ROS study notes (7) - Programming implementation of server-side Server

1. Model

Among them, /turtle_command is the custom service name, and std_srvs::Trigger is the type that has been defined in ROS.

You can use therossrv show std_srvs/Trigger command to query. The meaning of this command can be found in my other article "Common Commands of ROS".

2. Programming implementation

Step 1: Write cpp code

As before, create a new file in the /catkin_ws/src/learning_service/src/ directory and copy the following code into it:turtle_command_server.cpp

/**
 * 该例程将执行/turtle_command服务，服务数据类型std_srvs/Trigger
 */
 
#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;
}

Step 2: Configure the compilation rules of CMakeLists.txt

Similarly, copy the following compilation rules to the corresponding location of/catkin_ws/src/learning_service/CMakeLists.txt file, as shown in the figure below.

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

Step 3: Compile and run

catkin_make
roscore
rosrun turtlesim turtlesim_node
rosrun learning_service turtle_command_server

If you are prompted[rospack] Error: package 'learning_service' not found, it means that you have not modified the environment variables. You need to enter the source catkin_ws/devel/setup.bash command first and then run it againrosrun learning_service turtle_command_serverCommand.

At this time, the little turtle is waiting for us to send a request. Enter the following command to send the request. Because the content of request in std_srvs::Trigger is empty, the content of the curly braces is also empty. You can use the rossrv show std_srvs/Trigger command to query.

rosservice call /turtle_command "{}"

You can see the turtles making circular motions.

PS: If you send it againrosservice call /turtle_command "{}", it will stop moving. If you send it again, it will continue to move... The reason is this line of code in cpp codepubCommand = !pubCommand; .