def expectWithHeader(self, expectedHeaderLen, callbackBody):
        """
        2nd level of waiting event
        read expectedHeaderLen that contain body size
        @param expectedHeaderLen: contains the number of bytes, which body length needs to be encoded
        @param callbackBody: next state use when expected date from expectedHeaderLen
        are received
        """
        self._callbackBody = callbackBody
        self.expect(expectedHeaderLen, self.expectedBody)