selectively run doxygen in cmake - cmake

Question

I've got a CMakeLists.txt file that handles Doxygen generation for a software project. Its contents are:
find_package(Doxygen)
if (DOXYGEN_FOUND)
set(doxyfile_in ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile.in)
set(doxyfile ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile)
configure_file(${doxyfile_in} ${doxyfile} #ONLY)
add_custom_target(doc ALL
${DOXYGEN_EXECUTABLE} ${doxyfile}
WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
COMMENT "Generating API documentation with Doxygen" VERBATIM)
endif()
I'd really like to only run doxygen if I'm doing an install build. Is there some straightforward way of communicating that to cmake?

Answer 1

First remove the ALL from add_custom_target().
add_custom_target(doc ALL
by
add_custom_target(doc
Then add
install(CODE
"EXECUTE_PROCESS(COMMAND ${CMAKE_COMMAND} --build ${CMAKE_BINARY_DIR} --target doc)")
src: https://cmake.org/cmake/help/latest/command/install.html#code
Examples
CMakeList.txt:
cmake_minimum_required (VERSION 3.5)
project(meta VERSION 1.0.0 LANGUAGES NONE)
# ....
enable_language(CXX)
add_executable(app main.cpp)
include(GNUInstallDirs)
set(CMAKE_INSTALL_CONFIGDIR ${CMAKE_INSTALL_LIBDIR}/cmake/foo)
install(TARGETS app
EXPORT FooTargets
RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
)
install(EXPORT FooTargets
NAMESPACE foo::
DESTINATION ${CMAKE_INSTALL_CONFIGDIR}
)
# replace by your own code
add_custom_target(doxygen
COMMAND echo Hello Doxygen
VERBATIM
)
install(CODE
"EXECUTE_PROCESS(COMMAND ${CMAKE_COMMAND} --build ${CMAKE_BINARY_DIR} --target doxygen)")
main.cpp
int main() {
return 0;
}
Shell:
$ cmake -H. -Bbuild
...
$ cmake --build build --target install -- DESTDIR=install
Scanning dependencies of target app
[ 50%] Building CXX object CMakeFiles/app.dir/main.cpp.o
[100%] Linking CXX executable app
[100%] Built target app
Install the project...
-- Install configuration: ""
-- Installing: install/usr/local/bin/app
-- Installing: install/usr/local/lib/cmake/foo/FooTargets.cmake
-- Installing: install/usr/local/lib/cmake/foo/FooTargets-noconfig.cmake
Scanning dependencies of target doxygen
Hello Doxygen
Built target doxygen

Answer 2

I've come up with an approach that divides the work between add_custom_command() and add_custom_target():
find_package(Doxygen)
if (DOXYGEN_FOUND)
set(doxyfile_in ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile.in)
set(doxyfile ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile)
configure_file(${doxyfile_in} ${doxyfile} #ONLY)
add_custom_command(
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/doxygen.stamp
DEPENDS ${doxyfile}
WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
COMMAND ${DOXYGEN_EXECUTABLE} ${doxyfile}
COMMAND cmake -E touch ${CMAKE_CURRENT_BINARY_DIR}/doxygen.stamp
COMMENT "Generating API documentation with Doxygen"
VERBATIM)
add_custom_target(doc ALL
DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/doxygen.stamp)
endif()
It's not exactly what I originally was asking for, but it's good enough for my purposes. I'm not going to accept this answer just yet, in case someone chimes in with an answer that does more exactly address my initial request.