• ROS2的launch有何不同？

    ​一、提要

            在launch多节点执行方面，ROS2与ROS1有很大不同。首先，ros2与ros1不同之处，在于ROS2不再有*.launch脚本文件，取而代之的是 3 种格式得以实现： ​

    1. python 脚本
    2. xml 文件
    3. yaml 文件

    二、启动launch文件

    2.1 xml文件yaml文件启动方式

    ros2 launch <package_name> <launch_file_name>

    2.2 对于py文件，也可以直接启动 launch 文件，

    像这样: ​

    ros2 launch turtlesim_mimic_launch.py

    ​

    三、launch文件的内容

    ​3.1 launch 文件内容描述

    因为launch file 文件毕竟是多节点执行脚本，因此，与“ ros2 run ” 指令的内容大致相同，如包名、节点名。当然，要丰富一些。如：

    1. "turtlesim" exec="turtlesim_node" name="sim" namespace="turtlesim2">
    2.  
    3.     "background_r" value="$(var background_r)"/>
    4.  
    5.     "background_g" value="$(var background_g)"/>
    6.  
    7.     "background_b" value="$(var background_b)"/>
    8.  
    9.   

    3.2 关于include

         include 可以在一个 launch file 中包含另外的 launch file ​

    3.3 关于group

    group 可以把多个 node 组合在一起 ​

    四、实践编写一个 launch file

    4.1 必要的套路

    无论是 python、xml 还是 yaml，编写 launch file 的步骤差不多一样。 ​

    1. 设置命令行参数的默认值，
    2. 设置 launch file 的包含关系，通过 标签
    3. 设置 Node 信息，包括 name、namespace、parameter
    4. 如果需要设置 remmaping 则设置 remapping 关系

    ​4.2 举个例子

    以下是官方文档 一个例子，是针对海龟turtlesim的例子：

    1. from launch import LaunchDescriptionfrom launch_ros.actions import Node
    2.  
    3. def generate_launch_description():
    4.     return LaunchDescription([
    5.         Node(
    6.             package='turtlesim',
    7.             namespace='turtlesim1',
    8.             executable='turtlesim_node',
    9.             name='sim'
    10.         ),
    11.  
    12.         Node(
    13.             package='turtlesim',
    14.             namespace='turtlesim2',
    15.             executable='turtlesim_node',
    16.             name='sim'
    17.         ),
    18.  
    19.         Node(
    20.             package='turtlesim',
    21.             executable='mimic',
    22.             name='mimic',
    23.             remappings=[
    24.                 ('/input/pose', '/turtlesim1/turtle1/pose'),
    25.                 ('/output/cmd_vel', '/turtlesim2/turtle1/cmd_vel'),
    26.             ])
    27.  
    28.     ])

    以上代码很清晰，就是启动两个海龟节点，一个mimic节点；上述有些需要解释的概念有：命名空间啥作用？重映射有什么要点？（需要执行后，用指令行进行分析）

    4.3  remap 起什么作用

    remap 是一个非常有用的技巧，一般用来做 Topic 的映射的。某节点本来不认识这个Topic，但通过重起名，或叫伪名，或叫重新伪装（注意，这里订阅者只收某种名称的Topic），使某些接收到该Topic。

    from="/different_topic" to="/needed_topic"/>

    把 from 中的 topic，转换成 to 指定的 topic ​

    有 2 种用途：

    1. 把一个 node 原本来发布的 topic，映射为另外一个名字
    2. 把其他的 node 发布的原始 topic，映射为所需要的 topic

    ​

     4.4 带着参数执行 launch

    我们注意到上面的 launch file 中有 args，比如 background_r，命令行启动时可以将数据透传过去，通过 key:=value 形式。 如：

