send
Publish ROS 2 message to topic
Syntax
Description
Examples
Exchange Data with ROS 2 Publishers and Subscribers
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
Input Arguments
pub
— ros2publisher
object
ros2publisher
ros2publisher
object, specified as a handle, that publishes the specified topic.
msg
— ROS 2 message
Message
structure
ROS 2 message, specified as a structure, with compatible fields for that message type.
Extended Capabilities
C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.
Version History
Introduced in R2019b
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