[ros2] components registration from subdirectory CMakeLists file

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If I take the contents of the subdirectories' CMakeLists.txt file to the main CMakeLists.txt and remove the call to add_subdirectory, everything works fine.

This is probably because in CMake each call to add_subdirectory() creates a new scope, meaning CMake variables set in the subdirectory are not visible to the parent directory.

It looks like rclcpp_components_register_nodes()calls_rclcpp_components_register_package_hook This is a macro that calls ament_register_extension(). This appends to the CMake variable AMENT_EXTENSIONS_ament_package, presumably to register something to be called when ament_package() is called. Since this is a subdirectory, the call to ament_package() in the parent directory doesn't see the changed value.

You might be able to work around this setting that variable in the PARENT_SCOPE at the very end of the <subdirectory>/CMakeLists.txt, though, it's possible other variables used by rclcpp_components_register_nodes() will need to be set in the parent scope too.

set(AMENT_EXTENSIONS_ament_package ${AMENT_EXTENSIONS_ament_package} PARENT_SCOPE)

Alternatively, when a CMake project uses add_subdirectory() it might be a sign that it has gotten too big and should be split into smaller projects.