    ros2 launch <package_name> <launch_file_name> background_r:=255

    五、参考：三种写法

    以下代码用于参考，是同样内容的三种文件实现方法，因为比较长，希望大家慢慢对照阅读。

    5.1 基于python的案例 

    1. # example.launch.py
    2.  
    3. import os
    4.  
    5. from ament_index_python import get_package_share_directory
    6.  
    7. from launch import LaunchDescription
    8. from launch.actions import DeclareLaunchArgument
    9. from launch.actions import IncludeLaunchDescription
    10. from launch.actions import GroupAction
    11. from launch.launch_description_sources import PythonLaunchDescriptionSource
    12. from launch.substitutions import LaunchConfiguration
    13. from launch.substitutions import TextSubstitution
    14. from launch_ros.actions import Node
    15. from launch_ros.actions import PushRosNamespace
    16.  
    17.  
    18. def generate_launch_description():
    19.  
    20.     # args that can be set from the command line or a default will be used
    21.     background_r_launch_arg = DeclareLaunchArgument(
    22.         "background_r", default_value=TextSubstitution(text="0")
    23.     )
    24.     background_g_launch_arg = DeclareLaunchArgument(
    25.         "background_g", default_value=TextSubstitution(text="255")
    26.     )
    27.     background_b_launch_arg = DeclareLaunchArgument(
    28.         "background_b", default_value=TextSubstitution(text="0")
    29.     )
    30.     chatter_ns_launch_arg = DeclareLaunchArgument(
    31.         "chatter_ns", default_value=TextSubstitution(text="my/chatter/ns")
    32.     )
    33.  
    34.     # include another launch file
    35.     launch_include = IncludeLaunchDescription(
    36.         PythonLaunchDescriptionSource(
    37.             os.path.join(
    38.                 get_package_share_directory('demo_nodes_cpp'),
    39.                 'launch/topics/talker_listener.launch.py'))
    40.     )
    41.     # include another launch file in the chatter_ns namespace
    42.     launch_include_with_namespace = GroupAction(
    43.         actions=[
    44.             # push-ros-namespace to set namespace of included nodes
    45.             PushRosNamespace(LaunchConfiguration('chatter_ns')),
    46.             IncludeLaunchDescription(
    47.                 PythonLaunchDescriptionSource(
    48.                     os.path.join(
    49.                         get_package_share_directory('demo_nodes_cpp'),
    50.                         'launch/topics/talker_listener.launch.py'))
    51.             ),
    52.         ]
    53.     )
    54.  
    55.     # start a turtlesim_node in the turtlesim1 namespace
    56.     turtlesim_node = Node(
    57.             package='turtlesim',
    58.             namespace='turtlesim1',
    59.             executable='turtlesim_node',
    60.             name='sim'
    61.         )
    62.  
    63.     # start another turtlesim_node in the turtlesim2 namespace
    64.     # and use args to set parameters
    65.     turtlesim_node_with_parameters = Node(
    66.             package='turtlesim',
    67.             namespace='turtlesim2',
    68.             executable='turtlesim_node',
    69.             name='sim',
    70.             parameters=[{
    71.                 "background_r": LaunchConfiguration('background_r'),
    72.                 "background_g": LaunchConfiguration('background_g'),
    73.                 "background_b": LaunchConfiguration('background_b'),
    74.             }]
    75.         )
    76.  
    77.     # perform remap so both turtles listen to the same command topic
    78.     forward_turtlesim_commands_to_second_turtlesim_node = Node(
    79.             package='turtlesim',
    80.             executable='mimic',
    81.             name='mimic',
    82.             remappings=[
    83.                 ('/input/pose', '/turtlesim1/turtle1/pose'),
    84.                 ('/output/cmd_vel', '/turtlesim2/turtle1/cmd_vel'),
    85.             ]
    86.         )
    87.  
    88.     return LaunchDescription([
    89.         background_r_launch_arg,
    90.         background_g_launch_arg,
    91.         background_b_launch_arg,
    92.         chatter_ns_launch_arg,
    93.         launch_include,
    94.         launch_include_with_namespace,
    95.         turtlesim_node,
    96.         turtlesim_node_with_parameters,
    97.         forward_turtlesim_commands_to_second_turtlesim_node,
    98.     ])

