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Wait for subscriber to exist?

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

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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.

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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

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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.

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

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Last updated: Nov 24 '21

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