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Exchange Data with ROS 2 Publishers and Subscribers

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This example shows how to publish and subscribe to topics in a ROS 2 network.

The primary mechanism for ROS 2 nodes to exchange data is to send and receive messages. Messages are transmitted on a topic and each topic has a unique name in the ROS 2 network. If a node wants to share information, it must use a publisher to send data to a topic. A node that wants to receive that information must use a subscriber for that same topic. Besides its unique name, each topic also has a message type, which determines the type of messages that are allowed to be transmitted in the specific topic.

This publisher-subscriber communication has these characteristics:

  • Topics are used for many-to-many communication. Multiple publishers can send messages to the same topic and multiple subscribers can receive them.

  • Publisher and subscribers are decoupled through topics and can be created and destroyed in any order. A message can be published to a topic even if there are no active subscribers.

Besides how to publish and subscribe to topics in a ROS 2 network, this example also shows how to:

  • Wait until a new message is received, or

  • Use callbacks to process new messages in the background

Prerequisites: Get Started with ROS 2, Connect to a ROS 2 Network

Subscribe and Wait for Messages

Create a sample ROS 2 network with several publishers and subscribers.

exampleHelperROS2CreateSampleNetwork

Use ros2 topic list to see which topics are available.

ros2 topic list
/parameter_events
/pose
/rosout
/scan

Assume you want to subscribe to the /scan topic. Use ros2subscriber to subscribe to the /scan topic. Specify the name of the node with the subscriber. If the topic already exists in the ROS 2 network, ros2subscriber detects its message type automatically, so you do not need to specify it.

detectNode = ros2node("/detection");
pause(5)
laserSub = ros2subscriber(detectNode,"/scan");
pause(5)

Use receive to wait for a new message. Specify a timeout of 10 seconds. The output scanData contains the received message data. status indicates whether a message was received successfully and statustext provides additional information about the status.

[scanData,status,statustext] = receive(laserSub,10);

You can now remove the subscriber laserSub and the node associated to it.

clear laserSub
clear detectNode

Subscribe Using Callback Functions

Instead of using receive to get data, you can specify a function to be called when a new message is received. This allows other MATLAB code to execute while the subscriber is waiting for new messages. Callbacks are essential if you want to use multiple subscribers.

Subscribe to the /pose topic, using the callback function exampleHelperROS2PoseCallback, which takes a received message as the input. One way of sharing data between your main workspace and the callback function is to use global variables. Define two global variables pos and orient.

controlNode = ros2node("/base_station");
pause(5)
poseSub = ros2subscriber(controlNode,"/pose",@exampleHelperROS2PoseCallback);
global pos
global orient

The global variables pos and orient are assigned in the exampleHelperROS2PoseCallback function when new message data is received on the /pose topic.

function exampleHelperROS2PoseCallback(message)    
    % Declare global variables to store position and orientation
    global pos
    global orient
    
    % Extract position and orientation from the ROS message and assign the
    % data to the global variables.
    pos = [message.linear.x message.linear.y message.linear.z];
    orient = [message.angular.x message.angular.y message.angular.z];
end

Wait a moment for the network to publish another /pose message. Display the updated values.

pause(3)
disp(pos)
       0.00235920447111606       -0.0201184589892978        0.0203969078651195

disp(orient)
       -0.0118389124011118       0.00676849978014866        0.0387860955311228

If you type in pos and orient a few times in the command line you can see that the values are continuously updated.

Stop the pose subscriber by clearing the subscriber variable

clear poseSub
clear controlNode

Note: There are other ways to extract information from callback functions besides using globals. For example, you can pass a handle object as additional argument to the callback function. See the Create Callbacks for Graphics Objects documentation for more information about defining callback functions.

Publish Messages

Create a publisher that sends ROS 2 string messages to the /chatter topic.

chatterPub = ros2publisher(node_1,"/chatter","std_msgs/String");

Create and populate a ROS 2 message to send to the /chatter topic.

chatterMsg = ros2message(chatterPub);
chatterMsg.data = 'hello world';

Use ros2 topic list to verify that the /chatter topic is available in the ROS 2 network.

ros2 topic list
/chatter
/parameter_events
/pose
/rosout
/scan

Define a subscriber for the /chatter topic. exampleHelperROS2ChatterCallback is called when a new message is received, and displays the string content in the message.

chatterSub = ros2subscriber(node_2,"/chatter",@exampleHelperROS2ChatterCallback)
chatterSub = 
  ros2subscriber with properties:

        TopicName: '/chatter'
    LatestMessage: []
      MessageType: 'std_msgs/String'
    NewMessageFcn: @exampleHelperROS2ChatterCallback
          History: 'keeplast'
            Depth: 10
      Reliability: 'reliable'
       Durability: 'volatile'

Publish a message to the /chatter topic. Observe that the string is displayed by the subscriber callback.

send(chatterPub,chatterMsg)
pause(3)
ans = 
'hello world'

The exampleHelperROS2ChatterCallback function was called when the subscriber received the string message.

Disconnect From ROS 2 Network

Remove the sample nodes, publishers and subscribers from the ROS 2 network. Also clear the global variables pos and orient

clear global pos orient
clear

Next Steps