

  /**
   * Builds a link to revert an entity revision.
   *
   * @param \Drupal\Core\Entity\EntityInterface $entity_revision
   *   The entity to build a revert revision link for.
   *
   * @return array
   *   A link render array.
   */
  protected abstract function buildRevertRevisionLink(EntityInterface $entity_revision);

  /**
   * Builds a link to delete an entity revision.
   *
   * @param \Drupal\Core\Entity\EntityInterface $entity_revision
   *   The entity to build a delete revision link for.
   *
   * @return array
   *   A link render array.
   */
  protected abstract function buildDeleteRevisionLink(EntityInterface $entity_revision);

  /**
   * Returns a string providing details of the revision.
   *
   * E.g. Node describes its revisions using {date} by {username}. For the
   *   non-current revision, it also provides a link to view that revision.
   *
   * @param \Drupal\Core\Entity\ContentEntityInterface $revision
   *   The entity revision.
   * @param bool $is_current
   *   TRUE if the revision is the current revision.
   *
   * @return string
   *   Returns a string to provide the details of the revision.
   */
  protected abstract function getRevisionDescription(ContentEntityInterface $revision, $is_current = FALSE);

  /**
   * Loads all revision IDs of an entity sorted by revision ID descending.
   *
   * @param \Drupal\Core\Entity\ContentEntityInterface $entity
   *   The entity.
   *
   * @return mixed[]
   */
  protected function revisionIds(ContentEntityInterface $entity) {
    $entity_type = $entity
      ->getEntityType();
    $result = $this
      ->entityTypeManager()
      ->getStorage($entity_type
      ->id())
      ->getQuery()
      ->allRevisions()
      ->condition($entity_type
      ->getKey('id'), $entity
      ->id())
      ->sort($entity_type
      ->getKey('revision'), 'DESC')
      ->execute();
    return array_keys($result);
  }

  /**
   * Generates an overview table of older revisions of an entity.
   *
   * @param \Drupal\Core\Entity\ContentEntityInterface $entity
   *   An entity object.
   *
   * @return array
   *   A render array.
   */
  protected function revisionOverview(ContentEntityInterface $entity) {
    $langcode = $this
      ->languageManager()
      ->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)
      ->getId();

    /** @var \Drupal\Core\Entity\ContentEntityStorageInterface $entity_storage */
    $entity_storage = $this
      ->entityTypeManager()
      ->getStorage($entity
      ->getEntityTypeId());
    $revision_ids = $this
      ->revisionIds($entity);
    $entity_revisions = $entity_storage
      ->loadMultipleRevisions($revision_ids);
    $translatable = $entity
      ->getEntityType()
      ->isTranslatable();
    $header = [
      $this
        ->t('Revision'),
      $this
        ->t('Operations'),
    ];
    $rows = [];
    foreach ($entity_revisions as $revision) {
      $row = [];

      /** @var \Drupal\Core\Entity\ContentEntityInterface $revision */
      if (!$translatable || $revision
        ->hasTranslation($langcode) && $revision
        ->getTranslation($langcode)
        ->isRevisionTranslationAffected()) {
        $row[] = $this
          ->getRevisionDescription($revision, $revision
          ->isDefaultRevision());
        if ($revision
          ->isDefaultRevision()) {
          $row[] = [
            'data' => [
              '#prefix' => '<em>',
              '#markup' => $this
                ->t('Current revision'),
              '#suffix' => '</em>',
            ],
          ];
          foreach ($row as &$current) {
            $current['class'] = [
              'revision-current',
            ];
          }
        }
        else {
          $links = $this
            ->getOperationLinks($revision);
          $row[] = [
            'data' => [
              '#type' => 'operations',
              '#links' => $links,
            ],
          ];
        }
      }
      $rows[] = $row;
    }
    $build[$entity
      ->getEntityTypeId() . '_revisions_table'] = [
      '#theme' => 'table',
      '#rows' => $rows,
      '#header' => $header,
    ];

    // We have no clue about caching yet.
    $build['#cache']['max-age'] = 0;
    return $build;
  }

  /**
   * Get the links of the operations for an entity revision.
   *
   * @param \Drupal\Core\Entity\EntityInterface $entity_revision
   *   The entity to build the revision links for.
   *
   * @return array
   *   The operation links.
   */
  protected function getOperationLinks(EntityInterface $entity_revision) {
    $links = [];
    if ($this
      ->hasRevertRevisionAccess($entity_revision)) {
      $links['revert'] = $this
        ->buildRevertRevisionLink($entity_revision);
    }
    if ($this
      ->hasDeleteRevisionAccess($entity_revision)) {
      $links['delete'] = $this
        ->buildDeleteRevisionLink($entity_revision);
    }
    return array_filter($links);
  }

}