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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
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.
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;
}; 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.
Asked: 2014-05-27 11:11:11 -0500
Seen: 11,172 times
Last updated: Nov 15 '14
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