3: Creating a ROS 2 Pub-Sub Subscription using TypeScript

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In this tutorial, we implement a ROS 2 subscription using TypeScript and learn to use the ros2 topic CLI tool for listening in on and testing pub-sub communications.

edited 20220211 — upgraded to use NodeJS 16

In tutorial #2 we learned about ROS 2’s publish-subscribe communication mode and how to create a simple ROS 2 node and publisher using the rclnodejs library and TypeScript. In this tutorial we introduce how to receive messages using a ROS 2 subscription.

ROS Messages

In this tutorial and the previous tutorial we are working with the simple ROS String message. ROS 2 includes a rich set of standard messages which we will introduce in up-coming tutorials. Resources section below includes a link to the standard ROS messages. ROS messages are formally defined using the Interface Definition Language (IDL). The ROS 2 SDK includes tools for creating programming interfaces from message IDL files.

Here’s the IDL definition of std_msgs/msg/String that we are using. Note how it only has thedata field:

string data

Let’s Code

Some details about this project. The program will consist of:

• a ROS Node that manages the life-cycle of a subscription and all of it’s communications
• a ROS Publisher of string messages to the topic msg
• when a message arrives the subscription will print it to stdout
• the program will run indefinitely until the process is manually terminated, e.g., Ctrl-C from shell

Prerequisites

Getting started you’ll need a dev environment with

• Nodejs 10.22–17.x — I’m using Node 16.13.0 (LTS)
• ROS 2 Foxy (LTS) or Galactic release

If you haven’t setup your ROS 2 JavaScript coding environment see this article.

Lastly if you would like to skip ahead and see the end result, you can find a link to a github project in the Resources section found at the end of this article.

Step-1: Project Setup

Project setup is easy as we will continue developing our code in the Nodejsros2-js-examples project we created in Tutorial #2. From the root folder of our ros2-js-examples project create the src/example2 folder using the following Linux commandline tools.

mkdir -p src/example2
cd src/example2

Step-2: Creating a Node and Subscription

Create the file node-subscriber.ts file.

Here’s a quick explanation of the key lines of code:

Line-14: Create a ROS 2 Node with the name node_subscriber in the ros2_js_examples namespace.

let node =     rclnodejs.createNode('node_subscriber','ros2_js_example');

Line-15,18: Create a ROS 2 Subscription that listens to the msg topic for a message of type std_msgs/msg/String. The subscription is configured with a callback that simply echo’s the message’s data property to the stdout.

Line -23: Inform rclnodejs to begin routing incoming messages to this node.

rclnodejs.spin(node);

Step-3: Compile TypeScript

Invoke the TypeScript compiler tsc to transcode our node-subsciber.ts file into JavaScript. The resulting JavaScript file is created in the dist/example2 folder with the same name as it’s src TypeScript file.

npm run build

or for continuous compilation on file changes use:

npx tsc -w

Step-4: Run

Using the Nodejs runtime, let’s run our node-subscriber program. In your terminal shell enter:

node dist/example2/node-subscriber.js

Initially not much is happening, as the program is waiting for messages to arrive.

Now let’s run node-publisher to generate messages.

To do this open a new terminal shell and change directory to the ros2-js-examples/ folder. Then run the command:

node dist/example1/node-publisher.js

Once node-publisher is running and publishing messages, you should see subscription messages begin dribbling to stdout as shown below.

Using The ros2 topic Tool To Test Pub-Sub Messaging

In tutorial #2 we got our first look at how the ros2 CLI tools can be used to listen to published messages. Now let’s take a look at how we can use the ros2 topic tool to publish a message to a topic and subscribe to that topic.

Publishing to a Topic

Open a new terminal shell and enter:

ros2 topic pub /ros2_js_examples/msg std_msgs/msg/String ‘{data: hello world}’

The topic tool will startup and repeatedly publish a ‘hello world’ message every second until we terminate the process.

Subscribing to a Topic

Next, open a new terminal shell and enter:

ros2 topic echo /ros2_js_examples/msg std_msgs/msg/String

The topic tool will listen for published messages on the /ros2_js_examples/msg topic and echo the message to stdout.

Note that because the node operates in the /ros2_js_examples namespace we need to prepended the node namespace to the msg topic name when using the ros2 cli tools.

Using the ros2 topic tool is a convenient way to test publishing and subscribing to a message.

Parting Thoughts; Looking Forward

Anonymous pub-sub communications is the simplest ROS 2 communications mode to implement and typically the most common form of communications in the node graph. In our next tutorial we introduce the ROS 2 rviz2 visualization tool and put pub-sub communications to work.

Resources

1. ROS 2 standard interfaces (messages)

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