private function _getResult()
    {
        $def = $this->actionChain->getCurrentAction();
        $result = $def['result'];
        if (! $result) {
            return null;
        } elseif (is_string($result)) {
            return $result;
        } elseif (is_array($result)) {
            return array_shift($result);
        } else {
            throw new Exception('invalid action result.');
        }
    }