this.slackApi = slackApi;
  }

  public void open(String accessToken) throws IOException {
    if (webSocket != null) throw new IllegalStateException();

    RtmStartResponse rtmStartResponse = slackApi.rtmStart(accessToken);
    webSocket = slackApi.rtm(rtmStartResponse.url, this);
  }

  // TODO(jwilson): can I read the response body? Do I have to?
  //                the body from slack is a 0-byte-buffer

      return jsonAdapter.fromJson(response.body().source());
    }
  }

  /** See https://api.slack.com/methods/rtm.start. */
  public RtmStartResponse rtmStart(String accessToken) throws IOException {
    HttpUrl url = baseUrl.newBuilder("rtm.start")
        .addQueryParameter("token", accessToken)
        .build();
    Request request = new Request.Builder()
        .url(url)