【ROS实操六】launch的使用
一、简介
简介
launch 字面意为“启动”、“发射”，在ROS2中主要用于启动程序。launch模块由launch文件与 ros2 launch命令组成，前者用于打包并配置节点，后者用于执行launch文件。
作用：简化节点的配置与启动，提高程序的启动效率。
准备工作
1.新建工作空间ws02_tools，本章后续编码实现都基于此工作空间；
2.终端下进入工作空间的src目录，调用如下两条命令分别创建C++功能包和Python功能包。
ros2 pkg create cpp01_launch --build-type ament_cmake --dependencies rclcpp
ros2 pkg create py01_launch --build-type ament_python --dependencies rclpy
3.在使用Python版的launch文件时，涉及的API众多，为了提高编码效率，可以在VScode中设置launch文件的代码模板，将VScode的配置文件python.json修改为为如下内容：
{// Place your snippets for python here. Each snippet is defined under a snippet name and has a prefix, body and // description. The prefix is what is used to trigger the snippet and the body will be expanded and inserted. Possible variables are:// $1, $2 for tab stops, $0 for the final cursor position, and ${1:label}, ${2:another} for placeholders. Placeholders with the // same ids are connected.// Example:// "Print to console": {// "prefix": "log",// "body": [// "console.log('$1');",// "$2"// ],// "description": "Log output to console"// }"ros2 node": {"prefix": "ros2_node_py","body": ["\"\"\" "," 需求："," 流程："," 1.导包；"," 2.初始化ROS2客户端；"," 3.自定义节点类；"," ", " 4.调用spain函数，并传入节点对象；"," 5.资源释放。 ","","","\"\"\"","# 1.导包；","import rclpy","from rclpy.node import Node","","# 3.自定义节点类；","class MyNode(Node):"," def __init__(self):"," super().__init__(\"mynode_node_py\")","","def main():"," # 2.初始化ROS2客户端；"," rclpy.init()"," # 4.调用spain函数，并传入节点对象；"," rclpy.spin(MyNode())"," # 5.资源释放。 "," rclpy.shutdown()","","if __name__ == '__main__':"," main()",],"description": "ros2 node"},"ros2 launch py": {"prefix": "ros2_launch_py","body": ["from launch import LaunchDescription","from launch_ros.actions import Node","# 封装终端指令相关类--------------","# from launch.actions import ExecuteProcess","# from launch.substitutions import FindExecutable","# 参数声明与获取-----------------","# from launch.actions import DeclareLaunchArgument","# from launch.substitutions import LaunchConfiguration","# 文件包含相关-------------------","# from launch.actions import IncludeLaunchDescription","# from launch.launch_description_sources import PythonLaunchDescriptionSource","# 分组相关----------------------","# from launch_ros.actions import PushRosNamespace","# from launch.actions import GroupAction","# 事件相关----------------------","# from launch.event_handlers import OnProcessStart, OnProcessExit","# from launch.actions import ExecuteProcess, RegisterEventHandler,LogInfo","# 获取功能包下share目录路径-------","# from ament_index_python.packages import get_package_share_directory","","def generate_launch_description():"," ", " return LaunchDescription([])"],"description": "ros2 launch"}
}二、基本使用流程
在 ROS2 中，launch 文件可以使用Python、XML或YAML编写，不同格式的 launch 文件基本使用流程一致。接下来我们通过一个案例演示 launch 文件的基本编写编译流程，案例需求：编写并执行 launch 文件，可以启动两个turtlesim_node节点。
实现步骤如下：
- 编写launch文件；
- 编辑配置文件；
- 编译；
- 执行。
1.C++实现
1.编写launch文件
功能包cpp01_launch下，创建launch目录。launch文件可以是python文件、xml文件或yaml文件，不同类型的launch文件可以直接存储在launch目录下，或者为了方便管理，我们也可以在launch目录下新建py、xml和yaml三个文件夹分别存储对应类型的launch文件，并且建议不同格式的launch文件命名时分别使用_launch.py、_launch.xml、_launch.yaml或.launch.py、.launch.xml、.launch.yaml作为后缀名。
不同类型的 launch 文件示例如下：
Python文件：py00_base.launch.py
from launch import LaunchDescription
from launch_ros.actions import Nodedef generate_launch_description():turtle1 = Node(package="turtlesim", executable="turtlesim_node", name="t1")turtle2 = Node(package="turtlesim", executable="turtlesim_node", name="t2")return LaunchDescription([turtle1, turtle2])
XML文件：xml00_base.launch.xml
<launch><node pkg="turtlesim" exec="turtlesim_node" name="t1" /><node pkg="turtlesim" exec="turtlesim_node" name="t2" />
</launch>
YAML文件：yaml00_base.launch.yaml
launch:
- node:pkg: "turtlesim"exec: "turtlesim_node"name: "t1"
- node:pkg: "turtlesim"exec: "turtlesim_node"name: "t2"
launch 文件编写完毕后，其实已经可以直接使用 ros2 launch 文件路径 的方式执行了，终端中进入当前工作空间，输入如下指令：
ros2 launch src/cpp01_launch/launch/py/py00_base.launch.py
但是这种执行方式不建议。
2.编辑配置文件
CMakeLists.txt 中添加语句：
install(DIRECTORY launch DESTINATION share/${PROJECT_NAME})
无论该功能包的launch目录下有多少个launch文件，launch相关配置只需设置一次即可。
3.编译
终端中进入当前工作空间，编译功能包：
colcon build --packages-select cpp01_launch
4.执行
当前工作空间下，启动终端，输入如下指令：
. install/setup.bash
ros2 run cpp01_launch py00_base.launch.py
该指令运行的是 python 格式的 launch 文件，其他两个launch文件与之同理。最终，会启动两个 turtlesim_node 节点。
2.Python实现
1.编写launch文件
功能包py01_launch下，创建launch目录，launch目录下新建py、xml和yaml三个文件夹分别存储对应类型的launch文件，launch 文件实现与 rclcpp 完全一致。并且也可以使用ros2 launch 文件路径的方式执行。
2.编辑配置文件
编辑 setup.py 文件，需要在 data_files 属性中，添加相关 launch 文件的路径，修改后的内容如下：
from setuptools import setup
from glob import glob
package_name = 'py01_launch'setup(name=package_name,version='0.0.0',packages=[package_name],data_files=[('share/ament_index/resource_index/packages',['resource/' + package_name]),('share/' + package_name, ['package.xml']),# launch 文件相关配置('share/' + package_name, glob("launch/py/*.launch.py")),('share/' + package_name, glob("launch/xml/*.launch.xml")),('share/' + package_name, glob("launch/yaml/*.launch.yaml"))],install_requires=['setuptools'],zip_safe=True,maintainer='ros2',maintainer_email='ros2@todo.todo',description='TODO: Package description',license='TODO: License declaration',tests_require=['pytest'],entry_points={'console_scripts': [],},
)
无论该功能包的launch目录下有多少个launch文件，launch相关配置只需设置一次即可。
3.编译
终端中进入当前工作空间，编译功能包：
colcon build --packages-select py01_launch
4.执行
当前工作空间下，启动终端，输入如下指令：
. install/setup.bash
ros2 run py01_launch py00_base.launch.py
该指令运行的是 python 格式的 launch 文件，其他两个launch文件与之同理。最终，会启动两个 turtlesim_node 节点。
建议：对于带有启动文件的功能包，最好在功能包的package.xml中添加对包ros2launch的执行时依赖：
<exec_depend>ros2launch</exec_depend>
这有助于确保在构建功能包后ros2 launch命令可用。它还确保可以识别不同格式的 launch 文件。
下期见！