class NegotiatorBase

abstract class NegotiatorBase implements VersionNegotiatorInterface {
    
    /**
     * The entity type manager to load the revision.
     *
     * @var \Drupal\Core\Entity\EntityTypeManagerInterface
     */
    protected $entityTypeManager;
    
    /**
     * Constructs a version negotiator instance.
     *
     * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
     *   The entity type manager.
     */
    public function __construct(EntityTypeManagerInterface $entity_type_manager) {
        $this->entityTypeManager = $entity_type_manager;
    }
    
    /**
     * Gets the revision ID.
     *
     * @param \Drupal\Core\Entity\EntityInterface $entity
     *   The entity.
     * @param string $version_argument
     *   A value used to derive a revision ID for the given entity.
     *
     * @return int
     *   The revision ID.
     *
     * @throws \Drupal\jsonapi\Revisions\VersionNotFoundException
     *   When the revision does not exist.
     * @throws \Drupal\jsonapi\Revisions\InvalidVersionIdentifierException
     *   When the revision ID is not valid.
     */
    protected abstract function getRevisionId(EntityInterface $entity, $version_argument);
    
    /**
     * {@inheritdoc}
     */
    public function getRevision(EntityInterface $entity, $version_argument) {
        return $this->loadRevision($entity, $this->getRevisionId($entity, $version_argument));
    }
    
    /**
     * Loads an entity revision.
     *
     * @param \Drupal\Core\Entity\EntityInterface $entity
     *   The entity for which to load a revision.
     * @param int $revision_id
     *   The revision ID to be loaded.
     *
     * @return \Drupal\Core\Entity\EntityInterface|null
     *   The revision or NULL if the revision does not exists.
     *
     * @throws \Drupal\Component\Plugin\Exception\PluginNotFoundException
     *   Thrown if the entity type doesn't exist.
     * @throws \Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException
     *   Thrown if the storage handler couldn't be loaded.
     */
    protected function loadRevision(EntityInterface $entity, $revision_id) {
        
        /** @var \Drupal\Core\Entity\RevisionableStorageInterface $storage */
        $storage = $this->entityTypeManager
            ->getStorage($entity->getEntityTypeId());
        $revision = static::ensureVersionExists($storage->loadRevision($revision_id));
        if ($revision->id() !== $entity->id()) {
            throw new VersionNotFoundException(sprintf('The requested resource does not have a version with ID %s.', $revision_id));
        }
        return $revision;
    }
    
    /**
     * Helper method that ensures that a version exists.
     *
     * @param int|\Drupal\Core\Entity\EntityInterface $revision
     *   A revision ID, or NULL if one was not found.
     *
     * @return int|\Drupal\Core\Entity\EntityInterface
     *   A revision or revision ID, if one was found.
     *
     * @throws \Drupal\jsonapi\Revisions\VersionNotFoundException
     *   Thrown if the given value is NULL, meaning the requested version was not
     *   found.
     */
    protected static function ensureVersionExists($revision) {
        if (is_null($revision)) {
            throw new VersionNotFoundException();
        }
        return $revision;
    }

}