Getting Started with Swift for Android

Prerequisites

Before you begin Swift development for Android, make sure you have the following tools installed:

Xcode (For macOS): If you don't have Xcode installed, it is recommended to install it from the Mac App Store. Xcode includes Swift, and the installation of Xcode will automatically include Swift as part of the development tools.
Android Studio with Android SDK and NDK: Android Studio is the official integrated development environment (IDE) for Android app development. To install Android Studio with all required components perform the following steps:
1. Download Android Studio from the official website: Android Studio Download.
2. Run the downloaded installer and follow the setup instructions.
3. Launch Android Studio and complete the initial configuration, including the installation of the Android SDK.
4. Open SDK Manager by selecting Tools -> SDK Manager in the main menu.
5. In the SDK Tools tab, select the NDK(side by side) and CMake checkbox. Click OK to download and install.
6. Install Android Emulator if you want to run on Android emulators
SCADE IDE (Optional): If you are interested in using Scade for Swift development, you can download it from SCADE's official website: Scade Download.

Install Swift for Android

For macOS

Download Swift Toolchain for Android
Run the downloaded installer and follow the setup instructions.
You can verify installation by running the scd list-installed command in the terminal. It should print full path to the installed Swift toolchain for Android.

For Linux

Currently Swift for Android is available for macOS only. It will be available for Linux very soon.

Create new Android project

Open Android Studio.
Select File -> New -> New Project in the main menu.
Choose a suitable template; for this guide, select "Empty Views Activity."
NOTE: Ensure you choose "Empty Views Activity," not "Empty Activity."
Provide project details such as name (e.g., SwiftAndroidExample) and package name (e.g., com.example.swiftandroidexample).
Click "Next" to proceed.

Create Swift Package Manager (SPM) Project

Create a new SPM project within the app/src/main/swift subdirectory, run the following commands in the terminal from the root project directory:
```
mkdir -p app/src/main/swift
cd app/src/main/swift
swift package init --name SwiftAndroidExample
```
It will create a new SPM project named SwiftAndroidExample within the app/src/main/swift subdirectory.This project follows the standard structure of an SPM project and will be automatically built by Gradle as part of the Android project's build process with custom build tasks configured in the build.gradle file.
Change the product type to a dynamic library in the SPM project manifest. To do that, add the type: .dynamic parameter in the library description in the Package.swift file located in the SPM project directory:
```
products: [
    .library(
        name: "SwiftAndroidExample",
        type: .dynamic,                     // add this line
        targets: ["SwiftAndroidExample"]),
],
```

Configure Swift SPM project in build.gradle

To configure SPM project build, make the following changes in the app/build.gradle file:

Add the following code in the android section:
```
sourceSets {
    main {
        jniLibs.srcDirs = ["lib"]
    }
}
```
This informs Gradle to locate JNI libraries in the lib subdirectory.
In the dependencies section, add:
```
implementation fileTree(dir: 'lib', include: ['*.jar'])
```
This line instructs Gradle to include .jar libraries in the resulting APK.

At the end of app/build.gradle, insert the following custom Gradle task:

task buildSwiftProject(type: Exec) {
    commandLine 'scd',
            'archive', '--type', 'android',
            '--path', 'src/main/swift',
            '--platform', 'android-arm64-v8a',
            '--platform', 'android-x86_64'
}

tasks.whenTaskAdded { task ->
    if (task.name == 'assembleDebug' || task.name == 'assembleRelease') {
        task.dependsOn buildSwiftProject
    }
}

Initialize Swift Runtime and Load Swift Library

In MainActivity.java, add the following code in the onCreate method which performs required Swift runtime initialization and loads the SwiftAndroidExample JNI library containing Swift code from the SPM project:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // Initialize Swift runtime and Swift Foundation library
    try {
        org.swift.swiftfoundation.SwiftFoundation.Initialize(this, false);
    } catch (Exception err) {
        android.util.Log.e("Swift", "Can't initialize Swift Foundation: " + err.toString());
    }

    // Load JNI library containing Swift code
    System.loadLibrary("SwiftAndroidExample");
}

Declare and call native Swift methods in Java

In the MainActivity class, declare a native method named swiftHelloWorld:

public class MainActivity extends AppCompatActivity {
    // Declare a native method in Java, with the implementation in Swift
    private native String swiftHelloWorld();

    // ... Rest of your MainActivity code
}

At the end of the onCreate method of the MainActivity class, add the following code:

@Override
protected void onCreate(Bundle savedInstanceState) {
    // ... Swift initialization code added before

    // Call the swiftHelloWorld method implemented in Swift
    String msg = swiftHelloWorld();

    // Display the value returned from swift in the text view
    android.widget.TextView text = new android.widget.TextView(this);
    text.setText(msg);
    text.setGravity(android.view.Gravity.CENTER_VERTICAL |
                    android.view.Gravity.CENTER_HORIZONTAL);
    text.setLayoutParams(new android.widget.RelativeLayout.LayoutParams(
            android.widget.RelativeLayout.LayoutParams.MATCH_PARENT,
            android.widget.RelativeLayout.LayoutParams.MATCH_PARENT));
    setContentView(text);
}

Implement native Java methods in Swift

Add the following code in the SwiftAndroidExample.swift source located in the app/src/main/swift/Sources/SwiftAndroidExample subdirectory of the Android project root:

import CJNI

// Use @_silgen_name attribute to set native name for a function called from Java
@_silgen_name("Java_com_example_swiftandroidexample_MainActivity_swiftHelloWorld")
public func MainActivity_swiftHelloWorld(
    env: UnsafeMutablePointer, activity: JavaObject
) -> JavaObject {
    let jstr = env.pointee.pointee.NewStringUTF(env, "Swift Hello World!")
    return jstr!
}

This Swift code implements the swiftHelloWorld Java native method of the MainActivity class. The method is declared with the @_silgen_name attribute to set correct name for the method implementation. It creates a new Java string containing the message "Swift Hello World!" and returns it to the caller.

Run the SwiftAndroidExample application

After completing the steps described in the previous sections, you can run the application on Android and observe the results. It will display a text view containing the text "Swift Hello World!".

Sources

Complete source code of the example application:
https://github.com/scade-platform/swift-android-example
Github repository with all Swift Android Toolchain releases:
https://github.com/scade-platform/swift-android-toolchain/releases
Detailed article about using Swift on Android:
https://medium.com/@SCADE/using-swift-pm-libraries-in-the-android-studio-projects-7cef47c300bf

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