Doc is built unconditionally, every time

[Michi Henning]

Using add_doxygen() like this:

file(GLOB public_headers ${CMAKE_SOURCE_DIR}/include/core/*.h)
add_doxygen(
    ${LIBNAME}-doc
    INPUT
        ${public_headers}
    OUTPUT_DIRECTORY
        ${CMAKE_BINARY_DIR}/doc
    STRIP_FROM_PATH
        "${CMAKE_SOURCE_DIR}/src"
    STRIP_FROM_INC_PATH
        "${CMAKE_SOURCE_DIR}/include"
    EXCLUDE_PATTERNS
        */internal/*
    EXCLUDE_SYMBOLS
        *::internal*
        *::Priv
    ALL
)

With this, the documentation is re-generated every time I build the project, even if Doxyfile.in has not changed, and none of the source files from which the documentation is built have changed.

For large documentation sets, this is quite annoying.

[Michi Henning (michihenning)]

Here is how I used to do this previously:

find_package(Doxygen)
find_program(DOT_EXECUTABLE dot /usr/bin)
if (NOT DOXYGEN_FOUND OR NOT DOT_EXECUTABLE)
    message(WARNING "Cannot generate documentation: doxygen and/or graphviz not found")
else()
    configure_file(${PROJECT_SOURCE_DIR}/doc/Doxyfile.in ${PROJECT_BINARY_DIR}/doc/Doxyfile @ONLY IMMEDIATE)
    add_custom_command(OUTPUT ${PROJECT_BINARY_DIR}/doc/lib${LIBNAME}/index.html
                       COMMAND ${DOXYGEN_EXECUTABLE} ${PROJECT_BINARY_DIR}/doc/Doxyfile
                       DEPENDS ${PROJECT_BINARY_DIR}/doc/Doxyfile
                               ${PROJECT_SOURCE_DIR}/include/core/*.h)
    add_custom_target(doc ALL
                       DEPENDS ${PROJECT_BINARY_DIR}/doc/lib${LIBNAME}/index.html)
    install(DIRECTORY ${PROJECT_BINARY_DIR}/doc/lib${LIBNAME}
            DESTINATION ${CMAKE_INSTALL_PREFIX}/share/doc)
endif()

Note that add_custom_command has a depends on the generated Doxyfile and the input files, and add_custom_target doesn't depend on Doxyfile, but on the generated index.html file.