How to move a file download from configure step to build step? - cmake

Question

Let's say I have the block below in my CMakeLists.txt.
file (DOWNLOAD http://.../file.zip "${CMAKE_BINARY_DIR}/file.zip")
execute_process (
COMMAND "${CMAKE_COMMAND}" -E tar -xf file.zip
WORKING_DIRECTORY "${CMAKE_BINARY_DIR}"
)
add_custom_command (
OUTPUT output.txt
COMMAND "${MY_COMMAND}" file-found-in-zip.txt output.txt
WORKING_DIRECTORY "${CMAKE_BINARY_DIR}"
)
The basic steps are:
Download.
Extract.
Add a custom command that uses a file that was extracted in step 2.
During the build step, the custom command may or may not be executed, but the download and extraction always will. How can I make the download and extraction conditional, such that it happens only if the custom command that needs it will be executed?

Answer 1

I guess something along:
file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/download.cmake
"file(DOWNLOAD http://.../file.zip ${CMAKE_CURRENT_BINARY_DIR}/file.zip"
)
add_custom_command(
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/file.zip
COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_BINARY_DIR}/download.cmake
VERBATIM
)
execute_process (
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/file-found-in-zip.txt
DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/file.zip
COMMAND ${CMAKE_COMMAND} -E tar -xf file.zip
WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
VERBATIM
)
add_custom_command (
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/output.txt
DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/file-found-in-zip.txt
COMMAND "${MY_COMMAND}" file-found-in-zip.txt output.txt
WORKING_DIRECTORY "${CMAKE_BINARY_DIR}"
VERBATIM
)
Or just:
file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/download.cmake
"file(DOWNLOAD http://.../file.zip ${CMAKE_CURRENT_BINARY_DIR}/file.zip"
)
add_custom_command(
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/output.txt
COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_BINARY_DIR}/download.cmake
COMMAND ${CMAKE_COMMAND} -E tar -xf file.zip
COMMAND "${MY_COMMAND}" file-found-in-zip.txt output.txt
WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
VERBATIM
)