@classmethod<EOL><INDENT>def _validate_first_message(cls, msg):<DEDENT>
data = cls._unpack_message(msg)<EOL>logger.debug(data)<EOL>if data != cls.RTM_HANDSHAKE:<EOL><INDENT>raise SlackApiError('<STR_LIT>'.format(data))<EOL><DEDENT>logger.info('<STR_LIT>')<EOL>
Check the first message matches the expected handshake.

        Note:
          The handshake is provided as :py:attr:`RTM_HANDSHAKE`.

        Arguments:
          msg (:py:class:`aiohttp.Message`): The message to validate.

        Raises:
          :py:class:`SlackApiError`: If the data doesn't match the
            expected handshake.