通过slam同步建图程序看ros2节点参数传递生效顺序

缘起：

ros2 节点参数可以通过启动节点添加，也可以通过launch文件添加，还可以通过配置文件.yaml文件添加，或者是几种方式套用添加，给初学者带来很大的困惑具体是哪个生效了呢，我们拿slam_toolbox  包的online_sync_launch.py来剖析。

程序安装参考这里

(SLAM) 建图时导航 — Navigation 2 1.0.0 文档

我们这里采用源码编译安装，方便随时修改代码编译运行看效果

源码下载地址：https://github.com/SteveMacenski/slam_toolbox/tree/foxy-develhttps://github.com/SteveMacenski/slam_toolbox/tree/foxy-devel

 文件目录结构

 online_sync_launch.py  # 同步建图launch文件源码

import os

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, LogInfo
from launch.conditions import UnlessCondition
from launch.substitutions import LaunchConfiguration, PythonExpression
from launch_ros.actions import Node
from ament_index_python.packages import get_package_share_directory
from nav2_common.launch import HasNodeParams


def generate_launch_description():
    use_sim_time = LaunchConfiguration('use_sim_time')
    params_file = LaunchConfiguration('params_file')
    default_params_file = os.path.join(get_package_share_directory("slam_toolbox"),
                                       'config', 'mapper_params_online_sync.yaml')

    declare_use_sim_time_argument = DeclareLaunchArgument(
        'use_sim_time',
        default_value='true',
        description='Use simulation/Gazebo clock')
    declare_params_file_cmd = DeclareLaunchArgument(
        'params_file',
        default_value=default_params_file,
        description='Full path to the ROS2 parameters file to use for the slam_toolbox node')

    # If the provided param file doesn't have slam_toolbox params, we must pass the
    # default_params_file instead. This could happen due to automatic propagation of
    # LaunchArguments. See:
    # https://github.com/ros-planning/navigation2/pull/2243#issuecomment-800479866
    has_node_params = HasNodeParams(source_file=params_file,
                                    node_name='slam_toolbox')

    actual_params_file = PythonExpression(['"', params_file, '" if ', has_node_params,
                                           ' else "', default_params_file, '"'])

    log_param_change = LogInfo(msg=['provided params_file ',  params_file,
                                    ' does not contain slam_toolbox parameters. Using default: ',
                                    default_params_file],
                               condition=UnlessCondition(has_node_params))

    start_sync_slam_toolbox_node = Node(
        parameters=[
          actual