react-native-automerge-generated

Native Automerge bindings for React Native via compiled Rust + UniFFI.

Replaces the WebAssembly-based Automerge backend with a native Rust implementation exposed through JSI, enabling:

• Hermes support — no longer requires JSC for WebAssembly
• Android support — WASM-based automerge only worked reliably on iOS
• No JSC patches — eliminates the createArrayBuffer patch for JSCRuntime.cpp
• No Metro hacks — no more slim→fullfat resolver redirects
• Better performance — direct native calls via JSI instead of WASM interpretation

Architecture

App TypeScript
       │
@automerge/automerge/slim  ←  UseApi(nativeApi)
       │
useapi-adapter.ts  (type conversion + API bridge)
       │
src/generated/automerge.ts  (auto-generated by ubrn)
       │  JSI (C++)
cpp/generated/  (auto-generated by ubrn)
       │  FFI (C ABI)
uniffi_automerge Rust crate  (UniFFI 0.29)
       │
automerge core (v0.7.3)

Hand-written components:

1. rust/ — UniFFI-annotated Rust wrapper (ported from automerge-swift)
2. src/useapi-adapter.ts — bridges generated API to @automerge/automerge/slim

Everything else (C++, ObjC++, Kotlin, generated TypeScript) is auto-generated by uniffi-bindgen-react-native.

Installation

npm install react-native-automerge-generated
# or
yarn add react-native-automerge-generated

iOS

cd ios && pod install

Android

Gradle sync should pick up the native module automatically.

Usage

import { UseApi } from '@automerge/automerge/slim';
import { nativeApi } from 'react-native-automerge-generated';

// Initialize once at app startup
UseApi(nativeApi);

// Then use automerge normally
import * as Automerge from '@automerge/automerge/slim';

let doc = Automerge.init();
doc = Automerge.change(doc, d => {
  d.key = 'value';
});

const saved = Automerge.save(doc);
const loaded = Automerge.load(saved);

Building from Source

Prerequisites

• Rust toolchain (1.89+): rustup install stable
• iOS targets: rustup target add aarch64-apple-ios aarch64-apple-ios-sim
• Android targets: rustup target add aarch64-linux-android armv7-linux-androideabi x86_64-linux-android
• Android NDK (via Android Studio)
• uniffi-bindgen-react-native: npm install (dev dependency)

Build

# iOS (builds universal static lib + generates bindings)
npm run build:ios

# Android (builds for all ABIs + generates bindings)
npm run build:android

# Check Rust compiles
cd rust && cargo check

Project Structure

rust/
  src/
    automerge.udl     UniFFI interface definition (50+ methods)
    doc.rs            Doc wrapper around automerge::AutoCommit
    sync_state.rs     SyncState wrapper
    scalar_value.rs   ScalarValue enum (String, Int, Uint, F64, etc.)
    value.rs          Value enum (Object | Scalar)
    patches.rs        Patch/PatchAction types
    obj_id.rs         ObjId custom type (ArrayBuffer)
    actor_id.rs       ActorId custom type (ArrayBuffer)
    change_hash.rs    ChangeHash custom type (ArrayBuffer)
    cursor.rs         Cursor custom type (ArrayBuffer)
    ...
  Cargo.toml
src/
  useapi-adapter.ts   UseApi bridge (NativeAutomerge + nativeApi)
  generated/
    automerge.ts      Auto-generated TypeScript bindings
    automerge-ffi.ts  Auto-generated FFI layer
  index.ts            Package entry point

Type Mapping

The adapter converts between the generated API types and what @automerge/automerge/slim expects:

| automerge/slim | Generated API | Conversion | |---|---|---| | ObjId (string "_root") | ArrayBuffer | base64 with o: prefix | | ChangeHash (hex string) | ArrayBuffer | hex encode/decode | | ActorId (hex string) | ArrayBuffer | hex encode/decode | | Cursor (string) | ArrayBuffer | base64 with c: prefix | | Uint8Array | Array<number> | Array.from() / new Uint8Array() | | JS primitives | ScalarValue tagged union | tag-based dispatch |

Current Limitations

• Stubbed methods: unmark(), spans(), getBlock(), updateBlock(), saveBundle(), encodeChange() throw or return empty values. These are not needed for core automerge operations.
• getChanges() returns concatenated bytes — the Rust encode_changes_since() returns all changes as a single byte array rather than individually split changes. This works for loadIncremental() / applyChanges() but callers expecting individual change objects should be aware.
• decodeChange() returns partial data — the native implementation returns change metadata (hash, actor, timestamp, message, deps) but not the full ops array or seq/startOp fields.
• No WASM fallback — this is a complete replacement, not a supplement.

Development Notes

Post-Build Hook

The build process includes a post-build script (scripts/add-useapi-export.sh) that automatically injects the nativeApi export into index.ts after ubrn generates the bindings. This is necessary because ubrn regenerates index.ts from a template on each build.

The export line:

export { nativeApi, NativeAutomerge, NativeSyncState } from './useapi-adapter';

This runs automatically after npm run build:ios or npm run build:android.

Testing

The package includes a comprehensive test suite with 40+ test cases covering all major Automerge operations.

# Install dependencies
npm install

# Build the native module first (required)
npm run build:ios  # or build:android

# Run tests
npm test

# Run tests in watch mode
npm run test:watch

# Run tests with coverage
npm test -- --coverage

Test coverage includes:

• Document creation, cloning, and manipulation
• Map, List, Text, and Counter operations
• Save/load persistence
• Merge operations and conflict resolution
• Change history and decodeChange()
• Sync protocol
• Complex nested structures

See tests/README.md for detailed test documentation.

License

Apache 2.0