Method does not return after subscription - EDIT : ROS shutdowns after ros::spinOnce()

edit

I am drawing waypoints on a map. I have a MapViewer class with the following constructor.

// MapViewer default constructor
MapViewer::MapViewer(std::queue<geometry_msgs::PointStamped::ConstPtr>* waypointQueue) {
  this->waypointQueue = waypointQueue;
  this->waypointColor = cv::Scalar(0,0,255);
  this->pathColor = cv::Scalar(0,255,0);
  this->mapFile = getenv("TURTLEBOT_NAVIGATION_MAP");
  this->waypointFile = getenv("TURTLEBOT_WAYPOINT_FILE");
  this->setMapMetaData();
  ROS_INFO("4");
  // TODO : Exception handling
}

My setMapMetaData method looks like this

// Get map meta data from /map_metadata topic (one time thing)
void MapViewer::setMapMetaData() {
  ros::NodeHandle n;
  ROS_INFO("1");
  ros::Subscriber sub = n.subscribe(
    "/map_metadata",
    100,
    &MapViewer::mapMetaDataCb,
    this
  );
  ROS_INFO("2");
  ros::spinOnce();
  ROS_INFO("3");
  return; // I don't think this return is necessary
}

My subscriber callback looks like this.

void MapViewer::mapMetaDataCb(const nav_msgs::MapMetaData::ConstPtr& mapMetaData) {
  this->mapMetaData = *mapMetaData;
  ROS_INFO("Map metadata set");
}

And when I run this, my output looks like this.

[ INFO] [1462745271.217146294]: Initializing ROS...
[ INFO] [1462745271.221878557]: 1
[ INFO] [1462745271.228107221]: 2
[ INFO] [1462745271.228156984]: 3

The "Initializing ROS..." comes from the main method. The problem is even though "3" is logged, "4" never is!!! Maybe I am missing something obvious but I have no clue as to what it is. Obviously since "3" is printed, ros::spinOnce must have returned. But apparently setMapMetaData has not returned.

Can someone help me figure out what I am missing?

EDIT

The method actually returns. Instead of ROS_INFO("4"), when I use std::cout << 4 << std::endl; it prints 4. The issue is (as a friend explained to me) that ROS is killed when I call spinOne() only once and stop calling it again.

I solved the problem by changing the setMapMetaData method to

// Get map meta data from /map_metadata topic (one time thing)
void MapViewer::setMapMetaData() {
  ROS_INFO("Subscribing to topic %s", MapViewer::MAP_METADATA_TOPIC.c_str());
  ros::NodeHandle n;
  ros::Subscriber sub = n.subscribe(
    MapViewer::MAP_METADATA_TOPIC,
    1,
    &MapViewer::mapMetaDataCb,
    this
  );

  while(!this->mapMetaDataSet) {
    // Block until map metadata is set
  }
}

And executed it in a separate thread after MapViewer instance was created. Because I subscribe to another topic in a different class (which calls ros::spin() ), the mapMetaDataCb() is also executed at that point.

I am not accepting this as an answer because I need to figure out why ROS is killed when I don't call spinOnce() anymore and how to counter it. Appreciate if you could shed some light into this.