Wait for new message
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 the following 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.
This example shows how to publish and subscribe to topics in a ROS 2 network. It also shows how to:
Wait until a new message is received, or
Use callbacks to process new messages in the background
Subscribe and Wait for Messages
Create a sample ROS 2 network with several publishers and subscribers.
ros2 topic list to see which topics are available.
ros2 topic list
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)
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
controlNode = ros2node("/base_station"); poseSub = ros2subscriber(controlNode,"/pose",@exampleHelperROS2PoseCallback); global pos global orient
The global variables
orient are assigned in the
exampleHelperROS2PoseCallback function when new message data is received on the
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.
-0.0178 0.0285 -0.0029
-0.0464 -0.0324 0.0222
If you type in
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 (MATLAB) documentation for more information about defining callback functions.
Create a publisher that sends ROS 2 string messages to the
chatterPub = ros2publisher(node_1,"/chatter","std_msgs/String");
Create and populate a ROS 2 message to send to the
chatterMsg = ros2message(chatterPub); chatterMsg.data = 'hello world';
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
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.
ans = 'hello world'
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
clear global pos orient clear
ros2subscriber object, specified
as a handle, that subscribes to a specific topic.
timeout— Timeout period
The amount of time before the
receiver function will error out
if a message is not received.
msg— ROS 2 message
ROS 2 message, specified as a
Message object handle.
Choosing between receive and using a callback:
receive when your program should wait until the next message
is received on the topic and no other processing should happen in the meantime.
If you want your program to keep running and be notified whenever a new message arrives, consider using a callback instead of receive.
If you want your program to periodically use the most recent data received by the
subscriber, consider accessing the
LatestMessage property instead of
using receive or a callback.