bonsai-native

bonsai-native is an OCaml native UI experiment for building iOS and Android apps with the Bonsai programming model.

The app UI is authored in OCaml. Platform code is only the renderer bridge:

Bonsai component
  -> bonsai_native node tree
  -> platform renderer + event table
  -> Android JNI / iOS Camlkit
  -> Jetpack Compose / UIKit
  -> native app UI

This is not a WebView and not a SwiftUI wrapper.

Packages

• bonsai_native: shared OCaml node DSL, JSON/event bridge, and Bonsai driver integration.
• bonsai_android: Android facade over bonsai_native; rendered by Kotlin Compose through JNI.
• bonsai_apple: iOS/macOS-facing API and renderer abstractions; UIKit backend lives under apple/uikit.

Backend package names intentionally remain explicit. Existing Android code can continue to open Bonsai_android, and iOS code can continue to open Bonsai_apple.

OCaml UI Example

let component graph =
  let open Bonsai.Let_syntax in
  let count, set_count = Bonsai.state 0 graph in
  let%arr count and set_count in
  Bonsai_android.vstack
    [ Bonsai_android.text (Int.to_string count)
    ; Bonsai_android.button "Increment" ~on_click:(set_count (count + 1))
    ]

On Android, Compose receives a native node tree JSON payload and sends event ids back to OCaml through JNI. OCaml owns the Bonsai driver and event table, so state updates are still Bonsai effects.

Repository Layout

• native/: shared bonsai_native implementation.
• src/: Android OCaml facade.
• android/: Gradle/Compose demo app.
• jni/: Android JNI bridge into OCaml.
• apple/: Apple OCaml package, UIKit backend, and iOS examples.
• examples/: Android demo entrypoints and smoke examples.
• scripts/: Android and iOS bootstrap/build helpers.
• docs/: architecture and platform build notes.

Android Quick Check

Use a switch with the Android cross compiler and Jane preview packages. The current working path is documented in docs/android-native-build.md.

export BONSAI_NATIVE_OPAM_SWITCH=/path/to/your/ocaml-android-switch

scripts/build-android-native.sh
cd android
rtk proxy ./gradlew :app:assembleDebug

The debug APK should contain:

lib/arm64-v8a/libbonsai_android_counter.so

Run the emulator smoke test:

scripts/test-android-emulator.sh

It installs the APK, launches the counter, taps Increment, and verifies the UI changes from 0 to 1. It also switches through the Android Todo and Search tabs, which mirror the current iOS demo tabs.

iOS Quick Check

The iOS package is under apple/ and builds through Camlkit/opam-cross-ios contexts. See docs/apple-native-build.md.

The simulator app target is:

opam exec -- dune build apple/examples/BonsaiNativeDemos.app \
  --workspace dune-workspace.simulator

Status

Working now:

• Shared OCaml DSL for text, button, text fields, stacks, scroll views, keyed lists, navigation stacks, images, custom views, and common modifiers.
• Android JNI native library built with OCaml 5.2.1, Core, Bonsai, and bonsai.ppx_bonsai.
• Android Compose renderer loads real native OCaml state and dispatches clicks back into Bonsai.
• Android and iOS demo apps expose the same Counter, Todo, and Search OCaml views.
• Apple source/backend scaffolding is included in the same repo.

Still early:

• Android release-size optimization.
• More complete iOS packaging automation.
• AppKit backend.
• Shared persistence/sync packages above the UI layer.