def consumer(self, fn):
        """Consumer decorator

        :param fn: coroutine consumer function

        Example:

        >>> api = StreamingAPI('my_service_key')
        >>> stream = api.get_stream()

        >>> @stream.consumer
        >>> @asyncio.coroutine
        >>> def handle_event(payload):
        >>>     print(payload)

        """
        if self._consumer_fn is not None:
            raise ValueError('Consumer function is already defined for this '
                             'Stream instance')
        if not any([asyncio.iscoroutine(fn), asyncio.iscoroutinefunction(fn)]):
            raise ValueError('Consumer function must be a coroutine')
        self._consumer_fn = fn