trait CachedPaginatedEntityIdListingControllerTrait in Apigee Edge 8

trait CachedPaginatedEntityIdListingControllerTrait {

  /**
   * The decorated entity controller from the SDK.
   *
   * We did not added a return type because this way all entity controller's
   * decorated() method becomes compatible with this declaration.
   *
   * @return \Apigee\Edge\Controller\PaginatedEntityIdListingControllerInterface
   *   An entity controller that extends these interfaces.
   */
  protected abstract function decorated();

  /**
   * Entity id cache used by the entity controller.
   *
   * @return \Drupal\apigee_edge\Entity\Controller\Cache\EntityIdCacheInterface
   *   The entity id cache.
   */
  protected abstract function entityIdCache() : EntityIdCacheInterface;

  /**
   * {@inheritdoc}
   */
  protected abstract function extractSubsetOfAssociativeArray(array $assoc_array, int $limit, ?string $start_key = NULL) : array;

  /**
   * {@inheritdoc}
   */
  public function getEntityIds(PagerInterface $pager = NULL) : array {
    if ($this
      ->entityIdCache()
      ->isAllIdsInCache()) {
      if ($pager === NULL) {
        return $this
          ->entityIdCache()
          ->getIds();
      }
      else {
        return $this
          ->extractSubsetOfAssociativeArray($this
          ->entityIdCache()
          ->getIds(), $pager
          ->getLimit(), $pager
          ->getStartKey());
      }
    }
    $ids = $this
      ->decorated()
      ->getEntityIds($pager);
    $this
      ->entityIdCache()
      ->saveIds($ids);
    if ($pager === NULL) {
      $this
        ->entityIdCache()
        ->allIdsInCache(TRUE);
    }
    return $ids;
  }

}