ROS2 中的 launch 文件入门的 6 个疑问

Posted 2021-10-06 frank909

本文记录了 ROS2 中 launch 文件学习过程中我个人觉得重要的 6 个基本问题，和大家分享，希望可以帮到初学者。

疑问1：launch 文件有什么用？

通过 launch 文件，ROS2 可以同时启动许多节点,这样简化了用命令行去多次启动不同的 Node。
​

疑问2：launch 文件怎么用？

完整的命令：
​

ros2 launch <package_name> <launch_file_name>

​

也可以直接启动 launch 文件，像这样:
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ros2 launch turtlesim_mimic_launch.py

​

turtlesim_mimic_launch.py 是一个 python 文件，在里面定义了一个 launch 文件的内容。
​

但 ROS1 launch 是不支持 py 文件的，而事实上 launch 文件在 ROS2 中有 3 种格式可以实现：
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1. python 脚本
2. xml 文件
3. yaml 文件

​

疑问3：launch 文件里面应该有什么？

node 相关

launch file 文件当中最重要的是 node 信息，它指定了包的信息、node 的名字还有可执行文件的路径。
​

<node pkg="turtlesim" exec="turtlesim_node" name="sim" namespace="turtlesim2">
    <param name="background_r" value="$(var background_r)"/>
    <param name="background_g" value="$(var background_g)"/>
    <param name="background_b" value="$(var background_b)"/>
  </node>

include

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

group

group 可以把多个 node 组合在一起
​

疑问4：如何编写一个 launch file?

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

​

官方文档有出示个一个例子

# example.launch.py

import os

from ament_index_python import get_package_share_directory

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.actions import IncludeLaunchDescription
from launch.actions import GroupAction
from launch.launch_description_sources import PythonLaunchDescriptionSource
from launch.substitutions import LaunchConfiguration
from launch.substitutions import TextSubstitution
from launch_ros.actions import Node
from launch_ros.actions import PushRosNamespace


def generate_launch_description():

    # args that can be set from the command line or a default will be used
    background_r_launch_arg = DeclareLaunchArgument(
        "background_r", default_value=TextSubstitution(text="0")
    )
    background_g_launch_arg = DeclareLaunchArgument(
        "background_g", default_value=TextSubstitution(text="255")
    )
    background_b_launch_arg = DeclareLaunchArgument(
        "background_b", default_value=TextSubstitution(text="0")
    )
    chatter_ns_launch_arg = DeclareLaunchArgument(
        "chatter_ns", default_value=TextSubstitution(text="my/chatter/ns")
    )

    # include another launch file
    launch_include = IncludeLaunchDescription(
        PythonLaunchDescriptionSource(
            os.path.join(
                get_package_share_directory('demo_nodes_cpp'),
                'launch/topics/talker_listener.launch.py'))
    )
    # include another launch file in the chatter_ns namespace
    launch_include_with_namespace = GroupAction(
        actions=[
            # push-ros-namespace to set namespace of included nodes
            PushRosNamespace(LaunchConfiguration('chatter_ns')),
            IncludeLaunchDescription(
                PythonLaunchDescriptionSource(
                    os.path.join(
                        get_package_share_directory('demo_nodes_cpp'),
                        'launch/topics/talker_listener.launch.py'))
            ),
        ]
    )

    # start a turtlesim_node in the turtlesim1 namespace
    turtlesim_node = Node(
            package='turtlesim',
            namespace='turtlesim1',
            executable='turtlesim_node',
            name='sim'
        )

    # start another turtlesim_node in the turtlesim2 namespace
    # and use args to set parameters
    turtlesim_node_with_parameters = Node(
            package='turtlesim',
            namespace='turtlesim2',
            executable='turtlesim_node',
            name='sim',
            parameters=[{
                "background_r": LaunchConfiguration('background_r'),
                "background_g": LaunchConfiguration('background_g'),
                "background_b": LaunchConfiguration('background_b'),
            }]
        )

    # perform remap so both turtles listen to the same command topic
    forward_turtlesim_commands_to_second_turtlesim_node = Node(
            package='turtlesim',
            executable='mimic',
            name='mimic',
            remappings=[
                ('/input/pose', '/turtlesim1/turtle1/pose'),
                ('/output/cmd_vel', '/turtlesim2/turtle1/cmd_vel'),
            ]
        )

    return LaunchDescription([
        background_r_launch_arg,
        background_g_launch_arg,
        background_b_launch_arg,
        chatter_ns_launch_arg,
        launch_include,
        launch_include_with_namespace,
        turtlesim_node,
        turtlesim_node_with_parameters,
        forward_turtlesim_commands_to_second_turtlesim_node,
    ])

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
​

<!-- example.launch.xml -->

<launch>

  <!-- args that can be set from the command line or a default will be used -->
  <arg name="background_r" default="0"/>
  <arg name="background_g" default="255"/>
  <arg name="background_b" default="0"/>
  <arg name="chatter_ns" default="my/chatter/ns"/>

