class MediaRevisionAccessCheck

class MediaRevisionAccessCheck implements AccessInterface {

  /**
   * The media storage.
   *
   * @var \Drupal\Core\Entity\ContentEntityStorageInterface
   */
  protected $mediaStorage;

  /**
   * The media access control handler.
   *
   * @var \Drupal\Core\Entity\EntityAccessControlHandlerInterface
   */
  protected $mediaAccess;

  /**
   * A static cache of access checks.
   *
   * @var array
   */
  protected $access = [];

  /**
   * Constructs a new MediaRevisionAccessCheck.
   *
   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
   *   The entity type manager.
   */
  public function __construct(EntityTypeManagerInterface $entity_type_manager) {
    $this->mediaStorage = $entity_type_manager
      ->getStorage('media');
    $this->mediaAccess = $entity_type_manager
      ->getAccessControlHandler('media');
  }

  /**
   * Checks routing access for the media item revision.
   *
   * @param \Symfony\Component\Routing\Route $route
   *   The route to check against.
   * @param \Drupal\Core\Session\AccountInterface $account
   *   The currently logged in account.
   * @param int $media_revision
   *   (optional) The media item revision ID. If not specified, but $media is,
   *   access is checked for that object's revision.
   * @param \Drupal\media\MediaInterface $media
   *   (optional) A media item. Used for checking access to a media items
   *   default revision when $media_revision is unspecified. Ignored when
   *   $media_revision is specified. If neither $media_revision nor $media are
   *   specified, then access is denied.
   *
   * @return \Drupal\Core\Access\AccessResultInterface
   *   The access result.
   */
  public function access(Route $route, AccountInterface $account, $media_revision = NULL, MediaInterface $media = NULL) {
    if ($media_revision) {
      $media = $this->mediaStorage
        ->loadRevision($media_revision);
    }
    $operation = $route
      ->getRequirement('_access_media_revision');
    return AccessResult::allowedIf($media && $this
      ->checkAccess($media, $account, $operation))
      ->cachePerPermissions()
      ->addCacheableDependency($media);
  }

  /**
   * Checks media item revision access.
   *
   * @param \Drupal\media\MediaInterface $media
   *   The media item to check.
   * @param \Drupal\Core\Session\AccountInterface $account
   *   A user object representing the user for whom the operation is to be
   *   performed.
   * @param string $op
   *   (optional) The specific operation being checked. Defaults to 'view'.
   *
   * @return bool
   *   TRUE if the operation may be performed, FALSE otherwise.
   */
  public function checkAccess(MediaInterface $media, AccountInterface $account, $op = 'view') {
    if (!$media || $op !== 'view') {

      // If there was no media to check against, or the $op was not one of the
      // supported ones, we return access denied.
      return FALSE;
    }

    // Statically cache access by revision ID, language code, user account ID,
    // and operation.
    $langcode = $media
      ->language()
      ->getId();
    $cid = $media
      ->getRevisionId() . ':' . $langcode . ':' . $account
      ->id() . ':' . $op;
    if (!isset($this->access[$cid])) {

      // Perform basic permission checks first.
      if (!$account
        ->hasPermission('view all media revisions') && !$account
        ->hasPermission('administer media')) {
        $this->access[$cid] = FALSE;
        return FALSE;
      }

      // There should be at least two revisions. If the revision ID of the
      // given media item and the revision ID of the default revision differ,
      // then we already have two different revisions so there is no need for a
      // separate database check.
      if ($media
        ->isDefaultRevision() && $this
        ->countDefaultLanguageRevisions($media) == 1) {
        $this->access[$cid] = FALSE;
      }
      elseif ($account
        ->hasPermission('administer media')) {
        $this->access[$cid] = TRUE;
      }
      else {

        // First check the access to the default revision and finally, if the
        // media passed in is not the default revision then access to that, too.
        $this->access[$cid] = $this->mediaAccess
          ->access($this->mediaStorage
          ->load($media
          ->id()), $op, $account) && ($media
          ->isDefaultRevision() || $this->mediaAccess
          ->access($media, $op, $account));
      }
    }
    return $this->access[$cid];
  }

  /**
   * Counts the number of revisions in the default language.
   *
   * @param \Drupal\media\MediaInterface $media
   *   The media item for which to count the revisions.
   *
   * @return int
   *   The number of revisions in the default language.
   */
  protected function countDefaultLanguageRevisions(MediaInterface $media) {
    $entity_type = $media
      ->getEntityType();
    $count = $this->mediaStorage
      ->getQuery()
      ->allRevisions()
      ->condition($entity_type
      ->getKey('id'), $media
      ->id())
      ->condition($entity_type
      ->getKey('default_langcode'), 1)
      ->count()
      ->execute();
    return $count;
  }

}