launch
    LaunchDescription                        # 最终返回

    actions
        DeclareLaunchArgument                # 启动参数
        IncludeLaunchDescription            # 导入启动文件
        ExecuteProcess                        # 运行指定命令
        TimerAction                    

    conditions
        IfCondition

    substitutions
        LaunchConfiguration                    # 导入启动文件中的参数
        PathJoinSubstitution                # 链接路径
        TextSubstitution                    # 参数值
        PythonExpression

    launch_description_sources
        PythonLaunchDescriptionSource        # Python启动文件

lauch_ros
    actions
        Node                                # 节点

    FindPackageShare                        # 查找软件包