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Android (Jetpack Compose)

Android renders as real Jetpack Compose. Like SwiftUI, Compose is a compiler-plugin framework — you can't author a @Composable from C# — so this is a native-shim backend: a Kotlin shim (native/SwiftDotNetComposeBridge, Bridge.kt) mirrors the Swift bridge and interprets the node tree into Compose over JNI.

  • Verified on the emulator (Pixel 9a AVD): the shared ContentView renders identically to iOS.

The Compose bridge

Bridge.kt interprets the entire control set — Text/Button/stacks/ZStack/ScrollView/Grid/List/Form/ Section/Group/DisclosureGroup/TabView(+paged carousel)/Tab/Menu, all inputs, NavigationStack/Link, Sheet, Alert, Image/Label, ProgressView/Gauge, Link, shapes — plus the modifier set.

  • Nav = a lightweight NavStack via CompositionLocal + Scaffold/TopAppBar.
  • Sheet = ModalBottomSheet; Alert = AlertDialog; ActionSheet = ModalBottomSheet with full-width option rows. AlertDialog has two button slots, so an alert with more than two buttons stacks them inside the confirm slot.
  • SF Symbols → an emoji map (avoids a material-icons dependency).
  • Modifiers apply via a Modified() wrapper (box modifiers + CompositionLocalProvider for font/color); shapes fill from the foregroundColor modifier.
  • Grid and AbsoluteLayout are custom Layouts. LazyVerticalGrid sizes columns but has no row span or explicit cell, and Compose has no absolute-positioning container — so both are written as Layout composables whose measure/place passes port SkiaNode.ResolveTracks and GridEngine line for line. Fixed/Flexible track sizes and point bounds are converted dp → px before sizing. See Grid.

Building & wiring

# 1. Build the .aar (JDK 21 + Android SDK)
native/SwiftDotNetComposeBridge/gradlew -p native/SwiftDotNetComposeBridge assembleRelease
# 2. Build the app
dotnet build sample/SampleApp -f net10.0-android

Toolchain: Gradle wrapper 8.14.3, AGP 8.11.1, Kotlin 2.1.0, compileSdk 36, Compose BOM 2025.01.00.

src/SwiftDotNet (net10.0-android TFM) binds the .aar via <AndroidLibrary … Bind="true"> and references Xamarin.AndroidX.Compose.* (Ui/Foundation, Material3, Activity.Compose). AndroidBridge : IBridge calls the bound Com.Swiftdotnet.Bridge.SwiftDotNetBridge static methods; events come back via a C# EventProxy : Java.Lang.Object, IEventCallback.

The reusable host base is SwiftDotNetActivity : ComponentActivity (a ComponentActivity is needed so the ComposeView gets its ViewTree owners).

Gotchas

  • Rebind after rebuilding the .aar, and clean obj/bin REPO-WIDE. Copy the freshly-built AAR from native/…/build/outputs/aar/ to build/, then:

    find . -type d \( -name obj -o -name bin \) -not -path "./native/*" -prune -exec rm -rf {} +

    Clearing only src/SwiftDotNet and sample/SampleApp is not sufficient once another head (e.g. SwiftDotNet.Skia.Maui) has been built. The failure mode is nasty because it doesn't look like a binding problem: the AAR extracts fine to obj/.../library_project_jars/, but the jar never reaches obj/.../class-parse.rsp, so zero bindings are generated and you get CS0246: 'Com' could not be found / 'IEventCallback' could not be found in AndroidBridge.cs. --no-incremental does not fix it. If you suspect the AAR itself, check it with javap -classpath <extracted> com.swiftdotnet.bridge.SwiftDotNetBridge — the types must be public.

  • Compose strong-skipping (Kotlin 2.x default) compares composable params by reference identity — mutating a VNode in place is skipped. Fix: VNode.props/children/type must be mutableStateOf (the Compose analog of iOS @Observable).

  • Button name collision with Android.Widget.Button → alias it.

  • Composables are kept private so the .NET binding generator doesn't bind synthetic Composer params.

  • Kotlin VNode props are Any? — cast with as? (Swift's are PropValue).

  • Minor UX: the app currently shows both the Activity ActionBar and the Compose TopAppBar in nav (disable the ActionBar via theme). ColorPicker cycles a palette on tap (no native Android color picker).

