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

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(2)
laserSub = ros2subscriber(detectNode,"/scan");
pause(2)

Use receive to wait for a new message. Specify a timeout of 10 seconds. The output scanData contains the received message data.

scanData = 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");
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)

   -0.0156    0.0281    0.0175

disp(orient)

   -0.0493    0.0102   -0.0113

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 Callback Definition 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
/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