code: def expectWithHeader(self, expectedHeaderLen, callbackBody):
          
          self._callbackBody = callbackBody
          self.expect(expectedHeaderLen, self.expectedBody)
docs: 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