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Part 1 - exercise: Add additional application to the snap

This exercise requires having followed the part 1.

Let’s do a little exercise. Right now, our talker-listener only has one application. How about you add another one? Having the ROS 2 topic tools inside our snap would be great for some introspection.

Assignment

Modify the snapcraft.yaml file:

  • Add a new app entry for ros2topic
  • Add the corresponding dependencies

Build and install the new snap.

Outcome

By the end of this exercise, you will have a new command available:

ros2-talker-listener.ros2topic hz /chatter

  • Note that the ROS 2 topic tools are only going to work with message types that are included in the snap.

Solution

The solution
  • Add ros2topic as a stage-package to have it available at run time:
 parts:
  ros-demos:
    plugin: colcon
    source: https://github.com/ros2/demos.git
    source-branch: humble
    source-subdir: demo_nodes_cpp
-   stage-packages: [ros-humble-ros2launch]
+   stage-packages: [ros-humble-ros2launch, ros-humble-ros2topic]

We add our dependency (ros-humble-ros2topic) to the stage-packages list. Stage-packages are packages that are required to run the part. Here we decided to add our dependency to our already existing ros-demos part for simplicity. Alternatively, we could have created an additional empty part simply to add our stage-packages dependency.

  • Add an additional app:
apps:
  ros2-talker-listener:
    command: opt/ros/humble/bin/ros2 launch talker-listener talker_listener.launch.py
    daemon: simple
    plugs: [network, network-bind]
    extensions: [ros2-humble]
+ ros2topic:
+   command: opt/ros/humble/bin/ros2 topic
+   plugs: [network, network-bind]
+   extensions: [ros2-humble]

We added another application entry to our snap. We kept the plugs and extensions and simply exposed the ros2 topic command.

  • Build and install the new snap:
snapcraft
sudo snap install ros2-talker-listener_0.1_*.snap --dangerous

Make sure everything works:

$ ros2-talker-listener.ros2topic hz /chatter
average rate: 1.000 min: 0.999s max: 1.000s std dev: 0.00049s window: 3

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