protected function exportSites(array $sites, $nodeTypeFilter)
    {
        $this->xmlWriter->startDocument('1.0', 'UTF-8');
        $this->xmlWriter->startElement('root');

        foreach ($sites as $site) {
            $this->exportSite($site, $nodeTypeFilter);
        }

        $this->xmlWriter->endElement();
        $this->xmlWriter->endDocument();
    }