ROS Resources: Documentation | Support | Discussion Forum | Index | Service Status | ros @ Robotics Stack Exchange

Attention: Answers.ros.org is deprecated as of August the 11th, 2023. Please visit robotics.stackexchange.com to ask a new question. This site will remain online in read-only mode during the transition and into the foreseeable future. Selected questions and answers have been migrated, and redirects have been put in place to direct users to the corresponding questions on Robotics Stack Exchange. Additional details are available here.

0

Wait for subscriber to exist?

asked 2021-11-24 16:01:09 -0600

flag of United States of America

241 ●19 ●32 ●37 https://github.com/tylerjw

I would like for one node to wait for a subscriber to a topic to exist before publishing to it. Is there a straightforward way to do that in rclcpp?

I'm developing on rolling if that matters.

edit retag flag offensive close merge delete

Comments

I think @Geoff's answer suffices for your purpose, but FYI there are wait_for_[publishers/subscribers]methods in the RCL layer (refer to https://github.com/ros2/rcl/pull/907/). It would be nice if there were client library (rclcpp/rclpy) wrappers because that fits your use case exactly!

aprotyas ( 2021-11-25 04:02:58 -0600 )edit

Some advice: in a robust distributed design, there are almost no good reasons a publishing node needs to know the number of subscribers. Remember you don't control who connects, and message delivery is unreliable. How does your node use this information?

Mike Scheutzow ( 2021-11-25 08:09:27 -0600 )edit

The original reason for this was to recreate the behavior of latched topics. I've found another way to do that though.

tyler-picknik ( 2022-03-02 09:11:08 -0600 )edit

1 Answer

Sort by » oldest newest most voted

1

answered 2021-11-24 16:58:58 -0600

4203 ●18 ●103 ●63

The easiest method I can think of off the top of my head is to check the publisher's get_subscription_count() method before publishing, and skip publishing (or even preparing to publish) if the count is zero. You might also need to check get_intra_process_subscription_count() as well.

edit flag offensive delete link

Question Tools

2 followers

Stats

Asked: 2021-11-24 16:01:09 -0600

Seen: 1,722 times

Last updated: Nov 24 '21

Related questions

ROS Answers is licensed under Creative Commons Attribution 3.0 Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.

Powered by Askbot version 0.10.2