`linux/CMakeLists.txt`: support offline/sandboxed builds via OBJECTBOX_PREBUILT_DIR

[o-murphy]

Is there an existing issue?

• I have searched existing issues

Use case

objectbox_flutter_libs downloads libobjectbox.so at CMake configure time via
FetchContent_Populate. This makes it impossible to build in any network-isolated
environment — Flatpak/Flathub sandboxes, NixOS, Gentoo, Guix, and any distro build
system that restricts network access during the build.

The usual cmake workaround — setting FETCHCONTENT_SOURCE_DIR_OBJECTBOX-DOWNLOAD —
has no effect here. FetchContent_Populate (the pre-3.14 deprecated API) internally
spawns a cmake subbuild: a separate cmake process. Cache variables set in the
parent do not propagate to that subbuild. This is a documented cmake limitation.

Proposed solution

Check a cmake variable OBJECTBOX_PREBUILT_DIR before calling FetchContent_Populate.
If the variable points to an already-extracted archive, skip the download entirely:

if(DEFINED OBJECTBOX_PREBUILT_DIR AND EXISTS "${OBJECTBOX_PREBUILT_DIR}/lib/libobjectbox.so")
    set(objectbox-download_SOURCE_DIR "${OBJECTBOX_PREBUILT_DIR}")
else()
    FetchContent_GetProperties(objectbox-download)
    if(NOT objectbox-download_POPULATED)
        FetchContent_Populate(objectbox-download)
    endif()
endif()

Distro packagers would pass:

-DOBJECTBOX_PREBUILT_DIR=/path/to/extracted/objectbox-c

Describe alternatives you've considered

Current workaround

We maintain a local patch in the Flatpak manifest for
ebalistyka-app that checks a hardcoded
relative path. It works but is fragile and app-specific.

Additional context

This patch proposes no breaking changes

Flathub has growing Flutter adoption. Every Flutter app using objectbox needs this
patch to pass Flathub CI (which runs flatpak-builder --sandbox with no network).
A one-line upstream fix lets all downstream packagers drop their local patches.

[greenrobot-team]

Thanks for the suggestion!

Note for us: FetchContent_Populate is on its way out, see #701.

[o-murphy]

Thanks for the suggestion!

Note for us: FetchContent_Populate is on its way out, see #701.

Is there some other way to avoid downloads during build from sources?

[o-murphy]

Thanks for the suggestion!

Note for us: FetchContent_Populate is on its way out, see #701.

In this case we 'll be able to use sollution kinda that
(should be cross-platform)

if(DEFINED OBJECTBOX_PREBUILT_DIR AND EXISTS "${OBJECTBOX_PREBUILT_DIR}")
    set(objectbox-download_SOURCE_DIR "${OBJECTBOX_PREBUILT_DIR}")
    message(STATUS "Using prebuilt objectbox from ${OBJECTBOX_PREBUILT_DIR}")
    
    if(WIN32)
        set(OBJECTBOX_LIBRARY_PATH "${OBJECTBOX_PREBUILT_DIR}/bin/objectbox.dll")
    elseif(APPLE)
        set(OBJECTBOX_LIBRARY_PATH "${OBJECTBOX_PREBUILT_DIR}/lib/libobjectbox.dylib")
    else()
        set(OBJECTBOX_LIBRARY_PATH "${OBJECTBOX_PREBUILT_DIR}/lib/libobjectbox.so")
    endif()
    
    message(STATUS "  Library: ${OBJECTBOX_LIBRARY_PATH}")
else()
    FetchContent_MakeAvailable(objectbox-download)
endif()

[o-murphy]

Hi folks!

There you can find clarification about patch applying for objectbox_flutter_libs for versions: 5.3.1 and 5.3.2
https://github.com/o-murphy/flutpak/blob/main/known-patches/PATCHES.md