Robotics - [ROS2 Foundation 3] ROS2 Build System

Build Tool

A build tool operates on a set of packages

1. determines the dependency graph
2. invokes the specific build system for each package in topological order.
3. for a specific package, knows how to setup the environment for it, invokes the build, and sets up the environment to use the built package.

The build system operates on a single package: CMake, Make, Python setuptools. catkin and ament_cmake are based on CMake

Dependency Graph

• find_package helps the graph. FindFoo.cmake or FindFoo.cmake for the dependency must be in a prefix that CMake searches implicitly, like /usr, or a location provided through env vars CMAKE_PREFIX_PATH, or CMAKE_MODULE_PATHCMAKE_MODULE_PATH
• Install a shared_lib in a non-default location, that location needs to be in LD_LIBRARY_PATH.

ROS 2 Cpp And Python Package

Reference

1. We’ll create a ROS2 Cpp package, which contains a package.xml and CMakeLists.txt.

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    $ cd ~/ros2_ws/src/
    $ ros2 pkg create my_cpp_py_pkg --build-type ament_cmake
   

   • See:

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       my_cpp_py_pkg/
       # --> package info, configuration, and compilation
       ├── CMakeLists.txt
       ├── package.xml
       # --> Python stuff
       ├── my_cpp_py_pkg
       │   ├── __init__.py
       │   └── module_to_import.py
       ├── scripts
       │   └── py_node.py
       # --> Cpp stuff
       ├── include
       │   └── my_cpp_py_pkg
       │       └── cpp_header.hpp
       └── src
           └── cpp_node.cpp
     

2. For Python, no more setup.py and setup.cfg, everything will be done in the CMakeLists.txt.
   • Note that we have a sub directory called "my_cpp_py_pkg". Inside, the python code lives there. Like ROS1, they are executables chmod +x.
   • The module is visible to other packages: from my_cpp_py_pkg.module_to_import import ...

3. package.xml:

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    <?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">
    <name>my_cpp_py_pkg</name>
    <version>0.0.0</version>
    <description>TODO: Package description</description>
    <maintainer email="your@email.com">Name</maintainer>
    <license>TODO: License declaration</license>
   
    <buildtool_depend>ament_cmake</buildtool_depend>
    <buildtool_depend>ament_cmake_python</buildtool_depend>
   
    <depend>rclcpp</depend>
    <depend>rclpy</depend>
   
    <test_depend>ament_lint_auto</test_depend>
    <test_depend>ament_lint_common</test_depend>
   
    <export>
        <build_type>ament_cmake</build_type>
    </export>
    </package>
   

4. CMakeLists.txt:

    cmake_minimum_required(VERSION 3.5)
    project(my_cpp_py_pkg)
    # Default to C++14
    if(NOT CMAKE_CXX_STANDARD)
    set(CMAKE_CXX_STANDARD 14)
    endif()
    if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
    add_compile_options(-Wall -Wextra -Wpedantic)
    endif()
    # Find dependencies
    find_package(ament_cmake REQUIRED)
    find_package(ament_cmake_python REQUIRED)
    find_package(rclcpp REQUIRED)
    find_package(rclpy REQUIRED)
    # Include Cpp "include" directory
    include_directories(include)
    # Create Cpp executable
    add_executable(cpp_executable src/cpp_node.cpp)
    ament_target_dependencies(cpp_executable rclcpp)
    # Install Cpp executables
    install(TARGETS
    cpp_executable
    DESTINATION lib/${PROJECT_NAME}
    )
    # Install Python modules
    ament_python_install_package(${PROJECT_NAME})
    # Install Python executables
    install(PROGRAMS
    scripts/py_node.py
    DESTINATION lib/${PROJECT_NAME}
    )
    ament_package()
   

C++ & Python Interface Packages

An interface package defines ROS2 messages, services, and common utilities for other ROS2 packages. To create Python utilities, once a Python package is built and installed, and the workspace is sourced, its Python modules in install/MY_INTERFACE/local/lib/python3.10/dist-packages/MY_INTERFACE are automatically added to the Python path. What we need are as follow:

• Make sure the python package files are here:
  • Empty MY_INTERFACE/MY_INTERFACE/__init__.py
  • Module file: MY_INTERFACE/MY_INTERFACE/My_Module
• Add MY_INTERFACE/setup.py:

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    from setuptools import setup, find_packages
  
    package_name = "MY_PACKAGE"
  
    setup(
        name=package_name,
        version="0.0.1",
        packages=find_packages(include=[package_name, f"{package_name}.*"]),
        install_requires=["setuptools"],
        zip_safe=True,
        maintainer="TODO",
        maintainer_email="your_email@example.com",
        description="TODO",
        license="Apache-2.0",
        tests_require=["pytest"],
        entry_points={
            "console_scripts": [],
        },
    ) 
  

• CMakeLists.txt

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    install(
        DIRECTORY mumble_interfaces/
        DESTINATION local/lib/python3.10/dist-packages/mumble_interfaces
        FILES_MATCHING
    PATTERN "*.py")
    ament_package()
  

  • The destination install/MY_INTERFACE/local/lib/python3.10/dist-packages/MY_INTERFACE is carefully chosen, because that’s where generated srv, msg files go. Why? Because when there are packages with the same name at two different locations, Python will look into one, and throw a file-not-found error if files are not there.
  • We are NOT using ament_python_install_package because it’s meant for pure Python packages. We need to manually install MY_INTERFACE in install/MY_INTERFACE/lib/python3.10/site-packages
• User Code:

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from MY_INTERFACE import MyFunc

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- Or one can use: `python3 -c import MY_INTERFACE.My_Module` in the same console, because after sourcing `install/setup.bash`, the installed file is added to the Python Path. 
- One can check the python path with: `python3 -c "import sys; print(sys.path)"` or `echo $PYTHONPATH`

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