}

  /** See https://api.slack.com/docs/oauth. */
  public HttpUrl authorizeUrl(String scopes, HttpUrl redirectUrl, ByteString state, String team) {
    HttpUrl.Builder builder = baseUrl.newBuilder("/oauth/authorize")
        .addQueryParameter("client_id", clientId)
        .addQueryParameter("scope", scopes)
        .addQueryParameter("redirect_uri", redirectUrl.toString())
        .addQueryParameter("state", state.base64());

  /** Guarded by this. */
  private OAuthSession session;

  public SlackClient(SlackApi slackApi) {
    this.slackApi = slackApi;
  }

  /** Shows a browser URL to authorize this app to act as this user. */
  public void requestOauthSession(String scopes, String team) throws Exception {
    if (sessionFactory == null) {
      sessionFactory = new OAuthSessionFactory(slackApi);