QT_ANDROID_MULTI_ABI_FORWARD_VARS

Allows to share CMake variables in multi-ABI builds

This variable was introduced in Qt 6.4.2.

The QT_ANDROID_MULTI_ABI_FORWARD_VARS variable allows specifying the list of CMake variables that need to be forwarded from the main ABI project to ABI-specific subprojects. Due to the specifics of the Multi-ABI project build process, there is no generic way to forward the CMake cache variables that are specified either in the command line or in another similar way.

A typical use case for the variable is propagating CMake cache variables specified in the command line. For example, a project has two variables PROJECT_WIDE_VARIABLE1 and PROJECT_WIDE_VARIABLE2 that affect the project configuration:

cmake_minimum_required(VERSION 3.18)

project(MyProject LANGUAGES CXX)

find_package(Qt6 REQUIRED COMPONENTS Core)

qt_add_executable(MyApp main.cpp)

if(PROJECT_WIDE_VARIABLE1)
    target_sources(MyApp PRIVATE sourcefile1.cpp)
endif()
if(PROJECT_WIDE_VARIABLE2)
    target_sources(MyApp PRIVATE sourcefile2.cpp)
endif()

The above contents of CMakeLists.txt enable you to control how MyApp is built by setting the corresponding CMake variables from the command line:

qt-cmake -S<source directory> -B<build directory> \
    -DPROJECT_WIDE_VARIABLE1=ON \
    -DPROJECT_WIDE_VARIABLE2=ON \
    -DQT_ANDROID_MULTI_ABI_FORWARD_VARS="PROJECT_WIDE_VARIABLE1;PROJECT_WIDE_VARIABLE2"

When configuring the application for desktop, PROJECT_WIDE_VARIABLE1 and PROJECT_WIDE_VARIABLE2 are visible in CMake listings and scripts as global cache variables. This doesn't work for Android Multi-ABI builds because ABI-specific subprojects do not inherit the cache variables from the main-ABI project. This issue can be solved by passing the list of required variables to the QT_ANDROID_MULTI_ABI_FORWARD_VARS variable, so both PROJECT_WIDE_VARIABLE1 and PROJECT_WIDE_VARIABLE2 values will be propagated to the ABI-specific builds.

The variable can be also defined in the project's CMakeLists.txt:

...
qt_add_executable(MyApp main.cpp)
...
if(ANDROID)
    set(QT_ANDROID_MULTI_ABI_FORWARD_VARS "PROJECT_WIDE_VARIABLE1;PROJECT_WIDE_VARIABLE2")
endif()
...

Setting the variable in this way allows you to have a predefined set of variables that will always be forwarded to abi-specific projects.