CMake ExternalProject_Add() and FindPackage()

Question

Is there are proper way to find a library (via FindPackage()) which was built with ExternalProject_Add()?

The problem is that CMake cannot

Answer 1

You can force a build using the build_external_project function below.

It works by generating a simple helper project inside the build tree and then calling the cmake configuration and the cmake build on the helper.

Customize at will for the actual ExternalProject_add command.

Note that the trailing arguments are used to pass CMAKE_ARGS. Furthur enhancements are left as an exercise to the reader :-)

# This function is used to force a build on a dependant project at cmake configuration phase.
# 
function (build_external_project target prefix url) #FOLLOWING ARGUMENTS are the CMAKE_ARGS of ExternalProject_Add

    set(trigger_build_dir ${CMAKE_BINARY_DIR}/force_${target})

    #mktemp dir in build tree
    file(MAKE_DIRECTORY ${trigger_build_dir} ${trigger_build_dir}/build)

    #generate false dependency project
    set(CMAKE_LIST_CONTENT "
        cmake_minimum_required(VERSION 2.8)

        include(ExternalProject)
        ExternalProject_add(${target}
            PREFIX ${prefix}/${target}
            URL ${url}
            CMAKE_ARGS ${ARGN}
            INSTALL_COMMAND \"\"
            )

        add_custom_target(trigger_${target})
        add_dependencies(trigger_${target} ${target})
    ")

    file(WRITE ${trigger_build_dir}/CMakeLists.txt "${CMAKE_LIST_CONTENT}")

    execute_process(COMMAND ${CMAKE_COMMAND} ..
        WORKING_DIRECTORY ${trigger_build_dir}/build
        )
    execute_process(COMMAND ${CMAKE_COMMAND} --build .
        WORKING_DIRECTORY ${trigger_build_dir}/build
        )

endfunction()