ROS2 makes launch files crazy - Too soon to be migrating?

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FWIW, I think the ROS2 launch system is in a decent place and I have started porting a lot to it. I think it mostly suffers from poor documentation which means you have to look at the source code. The implementation is somewhat complex which makes the source code hard to digest. There are still some edge cases that are not supported, but I have been able to find equivalent functionality for most things I want to do.

You are right that it generally takes more lines to do the same thing. Note however if you were to condense down your Node definitions to a single line like they are in an XML, it would be shorter and more comparable.

You can use a PathJoinSubstitution to evaluate paths to files at launch time (IE using launch arguments). The big change in ROS2 was that robot_state_publisher no longer accepts a file name, but needs the URDF contents as text. That is why you are required to load it before hand. I tend to use xacro so the following example works for me. Note that to use ParameterValue you probably need to be on Galactic as I don't think that has been back ported yet (one of the edge cases I was talking about earlier).

My suggestion would be to switch your URDF to xacro and use the method below. If you are set on using the urdf you could try something like 'robot_description': ParameterValue(Command(['cat ',model_path,]), value_type=str) with the downside being cat is a linux command and probably won't work on windows. Ideally it would have a OS agnostic command to get the contents of a file... Maybe with H-turtle we will see it

Also shown in the example is the use of conditionals for choosing the gui vs non-gui joint_state_publisher

With all that being said, the XML launch system is still supported and could be used with minimal porting.

Hope this helps!

#!/usr/bin/env python3

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.substitutions import LaunchConfiguration, Command, PathJoinSubstitution
from launch.conditions import IfCondition, UnlessCondition
from launch_ros.actions import Node
from launch_ros.descriptions import ParameterValue # Need master or Galactic branch for this feature
from launch_ros.substitutions import FindPackageShare

def generate_launch_description():
  this_pkg = FindPackageShare('wheeled_platform_tutorial')

  model_path = LaunchConfiguration('model_path')
  params_path = LaunchConfiguration('params_path')
  use_sim_time = LaunchConfiguration('use_sim_time')
  use_gui = LaunchConfiguration('use_gui')

  return LaunchDescription([
    DeclareLaunchArgument(
      'model_path',
      default_value=PathJoinSubstitution([this_pkg,'xacro','test_rig_model_top.gazebo']),
      description='Full path to model xacro file including filename and extension'
    ),

    DeclareLaunchArgument(
      'params_path',
      default_value=PathJoinSubstitution([this_pkg,'params','test_rig_model.yaml']),
      description='Full path to parameter yaml file including filename and extension'
    ),

    DeclareLaunchArgument(
      'use_sim_time',
      default_value='false',
      description='Use sim (Gazebo) clock when True'
    ),

    DeclareLaunchArgument(
      'use_gui',
      default_value='True',
      description='True causes joint_state_publisher to launch joint_state_publisher_gui'
    ),

    Node(
      package='robot_state_publisher',
      executable='robot_state_publisher',
      name='robot_state_publisher',
      output='screen',
      parameters=[{
        'robot_description': ParameterValue(Command(['xacro ',model_path,' params_path:=',params_path]), value_type=str)
      }],
    ),

    Node(
      package='joint_state_publisher_gui',
      executable='joint_state_publisher_gui',
      name='joint_state_publisher',
      output='screen',
      condition=IfCondition(use_gui)
    ),

    Node(
      package='joint_state_publisher',
      executable='joint_state_publisher',
      name='joint_state_publisher',
      output='screen',
      condition=UnlessCondition(use_gui)
    )

  ])