class VersionByRel

class VersionByRel extends NegotiatorBase {
    
    /**
     * Version argument which loads the revision known to be the "working copy".
     *
     * In Drupal terms, a "working copy" is the latest revision. It may or may not
     * be a "default" revision. This revision is the working copy because it is
     * the revision to which new work will be applied. In other words, it denotes
     * the most recent revision which might be considered a work-in-progress.
     *
     * @var string
     */
    const WORKING_COPY = 'working-copy';
    
    /**
     * Version argument which loads the revision known to be the "latest version".
     *
     * In Drupal terms, the "latest version" is the latest "default" revision. It
     * may or may not have later revisions after it, as long as none of them are
     * "default" revisions. This revision is the latest version because it is the
     * last revision where work was considered finished. Typically, this means
     * that it is the most recent "published" revision.
     *
     * @var string
     */
    const LATEST_VERSION = 'latest-version';
    
    /**
     * {@inheritdoc}
     */
    protected function getRevisionId(EntityInterface $entity, $version_argument) {
        assert($entity instanceof RevisionableInterface);
        switch ($version_argument) {
            case static::WORKING_COPY:
                
                /* @var \Drupal\Core\Entity\RevisionableStorageInterface $entity_storage */
                $entity_storage = $this->entityTypeManager
                    ->getStorage($entity->getEntityTypeId());
                return static::ensureVersionExists($entity_storage->getLatestRevisionId($entity->id()));
            case static::LATEST_VERSION:
                // The already loaded revision will be the latest version by default.
                // @see \Drupal\Core\Entity\Sql\SqlContentEntityStorage::buildQuery().
                return $entity->getLoadedRevisionId();
            default:
                $message = sprintf('The version specifier must be either `%s` or `%s`, `%s` given.', static::LATEST_VERSION, static::WORKING_COPY, $version_argument);
                throw new InvalidVersionIdentifierException($message);
        }
    }

}