/// Set a callback to be called when each new message is received.
  /**
   * The callback receives a size_t which is the number of messages received
   * since the last time this callback was called.
   * Normally this is 1, but can be > 1 if messages were received before any
   * callback was set.
   *
   * Since this callback is called from the middleware, you should aim to make
   * it fast and not blocking.
   * If you need to do a lot of work or wait for some other event, you should
   * spin it off to another thread, otherwise you risk blocking the middleware.
   *
   * Calling it again will clear any previously set callback.
   *
   * This function is thread-safe.
   *
   * If you want more information available in the callback, like the subscription
   * or other information, you may use a lambda with captures or std::bind.
   *
   * \sa rmw_subscription_set_on_new_message_callback
   * \sa rcl_subscription_set_on_new_message_callback
   *
   * \param[in] callback functor to be called when a new message is received
   */