Begin consuming messages.

        Args:
            callback (Callable[None, google.cloud.pubsub_v1.message.Messages]):
                A callback that will be called for each message received on the
                stream.

def open(self, callback):
        """Begin consuming messages.

        Args:
            callback (Callable[None, google.cloud.pubsub_v1.message.Messages]):
                A callback that will be called for each message received on the
                stream.
        """
        if self.is_active:
            raise ValueError("This manager is already open.")

        if self._closed:
            raise ValueError("This manager has been closed and can not be re-used.")

        self._callback = functools.partial(_wrap_callback_errors, callback)

        # Create the RPC
        self._rpc = bidi.ResumableBidiRpc(
            start_rpc=self._client.api.streaming_pull,
            initial_request=self._get_initial_request,
            should_recover=self._should_recover,
        )
        self._rpc.add_done_callback(self._on_rpc_done)

        # Create references to threads
        self._dispatcher = dispatcher.Dispatcher(self, self._scheduler.queue)
        self._consumer = bidi.BackgroundConsumer(self._rpc, self._on_response)
        self._leaser = leaser.Leaser(self)
        self._heartbeater = heartbeater.Heartbeater(self)

        # Start the thread to pass the requests.
        self._dispatcher.start()

        # Start consuming messages.
        self._consumer.start()

        # Start the lease maintainer thread.
        self._leaser.start()

        # Start the stream heartbeater thread.
        self._heartbeater.start()