I am writing a motor controller package in C++ that subscribes to /cmdvel and publishes to /odom. The node runs correctly and rqtgraph shows the subscription and publishers are connected. The problem is the callback function for /cmdvel subscriber never gets called at all. The run() function is called in the constructor, which results in odomloop() being called to calculate and build odometry messages. The cmdvelloop() function just has code used for testing in it. Is the while loop in the run() function causing the callback function to never be called? Does this require a multithreaded executor to handle my run() loop that publishes odometry and another thread to handle the /cmdvel subscription callback? Or would simply moving the run() function call into the sub callback and removing the while loop fix the issue?

I have the code that works with ROS1, and I used the general idea of that for this version. The code is 500 lines long so I will show the general structure:

namespace mc{
mc::mc : Node("driver")

//  set parameters & initialize
 variables
   run()
   }
   void mc::cmdvel_callback(const
   geometry_msgs::msg::Twist::SharedPtr
   twist_msg) 
   {
   // wheel speed (m/s)
   float right_speed = twist_msg->linear.x + track_width *
  twist_msg->angular.z / 2.0;
   float left_speed = twist_msg->linear.x - track_width *
 twist_msg->angular.z / 2.0;

   // send motor commands to mc

   }
   void mc::cmdvel_loop()
   {
//test code
   }
   int mc::run(){
   // connect to serial port
   while (rclcpp::ok())
   {

       cmdvel_loop();
       odom_loop(); // reads encoder count & publishes odometry




    }
}
return 0;
{
int main(int argc, char* argv[])
{
rclcpp::init(argc, argv);
rclcpp::executors::SingleThreadedExecutor exec;
rclcpp::NodeOptions options;
auto node = std::make_shared<mc::mc>();
exec.add_node(node);
exec.spin();

rclcpp::shutdown();

return 0;
}