ROS 2 Custom Interfaces Skill

Purpose: Generate accurate, educational ROS 2 custom message and service interface definitions following official ROS 2 Humble patterns for the Physical AI textbook.

When to Use

• Creating lessons in Chapter 5 (Communication Mastery - Lesson 3)
• Writing interface design exercises
• Generating worked examples for custom robot data types
• Teaching .msg and .srv file format and build process

Live Documentation Access

CRITICAL: Before generating any code, fetch current ROS 2 documentation using Context7 MCP:

Tool: mcp__context7__resolve-library-id
libraryName: "ros2 custom interfaces"

Then:
Tool: mcp__context7__get-library-docs
context7CompatibleLibraryID: [resolved ID]
topic: "custom msg srv interfaces"

This ensures interface patterns match current ROS 2 Humble conventions.

Interface Definition Patterns

Custom Message (.msg) Pattern

# File: msg/RobotStatus.msg
# Custom message for robot status reporting

# Header with timestamp
builtin_interfaces/Time stamp

# Robot identification
string robot_name
int32 robot_id

# Status information
bool is_active
float64 battery_level      # 0.0 to 100.0
float64[] joint_positions  # Array of joint angles in radians

# Nested message (from geometry_msgs)
geometry_msgs/Pose current_pose

Custom Service (.srv) Pattern

# File: srv/MoveRobot.srv
# Custom service for robot movement commands

# Request
string target_frame
geometry_msgs/Point target_position
float64 speed_factor  # 0.0 to 1.0
---
# Response
bool success
string message
float64 time_to_reach  # Estimated seconds

Package Structure for Interfaces

my_robot_interfaces/
├── CMakeLists.txt
├── package.xml
├── msg/
│   ├── RobotStatus.msg
│   └── SensorData.msg
└── srv/
    ├── MoveRobot.srv
    └── GetStatus.srv

CMakeLists.txt (Essential Parts)

cmake_minimum_required(VERSION 3.8)
project(my_robot_interfaces)

find_package(ament_cmake REQUIRED)
find_package(builtin_interfaces REQUIRED)
find_package(geometry_msgs REQUIRED)
find_package(rosidl_default_generators REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/RobotStatus.msg"
  "msg/SensorData.msg"
  "srv/MoveRobot.srv"
  "srv/GetStatus.srv"
  DEPENDENCIES builtin_interfaces geometry_msgs
)

ament_package()

package.xml (Essential Parts)

<?xml version="1.0"?>
<package format="3">
  <name>my_robot_interfaces</name>
  <version>0.1.0</version>
  <description>Custom interfaces for robot communication</description>
  <maintainer email="you@example.com">Your Name</maintainer>
  <license>Apache-2.0</license>

  <buildtool_depend>ament_cmake</buildtool_depend>
  <buildtool_depend>rosidl_default_generators</buildtool_depend>

  <depend>builtin_interfaces</depend>
  <depend>geometry_msgs</depend>

  <member_of_group>rosidl_interface_packages</member_of_group>
</package>

Built-in Types Reference

Primitive Types

Type	Description	Python Equivalent
bool	Boolean	bool
byte	8-bit unsigned	int
char	8-bit char	str (single char)
float32	32-bit float	float
float64	64-bit float	float
int8/16/32/64	Signed integers	int
uint8/16/32/64	Unsigned integers	int
string	UTF-8 string	str

Array Types

Syntax	Description
float64[]	Unbounded array
float64[3]	Fixed-size array (3 elements)
float64[<=10]	Bounded array (max 10)

Common Standard Message Types

• std_msgs/String, std_msgs/Int32, std_msgs/Bool
• geometry_msgs/Point, geometry_msgs/Pose, geometry_msgs/Twist
• sensor_msgs/Image, sensor_msgs/LaserScan, sensor_msgs/JointState
• builtin_interfaces/Time, builtin_interfaces/Duration

Educational Requirements

Layer 1 (Manual Foundation)

• Explain why custom interfaces needed (type safety, documentation)
• Walk through .msg/.srv file format
• Demonstrate build process with colcon

Layer 2 (AI Collaboration)

• Design interface from natural language requirements
• Iterate on interface design with AI feedback
• Generate boilerplate package structure (Three Roles INVISIBLE)

Layer 3 (Intelligence Design)

• Interface versioning strategies
• Designing for extensibility
• Standard vs custom interface decision framework

Hardware Tier Compatibility

Note: Interface packages require compilation with colcon.

• Tier 1: Cloud ROS 2 workspace with colcon build
• Tier 2+: Local ROS 2 Humble installation

Build commands:

cd ~/ros2_ws
colcon build --packages-select my_robot_interfaces
source install/setup.bash

Common Mistakes to Prevent

1. Missing DEPENDENCIES: Always list dependent packages in CMakeLists.txt
2. Wrong member_of_group: Must include rosidl_interface_packages
3. Type mismatches: Python uses float for float32/float64, int for all integers
4. Forgetting to source: Must source install/setup.bash after build

Interface Design Decision Framework

Create Custom Message When:

• Standard messages don't capture your data structure
• You need semantic meaning (RobotStatus vs generic struct)
• Multiple nodes will use the same data format
• Type checking prevents bugs

Use Standard Messages When:

• Data fits existing types (Point, Pose, Twist)
• Interoperability with existing tools needed
• Rapid prototyping (avoid build cycle)

Integration with Other Skills

• ros2-publisher-subscriber: Using custom messages in pub/sub
• ros2-service-pattern: Using custom services
• ros2-launch-system: Building interface packages in workspace

Authoritative Source

All patterns verified against: https://docs.ros.org/en/humble/Tutorials/Beginner-Client-Libraries/Custom-ROS2-Interfaces.html