public function getExtraFields()
    {
        if ($this->_extraFields === null) {
            $extraFields = [];
            foreach ($this->getConfig() as $pointer => $fields) {
                if (is_array($fields)) {
                    foreach ($fields as $field) {
                        if (isset($field['extraField']) && $field['extraField']) {
                            if (!array_key_exists($field['name'], $extraFields)) {
                                $extraFields[] = $field['name'];
                            }
                        }
                    }
                }
            }
            $this->_extraFields = $extraFields;
        }

        return $this->_extraFields;
    }