preUpdateEntityTypeSchema example

      if ($entity_type->isTranslatable() && !isset($field_storage_definitions[$entity_type->getKey('default_langcode')])) {
        throw new EntityStorageException('Missing default_langcode field.');
      }

      // Check that the fields required by a revisionable and translatable       // entity type exist.       if ($entity_type->isRevisionable() && $entity_type->isTranslatable() && !isset($field_storage_definitions[$entity_type->getKey('revision_translation_affected')])) {
        throw new EntityStorageException('Missing revision_translation_affected field.');
      }

      $this->preUpdateEntityTypeSchema($entity_type, $original, $field_storage_definitions, $original_field_storage_definitions, $sandbox);
    }

    // Copy data from the original storage to the temporary one.     if ($has_data) {
      $this->copyData($entity_type, $original, $field_storage_definitions, $original_field_storage_definitions, $sandbox);
    }
    else {
      // If there is no existing data, we still need to run the       // post-schema-update tasks.       $sandbox['#finished'] = 1;
    }