  <!-- include another launch file -->
  <include file="$(find-pkg-share demo_nodes_cpp)/launch/topics/talker_listener.launch.py"/>
  <!-- include another launch file in the chatter_ns namespace-->
  <group>
    <!-- push-ros-namespace to set namespace of included nodes -->
    <push-ros-namespace namespace="$(var chatter_ns)"/>
    <include file="$(find-pkg-share demo_nodes_cpp)/launch/topics/talker_listener.launch.py"/>
  </group>

  <!-- start a turtlesim_node in the turtlesim1 namespace -->
  <node pkg="turtlesim" exec="turtlesim_node" name="sim" namespace="turtlesim1"/>
  <!-- start another turtlesim_node in the turtlesim2 namespace
      and use args to set parameters -->
  <node pkg="turtlesim" exec="turtlesim_node" name="sim" namespace="turtlesim2">
    <param name="background_r" value="$(var background_r)"/>
    <param name="background_g" value="$(var background_g)"/>
    <param name="background_b" value="$(var background_b)"/>
  </node>
  <!-- perform remap so both turtles listen to the same command topic -->
  <node pkg="turtlesim" exec="mimic" name="mimic">
    <remap from="/input/pose" to="/turtlesim1/turtle1/pose"/>
    <remap from="/output/cmd_vel" to="/turtlesim2/turtle1/cmd_vel"/>
  </node>
</launch>

yaml 要写也很容易。
​

# example.launch.yaml

launch:

# args that can be set from the command line or a default will be used
- arg:
    name: "background_r"
    default: "0"
- arg:
    name: "background_g"
    default: "255"
- arg:
    name: "background_b"
    default: "0"
- arg:
    name: "chatter_ns"
    default: "my/chatter/ns"


# include another launch file
- include:
    file: "$(find-pkg-share demo_nodes_cpp)/launch/topics/talker_listener.launch.py"

# include another launch file in the chatter_ns namespace
- group:
    - push-ros-namespace:
        namespace: "$(var chatter_ns)"
    - include:
        file: "$(find-pkg-share demo_nodes_cpp)/launch/topics/talker_listener.launch.py"

# start a turtlesim_node in the turtlesim1 namespace
- node:
    pkg: "turtlesim"
    exec: "turtlesim_node"
    name: "sim"
    namespace: "turtlesim1"

# start another turtlesim_node in the turtlesim2 namespace and use args to set parameters
- node:
    pkg: "turtlesim"
    exec: "turtlesim_node"
    name: "sim"
    namespace: "turtlesim2"
    param:
    -
      name: "background_r"
      value: "$(var background_r)"
    -
      name: "background_g"
      value: "$(var background_g)"
    -
      name: "background_b"
      value: "$(var background_b)"

# perform remap so both turtles listen to the same command topic
- node:
    pkg: "turtlesim"
    exec: "mimic"
    name: "mimic"
    remap:
    -
        from: "/input/pose"
        to: "/turtlesim1/turtle1/pose"
    -
        from: "/output/cmd_vel"
        to: "/turtlesim2/turtle1/cmd_vel"

疑问5：如何在 ros2 launch 时传递参数？

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

疑问6：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。
​

其实就是移花接木，或者说是瞒天过海。
​

<remap 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

​

ROS2 官方教程的示例中有如下代码

Node(
    package='turtlesim',
    executable='mimic',
    name='mimic',
    remappings=[
      ('/input/pose', '/turtlesim1/turtle1/pose'),
      ('/output/cmd_vel', '/turtlesim2/turtle1/cmd_vel'),
    ]
)

实践

官方教程有很详细的指引，这里简单概述一下。

1.创建 launch 文件

turtlesim_mimic_launch.py

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='turtlesim',
            namespace='turtlesim1',
            executable='turtlesim_node',
            name='sim'
        ),
        Node(
            package='turtlesim',
            namespace='turtlesim2',
            executable='turtlesim_node',
            name='sim'
        ),
        Node(
            package='turtlesim',
            executable='mimic',
            name='mimic',
            remappings=[
                ('/input/pose', '/turtlesim1/turtle1/pose'),
                ('/output/cmd_vel', '/turtlesim2/turtle1/cmd_vel'),
            ]
        )
    ])

2.启动

ros2 launch turtlesim_mimic_launch.py

你会看到两个乌龟界面。

3. 调试

发送命令让乌龟动起来.
​

新开一个终端，敲下面的命令。

ros2 topic pub -r 1 /turtlesim1/turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: -1.8}}"

我们仔细观察看到，发送的是 /turtlesim1/turtle1/cmd_vel 这个 Topic，但 2 只乌龟都运动起来，这是因为我们在 mimic 这个节点中做了remap 动作，相当于进行了消息的透传。
​

我们可以通过 rqt_graph 查看更详细的信息。
​

新开一个终端然后输入 rqt_graph。

它们之间的关系还是一目了然的。

参考

1.ROS2官方文档
2.ROS1官方文档