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Section 10.3 Writing ROS 2 Subscribers & Callback Functions

Event-Driven Callbacks
Unlike publishers, subscribers are event-driven. A subscriber does not poll or wait in a loop. Instead, it registers a callback function with ROS 2.
Whenever a new message arrives on the target topic:
  1. ROS 2 catches the incoming message payload.
  2. It automatically invokes your callback function, passing the message payload as an argument.
In short: a new message arriving on a topic triggers a ROS 2 middleware interrupt, which in turn invokes your callback(msg) function directly—there is no loop in your own code polling for new data. 
import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class SimpleSubscriber(Node):
    def __init__(self):
        super().__init__('simple_subscriber')
        # Create subscriber listening to 'chatter'
        self.subscription = self.create_subscription(
            String,
            'chatter',
            self.listener_callback,
            10
        )

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

Subsection 10.3.1 Section 10.3 Interactive Exercises

Subsubsection 10.3.1.1 Exercise 10.3.1: ActiveCode Exercise — Safety Laser Scan Processing

Because Runestone CodeLens standard execution environments run pure Python without ROS dependencies installed, we mock the incoming sensor_msgs/LaserScan data structure using standard Python classes.
Task: Complete the callback function laser_scan_callback(msg).
  1. Iterate through the array of range measurements in msg.ranges.
  2. Extract the minimum distance detected (ignoring readings \(\leq 0.0\) which represent sensor errors).
  3. If the minimum valid distance is less than 0.5 meters, flag a safety stop (EMERGENCY_STOP = True) and print an alert!
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