RobocompRosChatter.md

RoboComp Component & Ros Node chatter

Here we will create a RoboComp Component which can communicate with a Ros Node using Ros Middleware. The Ros Node which we will use can be found here.

Steps:

1. Analyze the Ros Node.
2. Generate an IDSL.
3. Generate our RoboComp Component.
4. Run it.

Analyze the Ros Node

We need to know what messages or services they are using. To find out, we can take a look at the package documentation and the msg and srv directory. Here is an image.msg example which is used in our Ros Node:

string name
uint32 size
int32[] image

In the Talker.cpp code, we can see:

// %Tag(PUBLISHER)%
  ros::Publisher chatter_pub = n.advertise<beginner_tutorials::image>("chatter", 1000);
// %EndTag(PUBLISHER)%

Talker (Ros Node) publishes this MSG through the topic called chatter. So We need an IDSL with a structure containing the types of image.msg and an interface with a topic called chatter.

Generate an IDSL

Let's generate our IDSL:

module RoboCompListener{
	sequence<int> imageVector;
	struct Image {
	 	string name;
	 	uint size;
	 	imageVector image;
	 };
	
	interface chat{
		void chatter(Image img);
	};
};

The names of the variables must be identical to those used in the message!

We have created a structure Image and a topic called chatter.

Generate our RoboComp Component

Create your .cdsl like this:

import "/robocomp/interfaces/IDSLs/Listener.idsl";
Component listener
{
	Communications
	{
		subscribesTo chat(ros);
	};
	language Cpp;
	gui Qt(QWidget);
};

And use RoboCompDSL to generate your component.

Run it

To see if everything worked, try to insert this code in your subscribe method:

printf("Name: %s, size: %d\n", img.name.c_str(), img.size);
for(int i=0;i<img.image.size();i++)
    printf("value of img.image[%d] = %d\n", i, img.image[i]);

Launch your component and enjoy!

We assume that Talker and roscore are running.

If you get an error like this:

[ERROR] [1471018647.418758838]: Client [/listener] wants topic /chatter to have datatype/md5sum [RoboCompListener/Image/5d96ac93c863cbaa85815010f7256b9a], but our version has [beginner_tutorials/image/163b37c985fb24d5121dd98a6a240a84]. Dropping connection.

Then you have not generated an identical message structure.