[ROS Study Notes] (VII) Implementing the Server

I. Objective

The server waits for a signal. Each time it receives a signal from the client, it toggles the turtle’s motion state (moving → stopped, stopped → moving).

II. Create the Server Code

In the ~/catkin_ws/src/learning_service/src directory, create a file named turtle_command_server.cpp.

/**
 * 该例程将执行/turtle_command服务，服务数据类型std_srvs/Trigger
 REFERENCE:www.guyuehome.com
 */
 
#include <ros/ros.h>
#include <geometry_msgs/Twist.h>
#include <std_srvs/Trigger.h>

ros::Publisher turtle_vel_pub;
bool pubCommand = false;

// service回调函数，输入参数req，输出参数res
bool commandCallback(std_srvs::Trigger::Request  &req,
         			std_srvs::Trigger::Response &res)
{
	pubCommand = !pubCommand;

    // 显示请求数据
    ROS_INFO("Publish turtle velocity command [%s]", pubCommand==true?"Yes":"No");

	// 设置反馈数据
	res.success = true;
	res.message = "Change turtle command state!";

    return true;
}

int main(int argc, char **argv)
{
    // ROS节点初始化
    ros::init(argc, argv, "turtle_command_server");

    // 创建节点句柄
    ros::NodeHandle n;

    // 创建一个名为/turtle_command的server，注册回调函数commandCallback
    ros::ServiceServer command_service = n.advertiseService("/turtle_command", commandCallback);

	// 创建一个Publisher，发布名为/turtle1/cmd_vel的topic，消息类型为geometry_msgs::Twist，队列长度10
	turtle_vel_pub = n.advertise<geometry_msgs::Twist>("/turtle1/cmd_vel", 10);

    // 循环等待回调函数
    ROS_INFO("Ready to receive turtle command.");

	// 设置循环的频率
	ros::Rate loop_rate(10);

	while(ros::ok())
	{
		// 查看一次回调函数队列
    	ros::spinOnce();
		
		// 如果标志为true，则发布速度指令
		if(pubCommand)
		{
			geometry_msgs::Twist vel_msg;
			vel_msg.linear.x = 0.5;
			vel_msg.angular.z = 0.2;
			turtle_vel_pub.publish(vel_msg);
		}

		//按照循环频率延时
	    loop_rate.sleep();
	}

    return 0;
}

Implementation process:

1. Initialize the ROS node
2. Create a server instance
3. Wait for service requests in a loop and invoke the callback function
4. Process the service request in the callback function and return response data

III. Configure the Build Rules for the Server Code

Open the learning_service package’s CMakeLists.txt and add the following code in the area shown.

add_executable(turtle_command_server src/turtle_command_server.cpp)
target_link_libraries(turtle_command_server ${catkin_LIBRARIES})

This compiles turtle_command_server.cpp into the turtle_command_server executable and links the required libraries.

IV. Compile and Run

Compile the server:

cd ~/catkin_ws
cadkin_make

Load the environment variables (if .bashrc already contains the added environment variables, you do not need to perform this step again):

source devel/setup.bash

Then run the following commands (launch each of the following three lines in a separate terminal):

roscore
rosrun turtlesim turtlesim_node
rosrun learning_service turtle_command_server

Open another terminal and enter rosservice call /turtle_command+空格+两次Tab to send a signal. The turtle starts moving; send the same signal again, and the turtle stops.