React Native — native-api-bindgen

How a call works

Intent.parseUri(...) / UIView.alloc().initWithFrame(...)
  → generated TypeScript → global.__nab (installed by one C++ Turbo Module)
  → Android: C++ runtime → JNI            iOS: Objective-C++ runtime → NSInvocation

Methods (not constructors or properties) also get a …Async() Promise variant that runs off the JS thread; on iOS, UIKit members run on the main queue.
long / NSInteger are bigint in strict-typescript mode, number in ergonomic-typescript.
Java interfaces (Android) and Objective-C protocols (iOS, Proto.implement({...})) can be implemented in JavaScript; Objective-C blocks are JavaScript functions; Kotlin suspend functions return Promises; byte[] is a Uint8Array; <NativeView> shows views created through the bindings.
Java exceptions, NSError ** failures and Objective-C exceptions become JavaScript errors with their native details.

One-time app setup

package.json codegenConfig pointing at the generated spec (iOS also registers NabModuleProvider).
Android: include the generated CMake fragment and provide the module in OnLoad.cpp.
iOS: pod 'NativeApiBindings', :path => '../native-api-bindings'.

The generated library embeds the runtime. To depend on it instead, install native-api-bindgen-runtime from npm (npm install native-api-bindgen-runtime@beta).

The repository's examples/react-native/slice app does all of this. Its Showcase tab does real tasks (device dashboard, text-to-speech, share sheet, clipboard, persistent note, haptics) and its self-test suite passes 20 tests on the Android emulator and 19 on the iOS simulator (see Examples).

Measured

Release builds: about 1.50 µs per JSI → JNI call (Android emulator) and 1.14 µs per JSI → Objective-C call on the JS thread (iOS simulator); UIKit calls that hop to the main thread take about 10.0 µs. Metro does not tree-shake: the example's release APK is 19,312,390 bytes with bindings for its slice and 57,776,278 bytes with the whole Android SDK generated (baseline 18,623,666 bytes) — generate only what you use.