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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.
Asked: 2021-11-24 16:01:09 -0500
Seen: 1,235 times
Last updated: Nov 24 '21
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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!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?
The original reason for this was to recreate the behavior of latched topics. I've found another way to do that though.