Programming implementation of ROS learning 6 -service node

One 、 Concept

service Service It is a way of synchronous communication between nodes ; Allow clients Client Node issue request Request; By the server Server Feedback response after node processing Response.

Two 、 establish Service Process overview of

1. establish Service Of .cpp Program

2. compile

3. Configure environment variables

4. perform Service Program

3、 ... and 、 The specific process

31. establish Service.cpp file

Create in the function pack of the corresponding workspace Service.cpp file , The file should include the following four parts ：

1. initialization ROS node

2. establish Serve example ( Callable ros The service type data already defined in can also be defined by yourself , How to view ros For the defined service types, see ：）

3. Wait for the service request of the client circularly , After receiving the request, enter the callback function

4. Complete the service function processing in the callback function , And feed back the response data

example ： Simulate the following service model

establish turtle_command_server.cpp file , In this example ros The built-in trigger

/***********************************************************************
Copyright 2020 GuYueHome (www.guyuehome.com).
***********************************************************************/

/**
 *  The routine will execute /turtle_command service , Service data type 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 Callback function , Input parameters req, Output parameters res
bool commandCallback(std_srvs::Trigger::Request  &req,
         			std_srvs::Trigger::Response &res)
{
	pubCommand = !pubCommand;

    //  Display request data 
    ROS_INFO("Publish turtle velocity command [%s]", pubCommand==true?"Yes":"No");

	//  Set feedback data 
	res.success = true;
	res.message = "Change turtle command state!"

    return true;
}

int main(int argc, char **argv)
{
    // ROS Node initialization 
    ros::init(argc, argv, "turtle_command_server");

    //  Create node handle 
    ros::NodeHandle n;

    //  Create a file called /turtle_command Of server, Register callback function commandCallback
    ros::ServiceServer command_service = n.advertiseService("/turtle_command", commandCallback);

	//  Create a Publisher, The release is called /turtle1/cmd_vel Of topic, The message type is geometry_msgs::Twist, The queue length 10
	turtle_vel_pub = n.advertise<geometry_msgs::Twist>("/turtle1/cmd_vel", 10);

    //  Loop waiting for callback function 
    ROS_INFO("Ready to receive turtle command.");

	//  Set the frequency of the cycle 
	ros::Rate loop_rate(10);

	while(ros::ok())
	{
		//  Check the callback function queue once 
    	ros::spinOnce();
		
		//  If the flag is true, Then issue speed command 
		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);
		}

		// Delay according to the cycle frequency 
	    loop_rate.sleep();
	}

    return 0;
}

There is also the publisher's program in this program , Think about it , Our general request is to request a service , Then you give an answer . So in the process, assume that the request is for the service that makes the tortoise move , Then answer . In this program, you can pay attention to how the programs of publishers and servers are combined .

3.2 compile

Add the following code to CmakeList.txt Of install Just above

add_executeable(turtle_command_server src/turtle_command_server.cpp)

target_link_libraries(turtle_command_server ${catkin_LIBRARIES})

Then go back to the main directory of the workspace, open the terminal, and catkin_make once

3.3 Configure environment variables

Return to the main directory of the workspace and open the terminal , then source devel/setup.bash once

Four 、 perform

Because the service makes the little turtle move , So start the little turtle first

roscore
rosrun turtlesim turtlesim_node

Then directly execute your service File can , The instructions are as follows ：

rosrun Function package name .cpp file name

rosrun learning_service turtle_command_server

But note that this is the server , Therefore, there needs to be a request before there is a service , So there must be a client to call your request . The name of the service just created in the program is ：/turtle1/cmd_vel

Therefore, when requesting, you have to ask for the service area . The instructions are as follows ：

rosservice call /turtle_command"{}"