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Chapter 4: Your First ROS 2 Code

You've explored ROS 2 from the command line. You've seen turtlesim move. You've watched data flow through topics. Now it's time to write the code that makes it happen.

In this chapter, you'll write your first ROS 2 nodes in Python. You'll create publishers (nodes that send data), subscribers (nodes that receive data), and understand how they coordinate through ROS 2's pub/sub infrastructure. By the end, you'll have two nodes talking to each other—and you'll have created a reusable skill that applies to every ROS 2 project you'll ever build.

Duration: 4 lessons, 4 hours total Layer Breakdown: L1: 40%, L2: 50%, L3: 10% (intro to creating reusable skills) Hardware Tier: Tier 1 (cloud ROS 2 recommended) Prerequisites: Chapters 1-3 (ROS 2 exploration) Reusable Skill Created: ros2-publisher-subscriber (foundation for all ROS 2 development)

Learning Objectives​

By the end of this chapter, you will be able to:

  • Build ROS 2 packages with proper workspace structure
  • Write a Python ROS 2 publisher that sends data at regular intervals
  • Write a Python ROS 2 subscriber that receives and processes data
  • Understand the rclpy Node API and node lifecycle
  • Debug pub/sub communication using ros2 topic echo and logging
  • Collaborate with AI to extend your code with new features
  • Recognize reusable patterns that you'll apply throughout Module 1+

Lessons​

Lesson 4.1: Workspaces and Packages (60 minutes)​

Create a ROS 2 workspace and your first package. Understand the directory structure (src/, build/, install/), the package.xml manifest, and how to build with colcon.

Core Concepts:

  • Workspace structure (src, build, install directories)
  • Package.xml metadata and dependencies
  • Building with colcon

Lesson 4.2: Writing a Publisher (60 minutes)​

Create a node that publishes "Hello World" messages every 500ms. Understand the rclpy.Node API, timers, and how publishers send data.

Core Concepts:

  • Publisher creation (create_publisher)
  • Timer-based callbacks
  • Node lifecycle (init, spin, shutdown)

Lesson 4.3: Writing a Subscriber (60 minutes)​

Create a node that listens for those messages and logs them. Understand callbacks, message handling, and subscriber creation.

Core Concepts:

  • Subscriber creation (create_subscription)
  • Callback pattern (processing incoming messages)
  • Message handling and logging

Lesson 4.4: Try With AI (60 minutes)​

Collaborate with AI to extend your pub/sub system. Ask AI to help add error handling, configuration, logging levels. Experience productive AI dialogue through action.

Core Concepts:

  • AI-assisted code generation and pattern discovery
  • Validating AI suggestions against your constraints
  • Iterating toward better design through dialogue
  • Creating a reusable skill from the pattern

4-Layer Teaching Method​

Layer%What's Covered
L1: Manual40%Step-by-step code walkthroughs, package structure explanations
L2: AI Collab50%AI collaboration to extend your code, validation, iteration
L3: Intelligence10%Introduction: recognizing reusable patterns, skill creation preview
L4: Spec-Driven0%Not yet—too early for complex specifications

This is your first hands-on coding chapter. You write real Python code, run real ROS 2 nodes, and experience AI collaboration for the first time.

Hardware Requirements​

Minimum Tier: Tier 1 (Cloud ROS 2)

TierEquipmentWhat You Can Do
1Laptop + BrowserCloud ROS 2 (TheConstruct) with Python execution and topic visualization
2+Local ROS 2Local installation with faster execution

Recommended for beginners: Cloud ROS 2 (TheConstruct) avoids local installation complexity. You get a terminal, Python IDE, and visualization all in one browser window.

Prerequisites​

  • Chapters 1-3 (understand ROS 2 concepts and CLI)
  • Python programming (functions, classes, basic OOP)
  • ROS 2 environment set up (from Chapter 3)

Mastery Gate​

Before proceeding to Chapter 5, you should be able to:

  • Create a ROS 2 package from scratch using ros2 pkg create
  • Write a publisher node that sends messages every N seconds
  • Write a subscriber node that receives messages and processes them
  • Verify communication using ros2 topic echo to see data flowing
  • Extend a node with AI assistance (add logging, error handling, configuration)
  • Understand the publisher/subscriber pattern well enough to explain it to someone else
  • Recognize that this pattern repeats across all ROS 2 projects

If you can do these, you're ready for Chapter 5.

Key Concept: Publisher/Subscriber Pattern​

This pattern appears in every ROS 2 system you'll build:

Publisher Node → ROS 2 Topic → Subscriber Node(s)

Publisher's job: Create data, publish it periodically Subscriber's job: Receive data, process it ROS 2's job: Route data from publisher to all subscribers

You'll apply this pattern to:

  • Sensor data (Chapter 5)
  • Motor commands (Chapter 6)
  • System status (Chapter 7 capstone)

Master this chapter, and you unlock the entire ROS 2 ecosystem.

Previous Chapter: ← Chapter 3: Meet ROS 2

Next Chapter: Chapter 5: Communication Mastery →

Module Overview: ← Back to Module 1

Start Lesson 4.1: Workspaces and Packages →