Modifier behaviour

  • .Repeating() animations use rememberInfiniteTransition + infiniteRepeatable (for -1) or repeatable with RepeatMode.Reverse/Restart. Like Web and Skia, a repeating animation pulses opacity 1 → 0.4 — the wire carries no from/to pair, so every backend agrees on that one convention. Consequence: BadgeView.Pulse reads as an opacity pulse rather than a size pulse (its .ScaleEffect(1.0) is identity), and SkeletonView's shimmer is a fading static gradient, not a travelling highlight. A spring curve combined with a repeat degrades to a tween — springs have no duration and can't feed infiniteRepeatable.
  • .Keyframes(…) maps to a real keyframes<Float> spec per track — see keyframe animations. Each track becomes its own animateFloat (looping ones on rememberInfiniteTransition, one-shots on an Animatable replayed by the on: trigger), and opacity/transform/size land as alpha, graphicsLayer, offset and width/height. Two wire-order details: Compose's using easing applies to the segment starting at a keyframe where the wire records the curve a stop is arrived on, so each stop hands its curve to the one before it; and autoreverse is emitted as a mirrored return leg rather than RepeatMode.Reverse, which would also reverse each segment's easing. A timeline's opacity track takes over from the .Repeating() pulse.
  • Image.FromUrl loads dependency-free (coroutine + URL.openConnection + BitmapFactory, cached by URL in-process). No Coil dependency was added. The bridge AAR's manifest declares android.permission.INTERNET, which merges into every consuming app. No disk cache, no downsampling, no in-flight dedup — two nodes with the same URL fetch twice before the first caches.
  • .Material is a translucent tint, not a backdrop blur — and this is deliberate. Modifier.blur / RenderEffect.createBlurEffect blur a composable's own content, whereas .Material is a SwiftUI backdrop blur. Using them would smear the node's children, which is worse than the tint. Real backdrop blur on Android needs Window.setBackgroundBlurRadius, which is window-level and can't be expressed per-node.

Safe area / edge-to-edge

SwiftDotNetActivity.OnCreate calls EdgeToEdge.Enable(this) before base.OnCreate, so the app draws behind the system bars — required on Android 15+ for apps targeting SDK 35, and what makes WindowInsets.safeDrawing report real values in the first place. Keep content clear of the bars with .SafeAreaPadding(...).

  • .SafeAreaPadding(edges, regions) → Modifier.windowInsetsPadding over WindowInsets.safeDrawing, unioned with WindowInsets.ime when the Keyboard region is asked for, narrowed via WindowInsetsSides.
  • .IgnoresSafeArea(...) → Modifier.consumeWindowInsets. Compose content is already edge-to-edge, so there is no padding to remove; consuming is what stops a descendant from re-applying insets an ancestor deliberately bled into.
  • RootHostView reports safeDrawing + ime (in dp) to C# on the reserved $safeArea event id, keyed by a LaunchedEffect so it fires only when a value actually changes.

Gotcha: IME insets require windowSoftInputMode="adjustResize" (the default). A host activity that sets adjustPan reports a keyboard height of 0.

🧩 Not emulator-verified — the Kotlin compiles and the wire contract is tested, but real inset values, rotation and keyboard behavior are unconfirmed.

CameraView and Map are not in the AAR

native/camera/CameraRenderer.kt and native/maps/MapRenderer.kt sit outside native/SwiftDotNetComposeBridge/src/main/kotlin/, the only source root the AAR compiles, and build.gradle.kts declares neither a sourceSets block nor the CameraX / ML Kit / MapLibre dependencies their headers require. Both files are authored but not shipped — confirmed by their absence from classes.jar. Pulling them in means adding those dependencies to the bridge AAR that every consumer would then carry, which is a design decision, not a build fix.

Maps

A Compose/MapLibre renderer is authored in native/maps (MapRenderer.kt) against the same native registerRenderer seam, but it is not compiled into the bridge AAR — see above. See Maps.

Hot reload

🧩 Expected, not run (needs an emulator). dotnet watch run -f net10.0-android should work with no extra setup — the Kotlin shim is not involved, since a reload arrives as an ordinary replace patch. See Hot Reload.