Requesting feedback: new for-loop action for ROS 2 launch files to repeat entities

ROS ROS General
,
5

Thanks for the feedback. I think that makes a lot of sense; I generalized it into a ForEach action to make it more flexible and much more powerful. It can now iterate over sets of parameters. It’s basically a string list of YAML structures/dictionaries, similar to what Nav2’s ParseMultiRobotPose does.

See the updated PR: Add ForEach action to repeat entities using iteration-specific values by christophebedard · Pull Request #802 · ros2/launch · GitHub.

Examples:

XML 
<launch>
    <arg name="robots" default="{name: 'robotA', id: 1};{name: 'robotB', id: 2}" />
    <for_each values="$(var robots)" >
        <log message="'$(for-var name)' id=$(for-var id)" />
    </for_each>
</launch>
YAML 
launch:
    - arg:
        name: robots
        default: "{name: 'robotA', id: 1};{name: 'robotB', id: 2}"
    - for_each:
        values: $(var robots)
        children:
            - log:
                message: "'$(for-var name)' id=$(for-var id)"
Python 
import launch


def for_each(id: int, name: str):
    return [
        launch.actions.LogInfo(msg=f"'{name}' id={id}"),
    ]


def generate_launch_description():
    return launch.LaunchDescription([
        launch.actions.DeclareLaunchArgument(
            'robots', default_value="{name: 'robotA', id: 1};{name: 'robotB', id: 2}"),
        launch.actions.ForEach(launch.substitutions.LaunchConfiguration('robots'), function=for_each),
    ])

Default values are also supported, which I see is useful from looking at ParseMultiRobotPose. In Python, just set a default value for the corresponding callback function parameter. When using one of the frontends, provide an optional default value to the substitution: $(for-var name 'default').

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