关于rospy和roscpp订阅中的spin

区别

使用后发现rospy的订阅者不需rospy.spin()就可以进行订阅信息并调用回调函数。而roscpp则必须要有ros::spin()或者 ros::spinOnce()才会调用。
对于rospy，把发送端频率设为10,即1秒发送10次，接受端的测试代码：

点击查看代码

import rospy
import time
from std_msgs.msg import String
def callback(data):
    rospy.loginfo(rospy.get_caller_id() + "I heard %s", data.data)

def listener():

    # In ROS, nodes are uniquely named. If two nodes with the same
    # name are launched, the previous one is kicked off. The
    # anonymous=True flag means that rospy will choose a unique
    # name for our 'listener' node so that multiple listeners can
    # run simultaneously.
    rospy.init_node('listener', anonymous=True)
    rate = rospy.Rate(20)

    print("before subscriber.")
    rospy.Subscriber("chatter", String, callback)
    print("after subscriber.")
    # while not rospy.is_shutdown():
    #     print("not shutdown.")
    #     rate.sleep()

    # print("outside")
    # rospy.spin()
    # print("outside")

    time.sleep(0.2)
    # spin() simply keeps python from exiting until this node is stopped

if __name__ == '__main__':
    listener()

输出：

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hqtech@hqtech-ROG-Zephyrus-M16:~/zzy_ws$ rosrun driver listener.py 
before subscriber.
after subscriber.
[INFO] [1662099023.089818]: /listener_29781_1662099022854I heard hello world 1662099023.0857892
[INFO] [1662099023.189953]: /listener_29781_1662099022854I heard hello world 1662099023.185761
hqtech@hqtech-ROG-Zephyrus-M16:~/zzy_ws$ 

time.sleep(0.2)恰好接收到两次。由此可见，rospy的订阅端并不会由rospy.spin()控制是否执行，而仅由程序阻塞，或者说运行的时间控制。回调执行时机只能说在rospy.Subscriber("chatter", String, callback)之后，只要程序还没结束并且收到消息就会执行。我猜测底层应该是新开了一个线程，但我们不能控制它执行的时机。官网的注释写的是很准确的。

关于ros::spin()和 ros::spinOnce()

对于ros::spin()比较直接，就是程序运行到此处直接阻塞，Ctrl + C才会结束，执行时机是运行到ros::spin()时。而对于ros::spinOnce()运行时机和ros::spin()一样，但他记录的是执行 ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);之后到ros::spinOnce()之间的时间所收到消息，执行对应次数的回调函数。通过控制这段时间，或者循环和rate.sleep();可以达到我们想要的效果。使用比较灵活。
接收端测试代码：

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#include "ros/ros.h"
#include "std_msgs/String.h"
#include <unistd.h>

void chatterCallback(const std_msgs::String::ConstPtr& msg) {
    ROS_INFO("I heard: [%s]", msg->data.c_str());
}

int main(int argc, char **argv) {
    ros::init(argc, argv, "listener");
    ros::NodeHandle n;
    ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);
    ros::Rate loop_rate(20);
    sleep(static_cast<unsigned int>(1));
    ros::spinOnce();
    printf("after spin.\n");
//    while (ros::ok()) {
//        printf("before spin");
//        sleep(1);
//        ros::spinOnce();
//        ros::spin();
//        printf("after spin\n");
//        loop_rate.sleep();
//    }
    return 0;
}

输出：

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hqtech@hqtech-ROG-Zephyrus-M16:~/zzy_ws$ rosrun driver listener
[ INFO] [1662100104.940729044]: I heard: [hello world 1662100104.2338932]
[ INFO] [1662100104.942940732]: I heard: [hello world 1662100104.3338606]
[ INFO] [1662100104.942992571]: I heard: [hello world 1662100104.4338796]
[ INFO] [1662100104.943017042]: I heard: [hello world 1662100104.5338557]
[ INFO] [1662100104.943038588]: I heard: [hello world 1662100104.633874]
[ INFO] [1662100104.943063684]: I heard: [hello world 1662100104.7338858]
[ INFO] [1662100104.943087774]: I heard: [hello world 1662100104.8338528]
[ INFO] [1662100104.943117424]: I heard: [hello world 1662100104.9339075]
after spin.
hqtech@hqtech-ROG-Zephyrus-M16:~/zzy_ws$ 

初学者记录一下，欢迎其他同学一起讨论。