Swift for Android vs KMP

Monorepo containing a Swift-powered REST API, an Android app (Jetpack Compose + Swift JNI), and a shared Swift/Kotlin Multiplatform iOS client.

Prerequisites

• Swift toolchain via Swiftly (matching the Android SDK snapshot)
• Android SDK/NDK + adb
• JDK 21 (newer versions break the SwiftJava toolchain)
• Node.js 18+

Getting Started

1. API – cd server && npm install && npm start
2. Swift package – cd Android+Swift/Coasters && ./gradlew :swiftcore:copyJniLibs
3. CLI – in Android+Swift/Core run swift run CoasterCLI (server must be running)
4. Android app – from Android+Swift/Coasters run ./gradlew :app:assembleDebug (or open in Android Studio)
5. iOS app – open iOS+KMP/iosApp/iosApp.xcodeproj and run from Xcode

SwiftAndroid Notes

• Source of truth for the Swift domain model lives in Android+Swift/Core/Sources/Core. Before building the Android app you must copy any changes into Android+Swift/Coasters/swiftcore/Sources/RollerCoasterCore (the vendored module Android consumes).
• After every Swift edit run the regeneration pipeline from Android+Swift/Coasters:

  ./gradlew :swiftcore:copyJniLibs   # rebuilds Swift, refreshes JNI libs + bindings
  ./gradlew clean assembleDebug      # or use Android Studio’s Clean/Rebuild

  Skipping these steps leaves Android running stale .so files or generated Java stubs.

Troubleshooting

• Gradle can’t find jni.h / JAVA_HOME – export JDK 21 in every shell: export JAVA_HOME=$(/usr/libexec/java_home -v 21).
• :swiftcore:copyJniLibs fails – double-check the Swift toolchain (Swiftly snapshot) and Swift SDK path; rerun the task after copying the updated Swift sources.
• Swift 6 concurrency errors in generated JNI code – the build ships with Sources/RollerCoasterCore/ConcurrencySupport.swift (and the matching file under Core/), which marks the raw JNI handles as @unchecked Sendable; make sure that file is present and you’re on the latest swift-java main before rerunning ./gradlew :swiftcore:copyJniLibs (any remaining Task {} warnings are benign).
• Android UI shows empty results – the app just wraps the Swift JSON feed; make sure the Express server (server/) is running and the emulator can reach http://10.0.2.2:3000.