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read topic without callback

asked 2014-05-27 11:11:11 -0500

crpizarr gravatar image

Is there a way to read the last message of a topic wihout using a callback? I mean, the tutorials teach to read a topic assigning a callback in a node, so that whenever a topic updates, the callback is executed. I want to read a topic when the user clicks a button, just once, something like this:

read_topic("/my_topic")

that returns the current data in the topic.

Thanks

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answered 2014-05-27 12:50:41 -0500

ahendrix gravatar image

You can get the next message in a topic using ros::topic::waitForMessage().

Note that internally this creates a subscriber on the topic you're interested in, waits for the next message to arrive, and then closes itself down and returns that message to you, so it's a fairly heavy operation, and requires that the topic it's listening to is either a latched topic, or is published frequently.

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Comments

Do you know if there is an aquivalent in Python?

DNiebuhr gravatar image DNiebuhr  ( 2021-02-22 08:27:17 -0500 )edit
2

answered 2014-05-27 11:38:20 -0500

updated 2014-05-27 11:38:38 -0500

What you're looking for is a polled topic which was proposed a while ago, but unfortunately there is currently no plan for it to be implemented.

You can get the same effect by having a variable that gets updated with the most recent message on the topic every time the callback is called, and then read that variable whenever you need the topic.

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answered 2014-11-15 11:39:55 -0500

yuyu gravatar image

This could be a way around? Example code

#include ros/ros.h>
#include boost/thread.hpp>
#include tf/transform_broadcaster.h>
#include turtlesim/Pose.h>

tf::Transform tfA;
tf::Transform tfB;
static boost::mutex mutex;

void topicACallback(const turtlesim::PoseConstPtr& msg){
  tf::Transform transform;
  transform.setOrigin( tf::Vector3(msg->x, msg->y, 0.0) );
  tf::Quaternion q;
  q.setRPY(0, 0, msg->theta);
  transform.setRotation(q);
  mutex.lock();
  tfA = transform;
  mutex.unlock();
}

void topicBCallback(const turtlesim::PoseConstPtr& msg){
  tf::Transform transform;
  transform.setOrigin( tf::Vector3(msg->x, msg->y, 0.0) );
  tf::Quaternion q;
  q.setRPY(0, 0, msg->theta);
  transform.setRotation(q);
  mutex.lock();
  tfB = transform;
  mutex.unlock();
}

void timerCallback(const ros::TimerEvent& event){
  tf::Transform tfA_local,tfB_local;
  mutex.lock();
  tfA_local = tfA;
  tfB_local = tfB;
  mutex.unlock();
  ROS_INFO("doing the rest of computation");
}

int main(int argc, char** argv){
  ros::init(argc, argv, "test");
  ros::NodeHandle node;
  ros::Subscriber sub1 = node.subscribe("/turtle1/pose", 10, &topicACallback);
  ros::Subscriber sub2 = node.subscribe("/turtle2/pose", 10, &topicBCallback);
  ros::Timer timer = node.createTimer(ros::Duration(0.1),timerCallback);
  ros::spin();
  return 0;
};
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Asked: 2014-05-27 11:11:11 -0500

Seen: 9,870 times

Last updated: Nov 15 '14