This ROS2 package contains some example C++ applications which demonstrate how to listen for various events generated by an rclcpp::Node object.

events_listener.cpp shows how to register callbacks on rclcpp::Publisher and rclcpp:Subscription objects to receive notification of DDS-level events which are exposed by the rclcpp layer.

Each callback must be specified through the options data-structures ( rclcpp::PublisherOptions, and rclcpp::SubscriptionOptions) when creating the endpoints.

The example uses a custom helper "listener class" to demonstrate how to tie this events with any custom C++ object. See node_listener.hpp.

In order to receive these notification the Node and its endpoints must be attached to a spinning executor.

graph_listener.cpp shows how to receive notification of changes to the "graph" database maintained by each Node.

This is achieved by using the rclcpp::Event interface along with various functions exposed by rclcpp::Node.

Contrary to other ROS2 events (data receival, publisher/subscription events, timers, etc...), "graph changed" events cannot be dispatched by an executor, and they thus require the application to spawn a thread to call rclcpp::Node::wait_for_graph_change() in parallel to a spinning executor.

1. Clone (or symlink) repository in a new workspace:

   mkdir ws-node_listener
   cd ws-node_listener
   git clone https://github.com/asorbini/ros2_node_listener

2. Load ROS2 Galactic, build the examples, and load them:

   source /opt/ros/galactic/setup.bash
   colcon build --symlink-install
   source install/setup.bash

3. (Optional) Make sure to use RTI Connext DDS:

   export RMW_IMPLEMENTATION=rmw_connextdds

4. Run events_listener example:

   ros2 run node_listener events_listener
   
   # Use ^C to terminate

5. Run graph_listener example:

   # Start listener in backgroun
   ros2 run node_listener graph_listener &
   
   # Generate some "graph changes" with `ros2 topic pub`
   ros2 topic pub "chatter" example_interfaces/msg/String
   
   # Use ^C to terminate the publisher (more changes will print)
   
   # Terminate listener
   fg
   # then ^C