implementing turtlebot bumper sensor

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Hi,

I'm trying to subscribe to turtlebotSensorState to get bumps_wheeldrops. I think I've done everything right but in gazebo simulator when the robot hits an object it's not stopping. below is my codes, any help is appreciated. Also,it doesn't matter that I put the callback before main, does it?

#include turtlebot_node/TurtlebotSensorState.h>

#include ros/rate.h>

void callback01(const turtlebot_node::TurtlebotSensorState::ConstPtr& msg){
turtlebot_node::TurtlebotSensorState bumperState = msg->bumps_wheeldrops;
}
//this part goes to main:
ros::Subscriber sens_sub_ = n.subscribe<turtlebot_node::TurtlebotSensorState>("sensor_state", 1000, callback01);

ros::Rate loop_rate(10);
while (ros::ok()){
    if (bumperState !=0){
            publish (0,0);
    }else
            publish(angular_,linear_):
loop_rate.sleep();
ros::spin();
}