    这是用 python 写的，最后返回的是 LaunchDescription 信息，这个等同于 xml 中 标签中的内容

    5.2 基于xml的写法

    2.  
    3. <launch>
    4.  
    5.   
    6.   <arg name="background_r" default="0"/>
    7.   <arg name="background_g" default="255"/>
    8.   <arg name="background_b" default="0"/>
    9.   <arg name="chatter_ns" default="my/chatter/ns"/>
    10.  
    11.   
    12.   <include file="$(find-pkg-share demo_nodes_cpp)/launch/topics/talker_listener.launch.py"/>
    13.   
    14.   <group>
    15.     
    16.     <push-ros-namespace namespace="$(var chatter_ns)"/>
    17.     <include file="$(find-pkg-share demo_nodes_cpp)/launch/topics/talker_listener.launch.py"/>
    18.   group>
    19.  
    20.   
    21.   <node pkg="turtlesim" exec="turtlesim_node" name="sim" namespace="turtlesim1"/>
    22.   
    23.   <node pkg="turtlesim" exec="turtlesim_node" name="sim" namespace="turtlesim2">
    24.     <param name="background_r" value="$(var background_r)"/>
    25.     <param name="background_g" value="$(var background_g)"/>
    26.     <param name="background_b" value="$(var background_b)"/>
    27.   node>
    28.   
    29.   <node pkg="turtlesim" exec="mimic" name="mimic">
    30.     <remap from="/input/pose" to="/turtlesim1/turtle1/pose"/>
    31.     <remap from="/output/cmd_vel" to="/turtlesim2/turtle1/cmd_vel"/>
    32.   node>
    33. launch>

    5.3 基于yaml的文件

    1. # example.launch.yaml
    2.  
    3. launch:
    4.  
    5. # args that can be set from the command line or a default will be used
    6. - arg:
    7.     name: "background_r"
    8.     default: "0"
    9. - arg:
    10.     name: "background_g"
    11.     default: "255"
    12. - arg:
    13.     name: "background_b"
    14.     default: "0"
    15. - arg:
    16.     name: "chatter_ns"
    17.     default: "my/chatter/ns"
    18.  
    19.  
    20. # include another launch file
    21. - include:
    22.     file: "$(find-pkg-share demo_nodes_cpp)/launch/topics/talker_listener.launch.py"
    23.  
    24. # include another launch file in the chatter_ns namespace
    25. - group:
    26.     - push-ros-namespace:
    27.         namespace: "$(var chatter_ns)"
    28.     - include:
    29.         file: "$(find-pkg-share demo_nodes_cpp)/launch/topics/talker_listener.launch.py"
    30.  
    31. # start a turtlesim_node in the turtlesim1 namespace
    32. - node:
    33.     pkg: "turtlesim"
    34.     exec: "turtlesim_node"
    35.     name: "sim"
    36.     namespace: "turtlesim1"
    37.  
    38. # start another turtlesim_node in the turtlesim2 namespace and use args to set parameters
    39. - node:
    40.     pkg: "turtlesim"
    41.     exec: "turtlesim_node"
    42.     name: "sim"
    43.     namespace: "turtlesim2"
    44.     param:
    45.     -
    46.       name: "background_r"
    47.       value: "$(var background_r)"
    48.     -
    49.       name: "background_g"
    50.       value: "$(var background_g)"
    51.     -
    52.       name: "background_b"
    53.       value: "$(var background_b)"
    54.  
    55. # perform remap so both turtles listen to the same command topic
    56. - node:
    57.     pkg: "turtlesim"
    58.     exec: "mimic"
    59.     name: "mimic"
    60.     remap:
    61.     -
    62.         from: "/input/pose"
    63.         to: "/turtlesim1/turtle1/pose"
    64.     -
    65.         from: "/output/cmd_vel"
    66.         to: "/turtlesim2/turtle1/cmd_vel"

    参考文章：

    Using Python, XML, and YAML for ROS 2 Launch Files — ROS 2 Documentation: Foxy documentation

    ROS2 中的 launch 文件入门的 6 个疑问 - 知乎 (zhihu.com)

    ​

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  • 原文地址：https://blog.csdn.net/gongdiwudu/article/details/125984181