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C++ to Swift to Android Integration

This example demonstrates how to call C++ code from Android through Swift. The app uses a C++ library that provides basic calculator functions (add and multiply), wraps them in Swift, and calls them from an Android Kotlin app.

Overview

The project is structured into three main parts:

  1. cpp-lib: A C++ library with basic calculator functions (add and multiply). This is built using CMake and packaged as an artifactbundle for consumption by Swift.

  2. swift-lib: A Swift package that wraps the C++ functions and exposes them to Android using swift-java. The Swift code calls the C++ functions and provides JNI bindings automatically.

  3. app: A standard Android application written in Kotlin that calls the Swift-wrapped C++ functions and displays the results.

Prerequisites

Before you can build and run this project, you need to have the following installed:

  • Basic setup: Follow the Prerequisites and Setup instructions in hello-swift-java/README.md to install JDK, Swiftly, Swift SDK for Android, and publish the swift-java packages locally.
  • Android NDK: Required to build the C++ library. Set the ANDROID_NDK_HOME environment variable to your NDK installation path.

Setup and Configuration

1. Build the C++ Library

Before building the Android app, you need to build the C++ library:

cd hello-cpp-swift/cpp-lib
./build-android-static.sh

This will create the prebuilt/HelloWorldCpp.artifactbundle directory containing the compiled C++ static libraries for all Android architectures (arm64-v8a, armeabi-v7a, x86_64).

Running the example

  1. Open the swift-android-examples project in Android Studio.

  2. Select the hello-cpp-swift:app Gradle target.

  3. Run the app on an Android emulator or a physical device.

  4. The app will display the results of C++ calculations (10 + 5 and 10 × 5) called through Swift.

Building from command line

# From the project root directory
./gradlew :hello-cpp-swift:app:assembleDebug

# Install on device/emulator
./gradlew :hello-cpp-swift:app:installDebug

How it works

  1. C++ Layer (cpp-lib/src/calculator.cpp):

    int add(int a, int b) {
    return a + b;
}

  2. Swift Layer (swift-lib/Sources/HelloCppSwift/Calculator.swift):

    import HelloWorldCpp

public func addNumbers(_ a: Int32, _ b: Int32) -> Int32 {
    return add(a, b)
}

  3. Android/Kotlin Layer (app/MainActivity.kt):

    val sum = com.example.hellocppswift.HelloCppSwift.addNumbers(10, 5)

The Swift code is automatically wrapped with JNI bindings using the swift-java JExtract plugin, making it callable from Kotlin/Java code.

Rebuilding the C++ Library

If you make changes to the C++ code, you need to rebuild the artifactbundle:

cd hello-cpp-swift/cpp-lib
./build-android-static.sh

Then rebuild the Android app to pick up the changes.