Creating a component package

info

The creation of a component package can be greatly simplified by using our Component Template. It comes with a boilerplate project which supports both C++ and Python.

A lot of the information in this page is also available in the README of the template repository.

Custom components can be defined in ROS 2 packages and used alongside the standard library of AICA components in AICA Core. Components can be implemented in C++ or Python. A component package can contain multiple components in either language.

Package infrastructure

A minimal directory structure for an example package named custom_component_package with C++ and Python component implementations is shown below.

custom_component_package
│
├── component_descriptions
│   ├── custom_component_package_cpp_component.json
│   └── custom_component_package_py_component.json
│
├── custom_component_package
│   └── py_component.py
│
├── include
│   └── custom_component_package
│       └── CppComponent.hpp
│
├── src
│   └── CppComponent.cpp
│
├── CMakeLists.txt
├── package.xml
└── setup.cfg

The following sections describe the package contents in more detail.

package.xml

The package manifest file defines the package metadata (name, version, description, maintainer and license) and package dependencies. An example including minimal dependencies is given below.

package.xml

<?xml version="1.0"?>
<?xml-model href="http://download.ros.org/schema/package_format3.xsd" schematypens="http://www.w3.org/2001/XMLSchema"?>
<package format="3">
    <version>0.0.1</version>
    <name>custom_component_package</name>
    <description>An example package for custom components</description>
    <maintainer email="john@example.com">John Doe</maintainer>
    <license>MIT</license>

    <buildtool_depend>ament_cmake_auto</buildtool_depend>
    <buildtool_depend>ament_cmake_python</buildtool_depend>

    <depend>modulo_components</depend>

    <export>
        <build_type>ament_cmake</build_type>
    </export>
</package>

CMakeLists.txt

The package build instructions are defined in a CMakeLists.txt file.

CMakeLists.txt

cmake_minimum_required(VERSION 3.15)
project(custom_component_package)

# find dependencies
find_package(ament_cmake_auto REQUIRED)
find_package(ament_cmake_python REQUIRED)

ament_auto_find_build_dependencies()

include_directories(include)

### Register and install C++ Components here ###

# install Python modules
ament_python_install_package(${PROJECT_NAME} SCRIPTS_DESTINATION lib/${PROJECT_NAME})

# export JSON component descriptions in the component_descriptions directory
install(DIRECTORY ./component_descriptions
        DESTINATION .)

ament_auto_package()

note

C++ components need to be added as library targets, registered with the rclcpp_components_register_nodes macro and finally exported with the install directive. See the C++ example in the next section.

setup.cfg

If the package contains any Python components, they need to be registered in a separate setup.cfg file. Refer to the Python example in the next section.

If a package contains no Python components, the setup.cfg file is not needed.

Source directories

Relative to the package root, C++ components should have header files defined in an include/<package_name> directory and source files defined in a src directory.

Python components should be defined in a subdirectory matching the package name.

caution

Any changes to the standard directory structure must be reflected in the CMakeLists.txt file accordingly.

Component descriptions

To fully support custom components in AICA Core, each new component should be fully described by a JSON file according to the component description schema.