Add multiBackend Android flavor for runtime OpenGL/Vulkan switching (#4288)

platform/android/MapLibreAndroid/src/cpp/CMakeLists.txt

@@ -2,8 +2,18 @@ cmake_minimum_required(VERSION 3.10)
 
 project(MapLibreAndroid)
 
-add_subdirectory(../../../../../
-                 ../../../../../${ANDROID_ABI})
+# (a) Multi-flavor build is self-contained in MultiBackend.cmake and only entered for the
+# Gradle `multiBackend` flavor (which passes -DMLN_ANDROID_MULTI_BACKEND=ON). It never touches
+# the shared top-level CMakeLists; the single-backend path below is unaffected.
+if(MLN_ANDROID_MULTI_BACKEND)
+    include(${CMAKE_CURRENT_SOURCE_DIR}/MultiBackend.cmake)
+    return()
+endif()
+
+# (b) Build core into THIS configure's own build dir (was: ../../../../../${ANDROID_ABI}, an
+# out-of-tree path both multi sub-builds collided on). Each configure now has its own dir, so
+# the multi sub-builds isolate automatically -- no MLN_ANDROID_TOP_BINARY_DIR guard needed.
+add_subdirectory(../../../../../ ${CMAKE_CURRENT_BINARY_DIR}/maplibre-core)
 
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)

platform/android/MapLibreAndroid/src/cpp/MultiBackend.cmake

@@ -0,0 +1,85 @@
+# Android multi-backend build (Gradle `multiBackend` flavor only).
+#
+# Builds libmaplibre.so (Vulkan) and libmaplibre-opengl.so (OpenGL) by invoking this directory's
+# stock single-backend build twice via ExternalProject, and surfaces both to AGP as SHARED
+# targets so both .so land in the multiBackend AAR.
+#
+# Reached via include() from src/cpp/CMakeLists.txt while project(MapLibreAndroid) is in effect,
+# so the add_library targets below land in the AGP-invoked MapLibreAndroid project (AGP's CMake
+# file-API query only sees targets declared there) and CMAKE_CURRENT_SOURCE_DIR stays src/cpp.
+include(ExternalProject)
+
+set(_src ${CMAKE_CURRENT_SOURCE_DIR})            # src/cpp -- the single-backend entry
+
+# Forward the exact Android toolchain AGP handed this configure to each sub-build. Each sub-build
+# takes the single-backend path (MLN_ANDROID_MULTI_BACKEND=OFF) and builds core into its own
+# BINARY_DIR (see src/cpp/CMakeLists.txt edit (b)), so the two sub-builds isolate automatically.
+set(_fwd
+    -DCMAKE_TOOLCHAIN_FILE=${CMAKE_TOOLCHAIN_FILE}
+    -DANDROID_ABI=${ANDROID_ABI}
+    -DANDROID_PLATFORM=${ANDROID_PLATFORM}
+    -DANDROID_NDK=${ANDROID_NDK}
+    -DANDROID_TOOLCHAIN=${ANDROID_TOOLCHAIN}
+    -DANDROID_STL=${ANDROID_STL}
+    -DANDROID_CPP_FEATURES=${ANDROID_CPP_FEATURES}
+    -DANDROID_SUPPORT_FLEXIBLE_PAGE_SIZES=${ANDROID_SUPPORT_FLEXIBLE_PAGE_SIZES}
+    -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
+    -DMLN_ANDROID_MULTI_BACKEND=OFF)
+
+# Distinct BINARY_DIR per backend => distinct core build dir => no shared static-lib / LTO dir.
+set(_gl ${CMAKE_BINARY_DIR}/multi-opengl)
+set(_vk ${CMAKE_BINARY_DIR}/multi-vulkan)
+
+ExternalProject_Add(maplibre-opengl-build
+    SOURCE_DIR ${_src}
+    BINARY_DIR ${_gl}
+    CMAKE_ARGS
+        ${_fwd}
+        -DMLN_WITH_OPENGL=ON
+        -DCMAKE_LIBRARY_OUTPUT_DIRECTORY=${_gl}/lib
+    BUILD_COMMAND ${CMAKE_COMMAND} --build <BINARY_DIR> --target maplibre
+    INSTALL_COMMAND ""
+    BUILD_ALWAYS 1)
+
+ExternalProject_Add(maplibre-vulkan-build
+    SOURCE_DIR ${_src}
+    BINARY_DIR ${_vk}
+    CMAKE_ARGS
+        ${_fwd}
+        -DMLN_WITH_VULKAN=ON
+        -DCMAKE_LIBRARY_OUTPUT_DIRECTORY=${_vk}/lib
+    BUILD_COMMAND ${CMAKE_COMMAND} --build <BINARY_DIR> --target maplibre
+    INSTALL_COMMAND ""
+    BUILD_ALWAYS 1)
+
+# Surface both externally-built .so to AGP via stub SHARED targets whose output file is
+# overwritten with the real backend .so. AGP packages targets it builds; ExternalProject outputs
+# are opaque to it and IMPORTED targets aren't packaged -- hence this bridge.
+set(_stub ${CMAKE_BINARY_DIR}/maplibre_multi_stub.cpp)
+file(WRITE ${_stub} "extern \"C\" void maplibre_multi_backend_stub() {}\n")
+
+add_library(maplibre-opengl SHARED ${_stub})    # -> libmaplibre-opengl.so
+add_dependencies(maplibre-opengl maplibre-opengl-build)
+add_custom_command(TARGET maplibre-opengl POST_BUILD
+    COMMAND ${CMAKE_COMMAND} -E copy
+        ${_gl}/lib/libmaplibre.so
+        $<TARGET_FILE:maplibre-opengl>
+    VERBATIM)
+
+add_library(maplibre-vulkan SHARED ${_stub})
+# Produce libmaplibre.so (not libmaplibre-vulkan.so): Vulkan is the multiBackend AAR's
+# default/primary library, matching the SDK-wide convention where Vulkan is the default
+# backend and OpenGL ships as the explicitly-named "-opengl" variant.
+set_target_properties(maplibre-vulkan PROPERTIES OUTPUT_NAME maplibre)
+add_dependencies(maplibre-vulkan maplibre-vulkan-build)
+add_custom_command(TARGET maplibre-vulkan POST_BUILD
+    COMMAND ${CMAKE_COMMAND} -E copy
+        ${_vk}/lib/libmaplibre.so
+        $<TARGET_FILE:maplibre-vulkan>
+    VERBATIM)
+
+# AGP's defaultConfig nativeTargets always lists "maplibre"; provide a custom target so
+# `cmake --build . --target maplibre` triggers both .so builds.
+add_custom_target(maplibre DEPENDS maplibre-opengl maplibre-vulkan)
+
+install(TARGETS maplibre-opengl maplibre-vulkan LIBRARY DESTINATION lib)