ros在关闭节点后发布消息

参考：1.https://answers.ros.org/question/27655/what-is-the-correct-way-to-do-stuff-before-a-node-is-shutdown/
2.http://wiki.ros.org/roscpp/Overview/Initialization%20and%20Shutdown
小车由于在关闭节点后无法发布Twist消息而不能停止，查找后发现可以使用自己的shutdown回调函数解决这一问题。思路是在ros::init中使用ros::init_options::NoSigintHandler选项，然后用signal函数注册自己的回调函数，用该回调函数控制原子变量g_request_shutdown从而获得节点的运行状态，最后用ros::shutdown()手动关闭节点。
在python中可以直接使用rospy.on_shutdown()来更改节点关闭时的行为。

#include 
#include "calc_motion.h"
#include "geometry_msgs/Twist.h"
#include 


// Signal-safe flag for whether shutdown is requested
sig_atomic_t volatile g_request_shutdown = 0;

void mySigIntHandler(int sig){
    g_request_shutdown = 1;
}

int main(int argc, char **argv){
    // 关闭时使用自己的回调函数
    ros::init(argc, argv, "motion", ros::init_options::NoSigintHandler);
    signal(SIGINT, mySigIntHandler);    

    ros::NodeHandle nh;
    geometry_msgs::Twist twist;
    CalcMotion cm;

    ros::Publisher pub = nh.advertise("/cmd_vel", 5);
    ros::Subscriber sub = nh.subscribe("/direction", 1, &CalcMotion::getDirection, &cm);
    
    ros::Rate r(10);
    while(!g_request_shutdown){
        ros::spinOnce();
        // ROS_DEBUG("Twist x = %f, z = %f", twist.linear.x, twist.angular.z);        
        cm.calcTwist(twist);
        pub.publish(twist);
        r.sleep();

    }
    twist.linear.x = twist.angular.z = 0;
    pub.publish(twist);     
    // 手动关闭ros节点
    ros::shutdown();
    return 0;
}