前言

书本在这一节前还有一个用面向对象编程的方法改写发布和订阅者，那个太无聊了而且没什么好说的，直接略过我们

Launch文件可以一次启动多个节点，ROS2分为XML、YAML、Python三种语言可以用来编写。

XML

新建topic_pkg/launch/pub_sub.launch.xml

<launch>

 <node pkg = "topic_pkg" exec = "publisher_node" name = "publisher_node" />

 <node pkg = "topic_pkg" exec = "subscriber_node" name = "subscriber_node" />
 
</launch>

改写CMakeLists.txt新增

install(
  DIRECTORY
    launch
  DESTINATION
    share/${PROJECT_NAME}
)

完成之后就可以查看效果了

colcon build
source install/setup.bash
ros2 launch topic_pkg pub_sub.launch.xml

YAML

新建topic_pkg/launch/pub_sub.launch.yaml

launch:
- node:
    pkg: "topic_pkg"
    exec: "publisher_node"
    name: "publisher_node"
- node:
    pkg: "topic_pkg"
    exec: "subscriber_node"
    name: "subscriber_node"

Cmake需要修改的地方和XML一致，所以不需要继续修改，也可以用同样的流程查看效果

Python

from launch_ros.actions import Node
from launch import LaunchDescription

def generate_launch_description():

    publisher_cmd = Node(
        package='topic_pkg',
        executable='publisher_node',
        name='publisher_node'
        )

    subscriber_cmd = Node(
        package='topic_pkg',
        executable='subscriber_node',
        name='subscriber_node'
        )

    ld = LaunchDescription()

    ld.add_action(publisher_cmd)
    ld.add_action(subscriber_cmd)
    
    return ld