CMake call add_subdirectory within custom command - cmake

Question

I'm working with a code generator that produces C++ and a CMakeLists.txt file, unfortunately I cannot use this in my main CMakeLists.txt file for testing purposes.
For example you have the following CMakeLists.txt file:
project(SomeProject CXX C)
add_custom_command(OUTPUT ${SRCS}
COMMAND ${CODEGEN_CLI_PATH} -i "${INPUT}" -o "${OUT}"
COMMENT "Generating sources"
WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
VERBATIM
)
add_custom_target(CODEGEN
DEPENDS
${SRCS}
)
# Needs to be executed after the custom command
add_subdirectory(${GENERATED_CMAKE_LISTS_LOCATION})
Is it possible to use functions such as add_subdirectory only after you execute custom commands for a particular target, such as CODEGEN?
I've already tried to execute it by adding an extra line to the existing custom command:
COMMAND ${CMAKE_COMMAND} -D DIR=${GENERATED_CMAKE_LISTS_LOCATION} -P add_subdirectories.cmake
Unfortuantly this doesn't work because it isn't allowed to execute functions like add_subdirectory in script mode.
Neither I can manage to call custom made functions (that are executing add_subdirectory) from add_custom_command that are located in the same file.

Answer 1

Nope, it is not possible. The add_subdirectory command is run during configuration step, while CODEGEN is a target that runs during build.
You seem to be doing something wrong, so the only advice I can give you is to use execute_process command to run commands you need. The execute_process command is executed during configuration stage, so it will be able to generate files you need before add_subdirectory.
But again, please describe your problem, why do you want CMake to do that.

Answer 2

I have a huge fixed unsigned char array that I compiled into a static library. The way I work around it is by:
if(NOT EXISTS ${PATH_TO_FOLDER}/smeagol.a)
add_subdirectory(smeagol)
endif()
I'm still looking for a nicer kung-fu way to do it using cmake. I feel that its out there, and I will update this answer once i find it.