How to publish only one message on a topic?

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Hi guys,

I have written a program which continuously publishes velocity on a topic. I want to publish only one velocity message at once. In case I change my velocity I need to publish one message per velocity change. How can I do it. Here is my code:

> #include "ros/ros.h"
> #include "nav_msgs/Odometry.h" 
> #include <ros/console.h>
> #include "geometry_msgs/Pose.h"
> #include "geometry_msgs/Twist.h"
> #include <tf/transform_datatypes.h>
> 
> 
> 
> 
> 
> double quatx; double quaty; double
> quatz; double quatw; double
> linearposx; double linearposy;
> 
> void ComPoseCallback(const
> nav_msgs::Odometry::ConstPtr& msg);
> 
> int main(int argc, char **argv) {
>     ros::init(argc, argv, "Test_FK"); 
>     ros::NodeHandle n;                 
>     ros::Publisher Velocity_pub = n.advertise<geometry_msgs::Twist>("/RosAria/cmd_vel",10);
>     ros::Subscriber ComPose_sub = n.subscribe("/RosAria/pose", 1000,
> ComPoseCallback);
> 
>        geometry_msgs::Twist velocity;    velocity.linear.x=1;   
> velocity.linear.y=0;   
> velocity.linear.z=0;   
> velocity.angular.x=0;   
> velocity.angular.y=0;   
> velocity.angular.z=0;
>         //std::cout<<"xPosition"<<linearposx<<std::endl;
> //ros::Rate loop_rate(10);
>        while (ros::ok())
>     {
>         Velocity_pub.publish(velocity);   
> ros::spinOnce();
>     
>    
>     }
>     return 0; }
> 
> void ComPoseCallback(const
> nav_msgs::Odometry::ConstPtr& msg)    
> {
>     ROS_INFO("Seq: [%d]", msg->header.seq);
>     ROS_INFO("Position-> x: [%f], y: [%f], z: [%f]",
> msg->pose.pose.position.x,msg->pose.pose.position.y,
> msg->pose.pose.position.z);
>     ROS_INFO("Orientation-> x: [%f], y: [%f], z: [%f], w: [%f]",
> msg->pose.pose.orientation.x,
> msg->pose.pose.orientation.y,
> msg->pose.pose.orientation.z,
> msg->pose.pose.orientation.w);
>         linearposx=msg->pose.pose.position.x; 
> linearposy=msg->pose.pose.position.y; 
> quatx= msg->pose.pose.orientation.x;  
> quaty= msg->pose.pose.orientation.y;  
> quatz= msg->pose.pose.orientation.z;  
> quatw= msg->pose.pose.orientation.w;
>        tf::Quaternion q(quatx, quaty, quatz, quatw);    tf::Matrix3x3 m(q); 
> double roll, pitch, yaw;   
> m.getEulerYPR(yaw, pitch, roll);
> //getRPY    ROS_INFO("Yaw: [%f],Pitch:
> [%f],Roll: [%f]",yaw,pitch,roll);
> 
>    return ; }

I am using pioneer 3dx robot which maintains its velocity once I give a velocity command. I don't need to send my velocity continuously.