Put his hands in the native code (C/C++) with the Android NDK can be a hard task for an Android developer used to Java/Kotlin and Gradle.
This sample shows several use cases about how to use CMake:
- to build native library without Android Studio
- to use prebuilt library (shared (.so) or static (.a))
- to depends on another native module from your native module
- to build Android application with native code.
The application allows you to do some computations on 2 numbers given in input. Computations are realized by native libraries, written in C/C++, embedded in the application. Each computation is realized by one of the library and each library is imported in a different way using CMake.
The project contains 4 modules:
- app
- subdirectory
- shared_library
- static_library
The app module contains the Android application code and depends on other modules to do
computations.
Other modules contains C/C++ code and each one does one different computation:
subdirectorydoes a multiplication on 2 numbers.shared_librarydoes an addition on 2 numbers.static_librarydoes a subtraction on 2 numbers.
This module contains the code of the Android application. It contains Kotlin code to manage a basic Android application but also C/C++ code to communicate with the native libraries this application depends on.
Dependencies on native libraries are defined in the CMakeList targeted by the build.gradle of the
module:
externalNativeBuild {
cmake {
path "src/main/cpp/CMakeLists.txt"
}
}
The C/C++ code uses JNI to create a bridge between Kotlin code and pure C/C++ code. It allows to write functions which are called by Kotlin code and from inside JNI function, to call any C/C++ function.
This module is imported by the app module with a CMake dependency. The CMakeList used by the app
module targets the CMakeList of this module with the command add_subdirectory.
The library is built as a shared library. A .so file is generated and contains all the code.
The include directory contains all public headers required by the app module to be able to use
it.
The library exposes its headers by using target_include_directories with PUBLIC visibility.
This module allows to generate a shared library, a .so file, by executing the library_built.sh
script.
This module is not built each time the app module is built. app module only targets the
pre-generated .so file and its headers inside the include directory.
To import headers in the app module, we use the command include_directories
This module allows to generate a static library, a .a file, by executing the library_built.sh
script.
This module is not built each time the app module is built. app module only targets the
pre-generated .a file and its headers inside the include directory.
To import headers in the app module, we use the command include_directories