How to subscribe the topic only once

asked 2017-09-06 04:15:23 -0500

hudaming
13 ●3 ●4 ●8

updated 2017-09-06 08:17:53 -0500

8729 ●137 ●239 ●256 https://github.com/lucasw

I have a problem about how to sucscribe the topic only once,In some time, I only hope to get the first data from the topic, rather than change the publisher. So how should I do? This is my code:

#include  <ros/ros.h>
#include <sensor_msgs/NavSatFix.h>
#include <geometry_msgs/PointStamped.h>
static geometry_msgs::PointStamped point;

void imageCallback(const sensor_msgs::NavSatFix::ConstPtr &msg)
{
  point.point.x=msg->latitude;
  point.point.y=msg->longitude;
  point.point.z=msg->altitude;
}



int main(int argc, char **argv)
{
  ros::init(argc, argv, "test");
  ros::NodeHandle nh;
  ros::Subscriber positions_sub = nh.subscribe("/POSITIONS", 3, &imageCallback);
  ros::spin();
}

In this code, I can get many points, but I only hope to get the first point. What should I change in tthe code ? THANKS!

answered 2017-09-06 05:31:18 -0500

gvdhoorn
86574 ●283 ●1432 ●1054 http://cor.tudelft.nl/

updated 2019-01-24 11:42:05 -0500

While @bvbdort's answer will probably work, there is a convenience method that essentially does all that for you in a single line: ros::topic:waitForMessage(..).

It also supports specifying timeouts, which NodeHandle to use and some other niceties.

Edit:

#include <ros/ros.h>
#include <sensor_msgs/NavSatFix.h>
#include <geometry_msgs/PointStamped.h>

int main(int argc, char *argv[])
{
  ros::init(argc, argv, "test");
  ros::NodeHandle nh;

  sensor_msgs::NavSatFixConstPtr navsatfix_msg = ros::topic::waitForMessage("/POSITIONS");

  geometry_msgs::PointStamped point;
  point.point.x = navsatfix_msg->latitude;
  point.point.y = navsatfix_msg->longitude;
  point.point.z = navsatfix_msg->altitude;

  // do something with 'point'
  ...
}

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Sorry, I can not get the solution. Can you give me an example? My hope is to loop the imageCallback only once. Thanks!

hudaming ( 2017-09-06 08:25:24 -0500 )edit

See my edit. This is somewhat pseudo-code though, I haven't (even) compile-tested it.

gvdhoorn ( 2017-09-06 08:41:54 -0500 )edit

@gvdhoorn Your code didn't work for me, because the return value of waitForMessage is a boost::shared_ptr. So in this case I'd need to change that to boost::shared_ptr<sensor_msgs::NavSatFix> msg = ros::topic::waitForMessage<sensor_msgs::NavSatFix>("/POSITIONS");. You might want to fix this.