Use clap in some examples

[mxgrey]

clap is a popular crate that makes it extremely easy to write CLI programs in Rust.

We should include some examples that use it, for users to reference. It would also be good to verify that the way Context::default_from_env() works is compatible with clap.

Maintainers can feel free to assign this ticket to me; I'm opening this mostly as a reminder to myself to do this.

[styczen]

@mxgrey This issue should be moved to new repository with examples (https://github.com/ros2-rust/examples)

[esteve]

@mxgrey I've transfered this issue to the examples repo. Thanks @styczen for the heads-up

[avanmalleghem]

@mxgrey , I'm working on this here : https://github.com/avanmalleghem/examples/tree/clap-demo

I have a working basic example publishing a number of messages you pass in parameter:

ros2 run examples_rclrs_clap_demo clap_demo --count 15

And you can receive them using for example:

ros2 run examples_rclrs_minimal_pub_sub minimal_subscriber

---

Now I try to understand what you are saying by :

It would also be good to verify that the way Context::default_from_env() works is compatible with clap.

If I understand well, one test would be to remap the topic, correct ?

For now, I tried this : ros2 run examples_rclrs_clap_demo clap_demo --count 15 --ros-args -r topic:=new_topic but end up with the following issue :

error: unexpected argument '--ros-args' found

Usage: clap_demo --count <COUNT>

For more information, try '--help'.
[ros2run]: Process exited with failure 2

That's just for me to know if I have to go further in this direction.

[mxgrey]

error: unexpected argument '--ros-args' found

Yes this is exactly the kind of problem that I suspected would show up.

I think we should add a utility function to rclrs that gives back the environment variables with ROS arguments removed, so users can pass just those arguments along to their argument parser.

Alternatively (or in addition) we could add a clap feature to rclrs that defines a Subcommand for ROS arguments. Users could then include that Subcommand in their main parser, and then their program's help text can accurately describe the available ROS commands to downstream users.

[avanmalleghem]

I think we should add a utility function to rclrs that gives back the environment variables with ROS arguments removed, so users can pass just those arguments along to their argument parser.

The very good news is that this function already exists https://docs.rs/rclrs/latest/rclrs/fn.extract_non_ros_args.html

I opened a PR with my example.