Migrate Python launch files to XML format

[andreas-loeffler]

Migrate Python launch files to XML format

Description

In a recent discussion on PR #309, @peci1 suggested that the current python launch files should be replaced with XML launch files.

Proposed Changes

The goal is to provide XML equivalents (or replacements) for the existing .launch.py files.

Affected Packages:

• joy: Convert existing .py launch files
• spacenav: Convert existing .py launch files
• wiimote: Convert existing .py launch files

Note: The ps3joy pkg would not be affected, since it already provides an XML launch file instead of a python equivalent, and primarily because it is still not ported to ros2

Open Question

@peci1, regarding your comment in #309 about deprecation:

1. Do u pref. removing the .launch.py files entirely?
2. Or should we keep them (temporarily) with a deprecation warning?

Reference

• Mentioned in PR fix node args used to launch joy nodes via launch file #309

I am happy to take a look at that!

[peci1]

Thanks for starting this!

Re ps3joy: that is XML launchfile, but ROS 1 XML launch file :-D Apparently, ps3joy is still not ported to ROS 2, so don't care about it.

I would also prefer if you could add launch files similar to those in joy to joy_linux to get parity.

Re deprecations: I thought a classic Python deprecation could be added to generate_launch_description(). However, this should (ideally) not appear on already released distros. I'm not sure how to proceed, maybe simple os.environ['ROS_DISTRO'] check could work and add the deprecation only in Lyrical and newer + Rolling. In 2029 when Jazzy support ends, we could remove the Python launches completely :-D

[andreas-loeffler]

Hi @peci1, I’ve been working on the launch file migrations and wanted to get your opinion on two technical points I have encountered

1. Migration of EventHandler based launch files over to XML

How would u proceed with the more "sophisticated" launch files (e.g https://github.com/ros-drivers/joystick_drivers/blob/ros2/joy/launch/joy-launch.py), relying on EventHandlers. As far as I know ros2 launch XMLs currently still do not support EventHandlers. While it was already mentioned back in 2022 (Issue ros2/launch#658 and PR Draft ros2/launch#659) there has been no further progress since.

Would u advice to skip the migration of py launch files if they cannot be implemented in a similar logic over in xml? Or leave the migration on hold till "parity" has been reached between xml and py launch files at least regarding EventHandlers? (Maybe I also missed smth and theres a "hacky" way to use some similar logic like EventHandlers in XMLs)

I might also post about this over in discourse, reaching "parity" between xml and py launch files, coz I'm quite curious abt what the mid/long term timeline and plan regarding the state of xml launch file support will be.

2. Merging "duplicated" launch files (spacenav)

https://github.com/ros-drivers/joystick_drivers/tree/ros2/spacenav/launch currently has 3 launch files implemented which are all very basic and only differ on the hardcoded params. I’d like to propose merging these into a single unified XML launch file that exposes these params as LaunchArguments (The params to be used for each case (no/static-deadband) could then be documented in the readme) Thus smth like the following and then migrate it over to xml:

# ....

def generate_launch_description():
    zero_when_static = LaunchConfiguration('zero_when_static')
    static_count_threshold = LaunchConfiguration('static_count_threshold')
    static_trans_deadband = LaunchConfiguration('static_trans_deadband')
    static_rot_deadband = LaunchConfiguration('static_rot_deadband')

    return LaunchDescription([
        DeclareLaunchArgument(
            'zero_when_static',
            default_value='true'
        ),
        DeclareLaunchArgument(
            'static_count_threshold',
            default_value='30'
        ),
        DeclareLaunchArgument(
            'static_trans_deadband',
            default_value='0.0'
        ),
        DeclareLaunchArgument(
            'static_rot_deadband',
            default_value='0.0'
        ),

        Node(
            package='spacenav',
            executable='spacenav_node',
            name='spacenav',
            namespace='',
            output='screen',
            parameters=[{
                'zero_when_static': zero_when_static,
                'static_count_threshold': static_count_threshold,
                'static_trans_deadband': static_trans_deadband,
                'static_rot_deadband': static_rot_deadband,
            }],
        ),
    ])

[peci1]

Hi, thanks for the analysis.

I tried to decode what the EventHandler is doing and if I'm not wrong, this is the same as on_exit="shutdown"in XML <node> tags. However, I'm not sure at all why is it there. Would you like to shutdown whole app just because your gamepad had a glitch? So I think it should be safe to omit the handler. On the other hand, I'd add respawn to the node.

Re spacenav: sure, merging the launch files makes sense. Or, maybe - creating a common one and then rewriting these 3 to use it. I think the value of these files is that they collect useful sets of arguments for various use-cases.