I am working on controlling the AR Drone with c++ script. I added the subscriber for checking the state of the drone. However, it runs once, when the spinOnce() is called and then it doesn't update the state. What is more, it stacks in the while loop before taking off, whereas where there is no spinOnce() the drone takes off without any issues.

#include "ros/ros.h"
#include "std_msgs/Empty.h"
#include "std_msgs/String.h"
#include <iostream>
#include <ardrone_autonomy/Navdata.h>
#include <geometry_msgs/Twist.h>
using namespace std;
using namespace ros;
using namespace std_msgs;


ardrone_autonomy::Navdata data;
int state = -1;

void receiveNavdata(const ardrone_autonomy::Navdata& data)
{
    //Take in state of ardrone
    cout<<"\nState: "<< data.state<<endl;   
    state=data.state;   
}

int main(int argc, char **argv){

  init(argc, argv, "talker");
NodeHandle rospy;
Empty empty;

Subscriber navData = rospy.subscribe("/ardrone/navdata", 1, receiveNavdata);

while( state == -1){
    ros::spinOnce();
    ros::Duration(0.1).sleep();
}   

Publisher reset;
if(state == 0){
    reset = rospy.advertise<Empty>("/ardrone/reset", 1);

    while(reset.getNumSubscribers() < 1)
        ros::Duration(0.1).sleep();

    reset.publish(empty);
}

while(state == 0)
    ros::Duration(0.1).sleep();

Publisher takeoff = rospy.advertise<Empty>("/ardrone/takeoff", 1);

while(takeoff.getNumSubscribers() < 1){
    cout<<"\nIN while"<<endl;
    ros::Duration(0.1).sleep();
}

cout<<"\nTakeOff"<<endl;

takeoff.publish(empty);
ros::Duration(5).sleep();

Publisher land = rospy.advertise<Empty>("/ardrone/land", 1);

while(land.getNumSubscribers() < 1)
    ros::Duration(0.1).sleep();

cout<<"\nLand"<<endl;

land.publish(empty);

ros::Duration(1).sleep();

return 0;
}