Exchange Data with ROS Publishers and Subscribers

The primary mechanism for ROS nodes to exchange data is sending and receiving messages. Messages are transmitted on a topic, and each topic has a unique name in the ROS network. If a node wants to share information, it uses a publisher to send data to a topic. A node that wants to receive that information uses a subscriber to that same topic. Besides its unique name, each topic also has a message type, which determines the types of messages that are capable of being transmitted under that topic.

This publisher and subscriber communication has the following characteristics:

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

  • Publishers 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.

The concept of topics, publishers, and subscribers is illustrated in the figure:

This example shows how to publish and subscribe to topics in a ROS network. It also shows how to:

  • Wait until a new message is received

  • Use callbacks to process new messages in the background

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

Subscribe and Wait for Messages

Start the ROS master in MATLAB® using the rosinit command.

rosinit
Initializing ROS master on http://bat1073807glnxa64:37527/.
Initializing global node /matlab_global_node_52521 with NodeURI http://bat1073807glnxa64:38835/

Create a sample ROS network with several publishers and subscribers using the provided helper function exampleHelperROSCreateSampleNetwork.

exampleHelperROSCreateSampleNetwork

Use rostopic list to see which topics are available.

rostopic list
/pose  
/rosout
/scan  

Use rostopic info to check if any nodes are publishing to the /scan topic. The command below shows that node_3 is publishing to it.

rostopic info /scan
Type: sensor_msgs/LaserScan
 
Publishers:
* /node_3 (http://bat1073807glnxa64:44501/)
 
Subscribers:
* /node_2 (http://bat1073807glnxa64:41109/)
* /node_1 (http://bat1073807glnxa64:44397/)

Use rossubscriber to subscribe to the /scan topic. If the topic already exists in the ROS network (as is the case here), rossubscriber detects its message type automatically, so you do not need to specify it.

laser = rossubscriber('/scan');
pause(2)

Use receive to wait for a new message. (The second argument is a time-out in seconds.) The output scandata contains the received message data.

scandata = receive(laser,10)
scandata = 
  ROS LaserScan message with properties:

       MessageType: 'sensor_msgs/LaserScan'
            Header: [1x1 Header]
          AngleMin: -0.5216
          AngleMax: 0.5243
    AngleIncrement: 0.0016
     TimeIncrement: 0
          ScanTime: 0.0330
          RangeMin: 0.4500
          RangeMax: 10
            Ranges: [640x1 single]
       Intensities: [0x1 single]

  Use showdetails to show the contents of the message

Some message types have visualizers associated with them. For the LaserScan message, plot plots the scan data. The MaximumRange name-value pair specifies the maximum plot range.

figure
plot(scandata,'MaximumRange',7)

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 exampleHelperROSPoseCallback.

robotpose = rossubscriber('/pose',@exampleHelperROSPoseCallback)
robotpose = 
  Subscriber with properties:

        TopicName: '/pose'
      MessageType: 'geometry_msgs/Twist'
    LatestMessage: [1x1 Twist]
       BufferSize: 1
    NewMessageFcn: @exampleHelperROSPoseCallback

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.

global pos
global orient

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

Wait for a few seconds to make sure that the subscriber can receive messages. The most current position and orientation data will always be stored in the pos and orient variables.

pause(2) 
pos
pos = 1×3

   -0.1187   -0.2222    0.0066

orient
orient = 1×3

   -0.1811    0.2013    0.1295

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 robotpose

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 (MATLAB) documentation for more information about defining callback functions.

Publish Messages

Create a publisher that sends ROS string messages to the /chatter topic (see Work with Basic ROS Messages).

chatterpub = rospublisher('/chatter', 'std_msgs/String')
chatterpub = 
  Publisher with properties:

         TopicName: '/chatter'
        IsLatching: 1
    NumSubscribers: 0
       MessageType: 'std_msgs/String'

pause(2) % Wait to ensure publisher is registered

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

chattermsg = rosmessage(chatterpub);
chattermsg.Data = 'hello world'
chattermsg = 
  ROS String message with properties:

    MessageType: 'std_msgs/String'
           Data: 'hello world'

  Use showdetails to show the contents of the message

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

rostopic list
/chatter
/pose   
/rosout 
/scan   

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

chattersub = rossubscriber('/chatter', @exampleHelperROSChatterCallback)
chattersub = 
  Subscriber with properties:

        TopicName: '/chatter'
      MessageType: 'std_msgs/String'
    LatestMessage: [0x1 String]
       BufferSize: 1
    NewMessageFcn: @exampleHelperROSChatterCallback

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

send(chatterpub,chattermsg)
pause(2)
ans = 
'hello world'

The exampleHelperROSChatterCallback function was called as soon as you published the string message.

Shut Down ROS Network

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

exampleHelperROSShutDownSampleNetwork
clear global pos orient

Shut down the ROS master and delete the global node.

rosshutdown
Shutting down global node /matlab_global_node_52521 with NodeURI http://bat1073807glnxa64:38835/
Shutting down ROS master on http://bat1073807glnxa64:37527/.

Next Steps