1. 新增launch文件

参考https://gitee.com/pibot/ros2_tutorials/blob/master/src/cpp_parameters/launch/cpp_parameters_launch.py
不同于ROS1的xml格式的launch文件，ROS2的launch文件直接是一个python文件

对照ROS1的launch

<node name="pibot_driver" pkg="pibot_bringup" type="pibot_driver" output="screen">
    <param name="base_frame" value="base_link" />
    <param name="baudrate" value="$(arg baudrate)" />
    <param name="cmd_vel_topic" value="cmd_vel" />
    <param name="odom_frame" value="odom" />
    ...
</node>

对应ROS2中LaunchDescription传入的Node实例，接受参数根据名字容易理解,
package executable name output parameters，且ROS1的launch都能找到对应的字段

在launch下新增bringup_launch.py文件，如下

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package="pibot_bringup",
            executable="pibot_driver",
            name="pibot_driver",
            output="screen",
            emulate_tty=True,
            parameters=[
                {"base_frame": "base_link"},
                {"port": "/dev/pibot"},
                {"baudrate": 115200}
            ]
        )
    ])

执行命令os2 launch pibot_bringup bringup_launch.py运行,提示无法找到launch文件

➜  . install/setup.bash                                                                                    
➜  ros2 launch  pibot_bringup bringup_launch.py                                                                                     
file 'bringup_launch.py' was not found in the share directory of package 'pibot_bringup' which is at '/home/pibot/ros2_ws/install/pibot_bringup/share/pibot_bringup'

对比https://gitee.com/pibot/ros2_tutorials/blob/master/src/cpp_parameters/CMakeLists.txt发现有个新增launch的install

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

新增后重新编译再次运行

➜  ros2 launch  pibot_bringup bringup_launch.py
[INFO] [launch]: All log files can be found below /home/pibot/.ros/log/2022-05-04-11-25-41-814156-david-ThinkPad-E420-45992
[INFO] [launch]: Default logging verbosity is set to INFO
[INFO] [pibot_driver-1]: process started with pid [46004]
[pibot_driver-1] [INFO] [1651634742.385702144] [pibot_driver]: BaseDriver startup...
[pibot_driver-1] [INFO] [1651634742.385829107] [pibot_driver]: subscribe cmd topic on [/cmd_vel]
[pibot_driver-1] [INFO] [1651634742.386822348] [pibot_driver]: init ok

2. 参数读取

串口文件名，波特率通过launch的参数传入的，我们新增在代码中读取该参数。
参考https://gitee.com/pibot/ros2_tutorials/blob/master/src/cpp_parameters/src/cpp_parameters_node.cpp 添加参数的声明

  node->declare_parameter<std::string>("port", "/dev/ttyACM0");
  node->declare_parameter<int32_t>("baudrate", 115200);

编译运行

ros2 launch  pibot_bringup bringup_launch.py

打开另外的窗口输入ros2 param get /pibot_driver port命令可以查看到参数的值

➜  ros2 param list
/pibot_driver:
  baudrate
  port
  qos_overrides./parameter_events.publisher.depth
  qos_overrides./parameter_events.publisher.durability
  qos_overrides./parameter_events.publisher.history
  qos_overrides./parameter_events.publisher.reliability
  use_sim_time
➜  ros2 param get /pibot_driver port
String value is: /dev/pibot

也可以输入ros2 run rqt_reconfigure rqt_reconfigure查看
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3. 添加参数约束

这些通过launch传入的参数，在初始化时使用，我们添加只读属性避免被改导致二义性

  rcl_interfaces::msg::ParameterDescriptor descriptor;
  descriptor.read_only = true;

  node->declare_parameter<std::string>("port", "/dev/ttyACM0", descriptor);
  node->declare_parameter<int32_t>("baudrate", 115200, descriptor);

4. 通讯相关

通讯(SerialTransport,SimpleDataframe)与ROS无关的，只需要修改相关日志输出的即可

本文代码https://gitee.com/pibot/pibot_bringup/tree/bc942bdb49ec5a358a0a13f8359eb5e7878e2350