Question: syntax to retrieve topic type

[rachelspykerman]

Hi,

I'm trying to retrieve the topic type for a particular topic name. I can get the topics, but unable to find documentation on how to get the topic type. I tried the following, but this does not work:

function getTopics() {
  var topicsClient = new ROSLIB.Service({
    ros : ros,
    name : '/rosapi/topics',
    serviceType : 'rosapi/Topics'
  });

  var request = new ROSLIB.ServiceRequest();
  topicsClient.callService(request, function(result) {
    console.log("Getting topics...", result);
    var resultsSorted = result.topics.sort();
    for(var i=0; i < resultsSorted.length; i++) {
        // Up until here, I successfully retrieve the topic names
        // Now I want to get the topic type for each topic name, this is not working
        var topicTypeClient = new ROSLIB.Service({
          ros : ros,
          name : '/rosapi/topic_type',
          serviceType : 'rosapi/Topics'
        });

        var request = new ROSLIB.ServiceRequest();
        topicTypeClient.callService(request, function(result) { 
          console.log('Topic Type: ', result.name);  // Guessing here as to how I would access the topic 
        });
    }
  });
};

How do I go about getting the topic types? Any suggestions would be greatly appreciated!

Thanks Racehl

[Behery]

Hi Rachel, there's a service in the rosbridge_suite called TopicType that does this. It's called /rosapi/topic_type and is defined here. You can call it from roslibjs as you did above for listing the topics.

[rachelspykerman]

So I tried the following, and it doesn't work. Any suggestions? Shouldn't I have to specify the topic name somewhere? Also the link you sent doesn't work (shows a page saying Not Found)

  var topicsClient = new ROSLIB.Service({
    ros : ros,
    name : '/rosapi/topics',
    serviceType : 'rosapi/Topics'
  });

  var request = new ROSLIB.ServiceRequest();
  topicsClient.callService(request, function(result) {
    console.log("Getting topics...", result);
    var resultsSorted = result.topics.sort();
    for(var i=0; i < resultsSorted.length; i++) {
        //console.log(resultsSorted[i]);
        var topicTypeClient = new ROSLIB.Service({
          ros : ros,
          name : '/rosapi/topic_type',
          serviceType : 'rosapi/TopicType'
        });

        var request1 = new ROSLIB.ServiceRequest();
        topicTypeClient.callService(request, function(result) {
          console.log('Topic Type: ', result.name);
        });
    }
  });
};

[Behery]

You specify the topic name as a parameter in the service request as follows:

var request = new ROSLIB.ServiceRequest({topic: 'name_of_topic'});

The definition for the service is here: https://github.com/RobotWebTools/rosbridge_suite/blob/develop/rosapi/srv/TopicType.srv (it was incorrect in my last comment, sorry)

There's also a tutorial here: http://wiki.ros.org/roslibjs/Tutorials/BasicRosFunctionality

[Behery]

I'm closing the issue, please reopen it or comment here if you still have problems :)

[rachelspykerman]

Sorry for the delay. I'm still running into issues with the following code:

function getTopics() {
  var topicsClient = new ROSLIB.Service({
    ros : ros,
    name : '/rosapi/topics',
    serviceType : 'rosapi/Topics'
  });

  var request = new ROSLIB.ServiceRequest();
  topicsClient.callService(request, function(result) {
    console.log("Getting topics...", result);
    var resultsSorted = result.topics.sort();
    for(var i=0; i < resultsSorted.length; i++) {
        //console.log(resultsSorted[i]);
        var topicTypeClient = new ROSLIB.Service({
          ros : ros,
          name : '/rosapi/topic_type',
          serviceType : 'rosapi/TopicType',
          topic : resultsSorted[i]
        });

        var request1 = new ROSLIB.ServiceRequest();
        topicTypeClient.callService(request, function(result) {
          console.log('Topic Type: ', result.topic_type);
        });
    }
  });
};

The basic ros functionality tutorial doesn't go over requesting topic types, only topic names (which I have working). The result.topic_type ends up being undefined (also tried result.name). Am I referencing the wrong parameter to access the topic type?

Rachel

[Behery]

it's a normal service that you can call. Add the topic name to the service request (like in my previous comment)

var topicTypeClient = new ROSLIB.Service({
          ros : ros,
          name : '/rosapi/topic_type',
          serviceType : 'rosapi/TopicType',
        });

        var request1 = new ROSLIB.ServiceRequest(
        {topic : resultsSorted[i]});
        topicTypeClient.callService(request, function(result) {
          console.log('Topic Type: ', result.type);
        });

and i just noticed that the topics service also returns a list of types. so you don't need to call the second service.

you can do this:

var topicTypeClient = new ROSLIB.Service({
          ros : ros,
          name : '/rosapi/topics',
          serviceType : 'rosapi/Topics',
        });
topicTypeClient.callService(request, function(result) {
          console.log('Topics: ', result.topics); //topic names
console.log('Topic Types: ', result.types); //topic types 
        });

the later service is defined here : https://github.com/RobotWebTools/rosbridge_suite/blob/develop/rosapi/srv/Topics.srv

[rachelspykerman]

Great thank you! I got it working now :)