public function getManifestContent($source)
    {
        $content = json_decode(file_get_contents($source . DIRECTORY_SEPARATOR
            . $this->getModel()->getManifestName()), true);
        if (json_last_error() === JSON_ERROR_NONE) {
            return $content;
        }
        throw new common_Exception('Portable element manifest is not a valid json file.');
    }