AndroidStudio rebuilds everything every time if mu

If I build my AndroidStudio for one ABI, it works as expected.

If I support two ABIs, say for 'armeabi-v7a' and 'arm64-v8a', then Android Studio will rebuild every file, every time, even if nothing changed.

I use CMake external build to for the native part of my app.

My grade file:

android {
    compileSdkVersion 26
    buildToolsVersion '26.0.2'
    defaultConfig {
        applicationId "com.steenriver.biplane"
        minSdkVersion 19
        targetSdkVersion 19
        versionCode 115
        versionName "1.15"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        ndk {
            //abiFilters 'x86', 'x86_64', 'armeabi', 'armeabi-v7a', 'arm64-v8a'
            abiFilters 'armeabi-v7a', 'arm64-v8a'
            //abiFilters 'arm64-v8a'
            //abiFilters 'armeabi-v7a'
        }
        externalNativeBuild {
            cmake {
                //ldFlags "-Wl,-fuse-ld=gold"
                cppFlags "-std=c++11 -DANDROID=1 -DPLAY=1 -DUSEES3=1 -DLOGTAG=biplane"
                arguments "-DCMAKE_VERBOSE_MAKEFILE=1", "-DANDROID_TOOLCHAIN=clang", "-DANDROID_ARM_NEON=TRUE", "-DANDROID_ARM_MODE=arm"
            }
        }

    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
        debug {
            externalNativeBuild {
                cmake {
                    cppFlags += "-DDEBUG -DSHOWGRAPHS"
                }
            }
        }
    }
    externalNativeBuild {
        cmake {
            path '../jni/CMakeLists.txt'
        }
    }
}

And my top level CMakeLists.txt:

cmake_minimum_required(VERSION 3.4.1)


# dependency: dblunt


set( dblunt_src_DIR $ENV{HOME}/src/dutch-blunt/src )
set( dblunt_build_DIR $ENV{HOME}/src/dutch-blunt/src/output )
file(MAKE_DIRECTORY ${dblunt_build_DIR})
add_subdirectory( ${dblunt_src_DIR} ${dblunt_build_DIR} )


# dependency: GBase

set( gbase_src_DIR $ENV{HOME}/apps/GBase/src )
set( gbase_build_DIR $ENV{HOME}/apps/GBase/src/output )
file(MAKE_DIRECTORY ${gbase_build_DIR})
add_subdirectory( ${gbase_src_DIR} ${gbase_build_DIR} )


# dependency: PI

set( pi_src_DIR ../../PI )
set( pi_build_DIR ../../PI/output )
file(MAKE_DIRECTORY ${pi_build_DIR})
add_subdirectory( ${pi_src_DIR} ${pi_build_DIR} )


# dependency uif
set( uif_src_DIR ../../../uif )
set( uif_build_DIR ../../../uif/output )
file(MAKE_DIRECTORY ${uif_build_DIR})
add_subdirectory( ${uif_src_DIR} ${uif_build_DIR} )


# dependency gpgoap
set( gpgoap_src_DIR $ENV{HOME}/src/GPGOAP )
set( gpgoap_build_DIR $ENV{HOME}/src/GPGOAP/output )
file(MAKE_DIRECTORY ${gpgoap_build_DIR})
add_subdirectory( ${gpgoap_src_DIR} ${gpgoap_build_DIR} )


# dependency: OpenDE
set( opende_src_DIR $ENV{HOME}/src/opende )
set( opende_build_DIR $ENV{HOME}/src/opende/output )
file(MAKE_DIRECTORY ${opende_build_DIR})
add_subdirectory( ${opende_src_DIR} ${opende_build_DIR} )


# dependency: OpenAL
set( openal_src_DIR $ENV{HOME}/src/openal-soft/jni )
set( openal_build_DIR $ENV{HOME}/src/openal-soft/jni/output )
file(MAKE_DIRECTORY ${openal_build_DIR})
add_subdirectory( ${openal_src_DIR} ${openal_build_DIR} )


# dependency: freealut
set( freealut_src_DIR $ENV{HOME}/src/freealut/src )
set( freealut_build_DIR $ENV{HOME}/src/freealut/src/output )
file(MAKE_DIRECTORY ${freealut_build_DIR})
add_subdirectory( ${freealut_src_DIR} ${freealut_build_DIR} )


# depencency: gpg (prebuilt, provided by Google)

add_library( gpg
             STATIC
             IMPORTED
)
set_target_properties( gpg
        PROPERTIES IMPORTED_LOCATION
        $ENV{HOME}/src/gpg-cpp-sdk/android/lib/gnustl/${ANDROID_ABI}/libgpg.a )


set ( CMAKE_SHARED_LINKER_FLAGS "-fuse-ld=gold ${CMAKE_SHARED_LINKER_FLAGS}" )

# now build app's shared lib
add_library(biplane SHARED
    main.cpp
    StateManager.cpp
    scorekeeper.cpp
)

target_include_directories(biplane PRIVATE
    $ENV{HOME}/src/gpg-cpp-sdk/android/include
    $ENV{HOME}/apps/GBase/src
    $ENV{HOME}/apps/GBase/src/Android
    $ENV{HOME}/src/stb
    ../../PI
    .
)

# add lib dependencies
target_link_libraries(biplane
    gpg
    pi
    uif
    gbase
    dblunt
    gpgoap
    opende
    freealut
    openal
    EGL
    GLESv3
    OpenSLES
    android
    log
    z
    m
)

How can I convince Android Studio to recompile only the files that changed?

This was caused by the fact that the build variants all had the same output directory for the .o files.

You need to split out the build directory into release/debug and into the different ABIs.

The way to do this, is by using the CMAKE_CURRENT_BINARY_DIR macro as such:

set( gpgoap_src_DIR $ENV{HOME}/src/GPGOAP )
add_subdirectory( ${gpgoap_src_DIR} ${CMAKE_CURRENT_BINARY_DIR}/gpgoap )

If you do this, then the arm7 debug object files will end up in: app/.externalNativeBuild/cmake/debug/armeabi-v7a so they will not clash with other ABIs.

Note that you cannot re-use output directories, so you do need to postfix them for each add_subdirectory() that you do.

One final remark: the cmake documentation says that the output directory is optional for add_subdirectory() but I found this not to be the case. You need to supply it at least two arguments.