Feature request

Feature description

• RMW can notify the event that any subscriptions connect / disconnect on the topic to each publisher.
• Based on this capability, in rclcpp and rclpy, user application can register user callback that will be called when subscription connects or disconnects on the topic for publisher.

ROS 1 Similar function: https://docs.ros.org/en/noetic/api/roscpp/html/classros_1_1NodeHandle.html#ae4711ef282892176ba145d02f8f45f8d

Use case

On visualization node, to suppress unnecessary CPU utilization, enable its visualization process only when RViz is connected.

Related Information / Reference

• image_pipeline comment out subscription match for temporary
• ROS2 publisher callback on subscription match
• pointcloud_to_laserscan does polling to check on subscription match

Implementation considerations

Since all tier-1 rmw implementation is DDS implementation, we can support this feature with on_publication_matched of DataWriterListener Interface. (we confirmed the code of Fast-DDS.)

• rmw Add matched event support #331
• rmw_implementation Add tests for rmw matched event rmw_implementation#216
• rmw_fastrtps Implement matched event rmw_fastrtps#645
• rmw_connextdds Support matched/unmatched event rmw_connextdds#101
• rmw_cyclonedds Implement matched event rmw_cyclonedds#435
• rcl Implement matched event rcl#1033
• rclcpp Implement matched event rclcpp#2105
• rclpy Implement matched event rclpy#1083
• Add matched event demo for rclcpp and rclpy demos#607
• Topic add matched event description ros2_documentation#3406
• Matched events release note update. ros2_documentation#3420