以发布和订阅topic为例理解ROS的套路

套路总结:
1.初始化并命名节点；
2.创建句柄；
3.发布消息或订阅消息，topic名称要对应；
4.循环。

talker.cpp↓

#include <ros/ros.h>
#include <topic_demo/gps.h>

int main(int argc, char **argv)
{
  ros::init(argc, argv, "talker");//初始化并命名节点
  ros::NodeHandle nh; //创建句柄，实例化node
  topic_demo::gps msg;// 创建GPS消息
  msg.x = 1.0;
  msg.y = 1.0;
  msg.state = "working";
  ros::Publisher pub = nh.advertise<topic_demo::gps>("gps_info", 1);//创建publisher，<...>里是要发布的消息的模板类型，"..."是topic的名称，1是待处理队列的长度
  ros::Rate loop_rate(1.0);//定义循环发布的频率1Hz
  while(ros::ok()){
    msg.x = 1.01 * msg.x;
    msg.y = 1.02 * msg.y;
    ROS_INFO("Talker: GPS: x = %f, y = %f", msg.x, msg.y);//ROS_INFO相当于cout和printf
    pub.publish(msg);//发布消息
    loop_rate.sleep();//根据定义的频率休眠1s
  };
  return 0;
}

listener.cpp

#include <ros/ros.h>
#include <topic_demo/gps.h>
#include <std_msgs/Float32.h>

void gpsCallback(const topic_demo::gps::ConstPtr &msg)
{
  std_msgs::Float32 distance;//distance是一个结构体，包含了data
  distance.data = sqrt(pow(msg->x, 2), pow(msg->y, 2));
  ROS_INFO("Listener: Distance to origin = %f, state = %s", distance.data, msg->state.c_str());//c_str()输出字符串的首地址，string s = "123"; printf("%s", s.c_str());输出"123"

int main(int argc, char **argv)
{
  ros::init(argc, argv, "listener");
  ros::NodeHandle n;
  ros::Subscriber sub = n.subscribe("gps_info", 1, gpsCallback);//"gps_info"为订阅的topic名称，与talker.cpp中发布的topic名称对应
  ros::spin();//反复调用当前可触发的回调函数，阻塞
  return 0;
}

gps.msg↓

float32 x
float32 y
string state