protected function http() {
        // 若$this->http不是\GuzzleHttp\Client实例，则创建一个
        if (!$this->http instanceof Client) {
            $this->http = new Client(
                array_merge(
                    [ 'base_uri' => $this->url ],
                    $this->options
                )
            );
        }

        return $this->http;
    }