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：
- python 脚本
- xml 文件
- yaml 文件
二、启动launch文件
2.1 launch 的用途
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。
2.2 launch启动指令
1）xml文件yaml文件
ros2 launch <package_name> <launch_file_name>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：
<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>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。
- 设置命令行参数的默认值，
- 设置 launch file 的包含关系，通过 标签
- 设置 Node 信息，包括 name、namespace、parameter
- 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：
from launch import LaunchDescriptionfrom 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'),
])
])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。
<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：
- 把一个 node 原本来发布的 topic，映射为另外一个名字
- 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
# 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
5.2 基于xml的写法
<!-- 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="$(var background_b)"/>
</node>
<!-- perform remap so both turtles listen to the same command topic -->
<node pkg="turtlesim" exec="mimic" name="mimic">
<remap from="/input/pose" to="/turtlesim1/turtle1/pose"/>
<remap from="/output/cmd_vel" to="/turtlesim2/turtle1/cmd_vel"/>
</node>
</launch>5.3 基于yaml的文件
# example.launch.yaml
launch:
# args that can be set from the command line or a default will be used
- arg:
name: "background_r"
default: "0"
- arg:
name: "background_g"
default: "255"
- arg:
name: "background_b"
default: "0"
- arg:
name: "chatter_ns"
default: "my/chatter/ns"
# include another launch file
- include:
file: "$(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"参考文章：
ROS2 中的 launch 文件入门的 6 个疑问 - 知乎 (zhihu.com)
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