一、说明

        我们知道，ros2 launch是用来同时启动多个节点的指令，与之配套的有一个,文件，在ROS2系统，该流程非常简单。

二、通过launch启动多节点

2.1 制作启动文件

        因为凡是在ros2下编译通过的包，在ros2下都有注册，因此，无需特别指定launch文件到底在哪个路径，你可以任意一个地方建立启动文件：如~/turtlesim_mimic_launch.py

        粘贴下面语句：        

from launch import LaunchDescription
from launch_ros.actions import Nodedef 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'),])])

2.2 启动launch文件

        1）启动的一般格式：

    ros2 launch  

        2）在当前目录执行：

    ros2 launch turtlesim_mimic_launch.py

        3）再执行发布指令： 

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}}"

        可以看到，打开两个不同的终端，并行展示出两个乌龟画面。

三、更复杂的launch指令

3.1 launch文件基本项

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

3.2 分析官方文档的一个案例

# example.launch.pyimport osfrom ament_index_python import get_package_share_directoryfrom 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 PushRosNamespacedef generate_launch_description():# args that can be set from the command line or a default will be usedbackground_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 filelaunch_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 namespacelaunch_include_with_namespace = GroupAction(actions=[# push-ros-namespace to set namespace of included nodesPushRosNamespace(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 namespaceturtlesim_node = Node(package='turtlesim',namespace='turtlesim1',executable='turtlesim_node',name='sim')# start another turtlesim_node in the turtlesim2 namespace# and use args to set parametersturtlesim_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 topicforward_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,])

3.3 语法拆解

1）我们通过一个包含头文件

2） 参数如何初始化

background_r_launch_arg = DeclareLaunchArgument(
        "background_r", default_value=TextSubstitution(text="0")
    )

3）包含其它的launch文件

    # include another launch file
    launch_include = IncludeLaunchDescription(
        PythonLaunchDescriptionSource(
            os.path.join(
                get_package_share_directory('demo_nodes_cpp'),
                'launch/topics/talker_listener.launch.py'))
    )

4）跨命名空间的包含

   # 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'))
            ),
        ]
    )

5）启动节点并且消息重映射

  # 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'),
            ]
        )

6）总返回：将上面的定义、映射等全算上放到list中

   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,
    ])