SwiftDroid

A SwiftUI-compatible runtime for Android, written in Swift. Write one codebase in Swift/SwiftUI. Run it natively on iOS and Android.

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Problem Statement

iOS and Android developers share a UI programming model (declarative, reactive, composable) but not a language or framework. The existing cross-platform options either sacrifice Swift semantics (transpilers like Skip convert Swift to Kotlin), sacrifice native performance (JS-based bridges), or require developers to learn a new language (Kotlin Multiplatform). SwiftDroid solves this by implementing the SwiftUI programming model as a pure Swift library that compiles natively to Android via the official Swift SDK for Android, with no generated code and no JVM.

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Architecture

Developer writes this once:

    struct HomeView: View {
        @State var count = 0
        var body: some View {
            VStack {
                Text("Count: \(count)")
                Button("Increment") { count += 1 }
            }
        }
    }

                    ┌────────────────────────────┐
                    │     SwiftDroid Core         │
                    │  (platform-agnostic Swift)  │
                    │                             │
                    │  @State  @Binding  @ObObj   │
                    │  View  ViewBuilder  Layout  │
                    └────────────┬───────────────┘
                                 │
              ┌──────────────────┼──────────────────┐
              │                                     │
    ┌─────────▼──────────┐             ┌────────────▼──────────┐
    │   SwiftDroidiOS    │             │  SwiftDroidAndroid     │
    │                    │             │                        │
    │  typealias to      │             │  NodeElement tree      │
    │  SwiftUI native    │             │      ↓                 │
    │  (zero overhead)   │             │  Layout engine         │
    │                    │             │  (propose/respond)     │
    │                    │             │      ↓                 │
    │                    │             │  AndroidRenderer       │
    │                    │             │  (swift-java bridge)   │
    │                    │             │      ↓                 │
    │                    │             │  Jetpack Compose       │
    └─────────┬──────────┘             └────────────┬──────────┘
              │                                     │
    ┌─────────▼──────────┐             ┌────────────▼──────────┐
    │   iOS (SwiftUI)    │             │  Android (native ARM)  │
    └────────────────────┘             └───────────────────────┘

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Comparison

	SwiftDroid	Skip	Compose Multiplatform	React Native
Input language	Swift	Swift	Kotlin	JS / TypeScript
Android execution	Native Swift	JVM (Kotlin)	JVM (Kotlin)	JS engine + bridge
iOS execution	SwiftUI native	SwiftUI native	Compose (non-native)	JS engine + bridge
Approach	Runtime library	Transpiler	Shared runtime	Runtime + bridge
Generated code	None	Kotlin files	None	JS bundle
Swift semantics	Exact	Approximated	N/A	N/A
Layout model	SwiftUI propose/respond	Compose constraints	Compose constraints	Flexbox (Yoga)
Maturity	Phase 0 — not ready	Production (2024)	Production (2023)	Production (2015)

Honest note: Skip is production-ready today and the correct choice if you need to ship to Android now. SwiftDroid is Phase 0. The architecture is better; the implementation takes longer. Read docs/internals/skip-analysis.md for the full trade-off analysis.

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Roadmap

Phase	Scope	Target	Status
0 — Foundation	SPM skeleton, research docs, public README	Weeks 1–3	Complete
1 — Reactivity	@State, @Binding, @ObservedObject, DependencyGraph	Weeks 4–10	Complete
2 — View Protocol	View, ViewBuilder, TupleView, NodeElement tree	Weeks 11–18	Complete
3 — Layout Engine	Propose/respond model, VStack/HStack/ZStack/Text	Weeks 19–32	Complete
4 — Android Renderer	swift-java bridge, Compose mapping, CounterApp demo	Weeks 33–44	In progress — done & tested: Button, render layer (DrawCommand/CommandRenderer), reactive ViewHost, the NodeElement→Compose mapping (ComposeRenderer), and a headless CounterApp demo (swift run CounterAppDemo). ✅ Native build verified: with open-source Swift 6.2.3 + Android SDK 6.2.3 + NDK r27c, the entire stack (core + Compose mapping + adapter + CounterApp) compiles AND links to a native Android ARM64 binary — file reports ELF 64-bit aarch64, interpreter /system/bin/linker64, no JVM, no transpilation. Reproducible via scripts/setup-android-sdk.sh + build-android.sh arm64. Remaining: run on an emulator/device, and the swift-java → Compose emit() + Gradle/JNI host for the live UI (see docs/internals/android-renderer.md)
5 — iOS Adapter	SwiftUI typealias layer, parity test suite	Weeks 45–52	Not started

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Getting Started

Requirements

• Swift 6.0+ (main-snapshot toolchain recommended)
• Xcode 16+ (for iOS builds)
• Swift SDK for Android (see below for Android builds)

Build on macOS

swift build
swift test

Build for Android

1. Install the Swift SDK for Android following the official guide.

2. Verify the SDK is installed:

   swift sdk list

3. Build:

   # Physical device (arm64)
   ./build-android.sh arm64
   
   # Emulator (x86_64)
   ./build-android.sh x86_64
   
   # Both
   ./build-android.sh all

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Contributing

Phase 0 contributions are most useful as:

• Documentation — corrections or additions to docs/internals/
• Issues — implementation questions, architecture feedback, use-case reports

Phase 1+ contributions: read docs/internals/swiftui-internals.md first. Every pull request must include unit tests. Platform-specific code must not appear in Sources/SwiftDroid/.

See the rules in task.md for session discipline.

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License

Apache 2.0. See LICENSE.