Connect/Disconnect notification/callback support about rmw HOT 44 OPEN

This would be a nice addition, probably as a new rmw_event_t as @ivanpauno suggests. We could interpret it as having an event notifying of changes on get_subscription_count()

We could do the same for subscriptions, though we should probably have that as a separate issue.

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In rmw_implementation, there is no test related to event currently (Including existing events).
So I need to add events test file.
Besides, rmw_implementation test depend on rmw_xxxx (e.g. rmw_fastrtps).
So I will implement rmw_fastrtps, rmw_connextdds, rmw_cyclonedds at first and then add test to rmw_implementation.

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@Barry-Xu-2018 thanks for checking, i was thinking that there would be some related code or test cases in there, so just make sure that everything we may change on that table at 1st. if we do not need to update, that is also fine, i will remove rmw_implementaion from the issue header.

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@ivanpauno @wjwwood @asorbini @MiguelCompany @alsora @SteveMacenski @mikeferguson
CC: @ros2/middleware_working_group @Barry-Xu-2018

This is to open discussion, could you share your thoughts?

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Adding this like a new rmw_event_t seems reasonable to me

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I think this is super useful - at the moment, you can't easily build something like rgbd_launch in ROS2 (where all of the various pipeline elements are lazy publishers/subscribers) - I didn't even realize this wasn't already ticketed somewhere...

I had some notes about use cases (and my current lazy subscriber workaround with a timer that checks get_subscription_count once a second) here: https://www.robotandchisel.com/2020/06/16/ubr1-on-ros2/

EDIT: there was also additional information in my post on 5 things ROS2 needed in 2020: https://www.robotandchisel.com/2020/07/29/five-things-ros2/

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I'm not familiar with the on_publication_matched API.
Would it be equivalent to checking get_subscription_count() on the publishers objects?
Obviously having a callback would be much better than continuously checking that API, but I just want to make sure that I understand how it would work.

In general this looks a great feature to add.

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Would it be equivalent to checking get_subscription_count() on the publishers objects?

the main difference is, application can be notified with event on connect/disconnect.

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We all agreed adding new event.
I submitted a PR to add connect/disconnect event and define rmw_matched_status_t for publisher and subscription event callback.
Please review it.

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@fujitatomoya

We should remove rmw_implementation in change list since the rmw interface isn't changed in #331

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We should remove rmw_implementation in change list since the rmw interface isn't changed in #331

@Barry-Xu-2018 I don't think so, since that repository is there the tests for this feature should be added.

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@MiguelCompany

I don't think so, since that repository is there the tests for this feature should be added.

Thank you for your kind reminder.
Yes. I will check and add tests.

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This issue has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/ros-2-tsc-meeting-minutes-2023-01-19/29423/1

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