Continuing the content of the previous chapter, this article talks about the source code in turtle_tf2_listener.cpp.

Just go to the source code:

#include <ros/ros.h>
#include <tf2_ros/transform_listener.h>
#include <geometry_msgs/TransformStamped.h>
#include <geometry_msgs/Twist.h>
#include <turtlesim/Spawn.h>

int main(int argc, char** argv){
  ros::init(argc, argv, "my_tf2_listener");

  ros::NodeHandle node;

// Calling the service generates a second turtle2

  ros::service::waitForService("spawn");
  ros::ServiceClient spawner = 
    node.serviceClient<turtlesim::Spawn>("spawn");
  turtlesim::Spawn turtle;
turtle.request.x = 4;        //The x-axis under the world coordinate system when turtle2 is generated is 4. Here you can change the value yourself and experience it carefully
  turtle.request.y = 2;           //The y-axis under the world coordinate system when turtle2 is generated is 2. Here you can change the value yourself and experience it carefully.
turtle.request.theta = 0;           //turtle2 The z-axis under the world coordinate system when generated is 0, and 0 degrees indicates that the glans is facing the right.
  turtle.request.name = "turtle2";
  spawner.call(turtle);

// Declare Publisher that controls turtle2 speed

  ros::Publisher turtle_vel =
    node.advertise<geometry_msgs::Twist>("turtle2/cmd_vel", 10);

//Instantiate a buffer that receives tf data buff

  tf2_ros::Buffer tfBuffer;

//Create a TransformListener object. After creating the listener, it will start receiving all tf2 conversions over the network.
//The scope of the TransformListener object should be persistent, otherwise its cache will not be filled and almost every query will fail. 
//A common method is to make the TransformListener object a member variable of the class.
//In this way, tfListener is equivalent to an instance of the TransformListener class, and tfBuffer is equivalent to its member variable
  tf2_ros::TransformListener tfListener(tfBuffer);

//The speed of publishing information is 10Hz, ros::Rate loop_rate( ) is written outside the loop

  ros::Rate rate(10.0);
  while (node.ok()){
    geometry_msgs::TransformStamped transformStamped;

// try-catch structure, can get thrown exception
    try{

// 1. Define the listener tf::TransformListener listener
// 2. Define the variable tf::StampedTransform transform
// 3. lookupTransform() is the core method of the tf library used to monitor the transformation between two coordinate systems and realize the transformation from /turtle2 coordinate system to turtle1 coordinate system
      // 4. ros::Time(0) represents the latest coordinate transformation. We can only subscribe to the latest tf transformation, but not the current tf transformation, which means we can only listen to the latest ros::Time(0) moment.
//       Can't listen to ros::Time::now(), because it takes time to create tf
      transformStamped = tfBuffer.lookupTransform("turtle2", "turtle1",
                               ros::Time(0));
    }
    catch (tf2::TransformException &ex) {
      ROS_WARN("%s",ex.what());
      ros::Duration(1.0).sleep();
      continue;
    }

    geometry_msgs::Twist vel_msg;

// pow(x,2) find the square of x
// sqrt(x) finds the square root of x
//Why do you think about the linear velocity of the x-axis and the angular velocity of the z-axis and you will know the reason?
    vel_msg.angular.z = 4.0 * atan2(transformStamped.transform.translation.y,
                                    transformStamped.transform.translation.x);
    vel_msg.linear.x = 0.5 * sqrt(pow(transformStamped.transform.translation.x, 2) +
                                  pow(transformStamped.transform.translation.y, 2));

// Publisher Turtle 2's speed
    turtle_vel.publish(vel_msg);
    
    rate.sleep();
  }
  return 0;
};

Finally: If you have read the analysis in the previous chapter turtle_tf2_broadcaster.cpp, we should know that turtle1 is at the origin of the world coordinate system, because : poseCallback callback function:

    transformStamped.transform.translation.x = msg->x;
    transformStamped.transform.translation.y = msg->y;

In the source code, msg->x;msg->y; means msg->x+0;msg->y+0; coincides with the origin of the world coordinate system (world)! ! It's crucial here, it's easy to understand this

The last one is the last one, the turtle 2 chases the turtle 1, the turtle 3 chases the turtle 2, and the source code of 3 turtles. The turtle_tf2_listener.cpp file is modified as follows:

turtle_tf2_demo_cpp.launch is modified as follows:

Running result: roslaunch turtle_tf2 turtle_tf2_demo_cpp.launch

The procedure requires precipitation and precipitation.