Controller Spawner launch extension

[SuperJappie08]

Is your feature request related to a problem? Please describe.
I was attempting to do some dynamic controller configuration and remapping from a launch file (using the xml front-end).
During this I ran into parsing issues with the dynamically created --controller-ros-args, since launch attempts to split the arguments in an unescapable way (hardcoded it works fine, so no real issue in a more normal case).
Furthermore, it is currently not easily possible to set controller parameters that are defined only in a launch file.

Describe the solution you'd like
A launch extension, like provided with ros_gz_sim and ros_gz_bridge in which special launch actions get exposed to spawn and unspawn hardware.
This could be part of the controller_manager package (similar to the Gazebo case) or a separate package (launch_ros2_control), similar to launch_ros.

Describe alternatives you've considered
Writing a Python launch file, which is inconvenient since it is introducing a lot more more complexity to do something relatively simple.
While using a launch front-end (like launch_xml or launch_yaml) could also easily suffice.

Additional context
I would be willing to help realize this feature.

[SuperJappie08]

I started experimenting with this idea, and this could be the concept for the spawner.

Python:

ControllerSpawner(
    controllers=[
        Controller(
            name="some_controller",
            remaps=[("from", "to")],  # These get converted to `--controller-ros-args -r <controller-name>:from:=to`,
            parameters=[...],  # Parameters specific for this controller, gets put in generated file, like how launch_ros does it
        ),
    ],
    # Optional other args,
   spawner_name='...',
   controller_manager=...,
   spawner_remaps=...,  # remaps for the spawner itself 
   controller_parameters=..., # Global parameters attached to all controllers
)

XML

<controller_spawner> <!-- A lot of the optional attributes, like name go here -->
  <controller name="<CONTROLLER_NAME>">
    <remap from="from" to="to"/>
    ....
    <param .../> <!-- accepts all the launch_ros Node Param forms, but specific for this controller -->
  </controller>
  <controller_param ... /> <!-- accepts all the launch_ros Node Param forms, but specific for all spawned controllers -->
  <spawner_remap .../> <!-- Remaps for the spawner -->
</controller_spawner>

It would probably also be useful to support conditions on the controllers.

[SuperJappie08]

I made a working version for Jazzy as a separate package called launch_ros2_control.
It currently only supports the most important features of the controller spawner.
(The main thing missing is remapping and parameters on the spawner itself).

I will add some more docs to it today.

[github-actions]

This issue is being labeled as stale because it has been open 45 days with no activity. It will be automatically closed after another 45 days without follow-ups.

[SuperJappie08]

(nudge to keep alive, in case somebody is interested)

[github-actions]

This issue is being labeled as stale because it has been open 45 days with no activity. It will be automatically closed after another 45 days without follow-ups.

[christophfroehlich]

Sorry for the late reaction. The launch extension is great, we definitely should go forward in this direction and replace the launch_utils python methods.
I'll bring this to tomorrows WG group meeting.

[SuperJappie08]

As a note, there is also a different way of creating such a spawn launch action, which would integrate the logic into it instead of starting the normal spawner node.

This was simpler for now, but the alternative is also an option.

(So my current implementation launch->action->spawner noder->spawning of controller, alternative launch->action->spawning of controller)