ROS2 Command Line - Topic Echo

🧠 Concept: Subscribers and Callbacks

A subscriber listens to a topic and calls your function every time a new message arrives. It's the receiving side of the publisher/subscriber pattern.

Key API:

self.sub = self.create_subscription(MessageType, '/topic', self.callback, queue_depth)

The callback receives the message automatically:

def callback(self, msg):
    self.get_logger().info(f'Received: {msg.data}')

Logging levels:

self.get_logger().info('normal message')
self.get_logger().warn('something unusual')
self.get_logger().error('something went wrong')

Problem Statement

Create a ROS2 Python node that listens to messages from a topic and processes them. This completes the communication loop started by the publisher.

Requirements

• Create a node named simple_subscriber
• Subscribe to topic /chatter
• Message type: std_msgs/String
• Log the received message to the console using self.get_logger().info()
• Format: "I heard: [message data]"

Input/Output Format

Input:

• /chatter (std_msgs/String) - Receives string messages

Output:

• Console Logs - Prints "I heard: Hello ROS2!" (or whatever is published)

⚠️ Common Pitfalls

• Forgetting to add self to the callback method definition.
• Mismatched message types between publisher and subscriber.

📚 Helpful Resources

• ROS2 Subscriber Documentation