[ROS2] Missing lazy subscription headers

[daisukes]

Feature request

Feature description

• migrating nodelet_topic_tools headers or equivalent alternative

  nodelet_demux.h
  nodelet_lazy.h
  nodelet_mux.h
  nodelet_throttle.h
  

• related issues/discussions
  • [ROS2] image_proc: replace SubscriberStatusCallback (from ROS1) with a Timer ros-perception/image_pipeline#780
  • migrate pcl_nodelet.hpp to pcl_node.hpp ros-perception/perception_pcl#385 (comment)
    • perception_pcl uses demux, lazy, and mux
  • https://answers.ros.org/question/286069/implementing-subscriberstatuscallback-within-ros2/
  • https://www.robotandchisel.com/2020/07/29/five-things-ros2/#subscriber-connect-callbacks

Implementation considerations

• maybe in include/rclcpp/topic_tools or any better place/way?
• currently, polling is needed for lazy subscription - workaround sample
• this enhancement would enable us to use subscription callback like ROS1
  • Connect/Disconnect notification/callback support rmw#330

[fujitatomoya]

@daisukes can you enlighten me more details about what the application want to do with example? i do not quite understand what is really required here...

this enhancement would enable us to use subscription callback like ROS1

especially why this is related to this feature?

[daisukes]

@fujitatomoya
Sure, one example is ROS2 porting of the perception_pcl package, which utilizes these headers.
Thanks to the introduction of type adaptation with Humble, I think many resource-intensive nodelets (image_pipeline, pointcloud_to_laserscan, etc) will be ported to ROS2 component, and many of them inherit classes in the nodelet_topic_tools headers, especially nodelet_lazy.h.

As for the lazy subscription (which is used in nodelte_lazy.h) it needs the feature (ros2/rmw#330) to archive the functionality without polling the subscription match count.

rclcpp_component (equivalent to ROS1 nodelet) is in this repo, so, it would be great if these headers are in the common include.

Does this answer your questions?

[fujitatomoya]

thanks for the explanation, now i see what is requested here.

lazy subscription sounds to me like more general feature, not dedicated to ROS 1 nodelet or ROS 2 component, it sounds one of the subscription option for me. but agree that rclcpp and rclpy would be the best place to handle these feature for the application, i think.

just to be clear, this feature is out of scope ros2/rmw#330 activity for now. after ros2/rmw#330 is completed, contribution will be really welcome 😄