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name mobile-native-modules
description Building native modules with Expo Modules API, Turbo Modules, JSI bridging, and Flutter platform plugins
standards-version 1.6.3

Mobile Native Modules

Trigger

Use this skill when the developer asks about:

  • Creating a native module or native extension
  • Expo Modules API (Swift/Kotlin module definitions)
  • Turbo Modules or JSI bridging for React Native
  • Native view components (Fabric components)
  • Flutter platform plugins (MethodChannel, EventChannel)
  • Bridging Swift/Kotlin/ObjC code to JS or Dart
  • Custom native functionality not available via existing packages

Required Inputs

Input Description
Framework expo (React Native) or flutter
Module purpose What native capability is needed
Platforms iOS only, Android only, or both

Workflow

1. Scaffold the Module

Run mobile_createNativeModule to generate the boilerplate:

Use MCP tool: mobile_createNativeModule
  framework: "expo"
  module_name: "Haptics"
  output_directory: "modules"

This creates:

  • Expo: TypeScript bindings, Swift module, Kotlin module, expo-module.config.json
  • Flutter: Dart API, Swift plugin, Kotlin plugin, platform interface

2. Expo Modules API (Recommended for React Native)

Module Definition (Swift)

import ExpoModulesCore

public class HapticsModule: Module {
  public func definition() -> ModuleDefinition {
    Name("Haptics")

    Function("impact") { (style: String) in
      let generator: UIImpactFeedbackGenerator
      switch style {
      case "heavy": generator = UIImpactFeedbackGenerator(style: .heavy)
      case "medium": generator = UIImpactFeedbackGenerator(style: .medium)
      default: generator = UIImpactFeedbackGenerator(style: .light)
      }
      generator.impactOccurred()
    }

    AsyncFunction("vibrate") { (duration: Double, promise: Promise) in
      AudioServicesPlaySystemSound(kSystemSoundID_Vibrate)
      promise.resolve(nil)
    }
  }
}

Module Definition (Kotlin)

package expo.modules.haptics

import expo.modules.kotlin.modules.Module
import expo.modules.kotlin.modules.ModuleDefinition

class HapticsModule : Module() {
  override fun definition() = ModuleDefinition {
    Name("Haptics")

    Function("impact") { style: String ->
      val vibrator = appContext.reactContext?.getSystemService(Context.VIBRATOR_SERVICE) as? Vibrator
      vibrator?.vibrate(VibrationEffect.createOneShot(50, VibrationEffect.DEFAULT_AMPLITUDE))
    }
  }
}

TypeScript Bindings

import HapticsModule from "./HapticsModule";

export function impact(style: "light" | "medium" | "heavy" = "light"): void {
  HapticsModule.impact(style);
}

export async function vibrate(duration: number): Promise<void> {
  return HapticsModule.vibrate(duration);
}

3. Turbo Modules (Advanced)

For performance-critical code that needs synchronous JSI access:

// NativeHaptics.ts
import type { TurboModule } from "react-native";
import { TurboModuleRegistry } from "react-native";

export interface Spec extends TurboModule {
  impact(style: string): void;
  vibrate(duration: number): Promise<void>;
}

export default TurboModuleRegistry.getEnforcing<Spec>("Haptics");

4. Flutter Platform Plugins

Dart API

class Haptics {
  static const MethodChannel _channel = MethodChannel('haptics');

  static Future<void> impact(String style) async {
    await _channel.invokeMethod('impact', {'style': style});
  }
}

Swift Implementation

public class HapticsPlugin: NSObject, FlutterPlugin {
  public static func register(with registrar: FlutterPluginRegistrar) {
    let channel = FlutterMethodChannel(name: "haptics", binaryMessenger: registrar.messenger())
    registrar.addMethodCallDelegate(HapticsPlugin(), channel: channel)
  }

  public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
    switch call.method {
    case "impact":
      let generator = UIImpactFeedbackGenerator(style: .medium)
      generator.impactOccurred()
      result(nil)
    default:
      result(FlutterMethodNotImplemented)
    }
  }
}

5. Native View Components

Expo (Fabric-compatible)

import ExpoModulesCore

public class MyMapView: ExpoView {
  let mapView = MKMapView()

  required init(appContext: AppContext? = nil) {
    super.init(appContext: appContext)
    addSubview(mapView)
  }

  override public func layoutSubviews() {
    mapView.frame = bounds
  }
}

Example Interaction

User: "I need to access the device gyroscope data that isn't available through Expo packages"

Assistant:

  1. Runs mobile_createNativeModule with module_name: "Gyroscope" to scaffold the module
  2. Implements Swift CMMotionManager gyroscope reading in the iOS module
  3. Implements Kotlin SensorManager gyroscope listener in the Android module
  4. Creates TypeScript bindings with useGyroscope() hook that returns { x, y, z } rotation rates
  5. Adds the module to expo-module.config.json and shows how to import it
  6. Runs npx expo prebuild --clean to regenerate native projects

Key References

Resource URL
Expo Modules API https://docs.expo.dev/modules/overview/
Expo Module Tutorial https://docs.expo.dev/modules/get-started/
Turbo Modules https://reactnative.dev/docs/the-new-architecture/pillars-turbomodules
Flutter Platform Channels https://docs.flutter.dev/platform-integration/platform-channels
JSI (JS Interface) https://reactnative.dev/docs/the-new-architecture/why

MCP Usage

Tool When
mobile_createNativeModule Scaffold module with Swift/Kotlin stubs and TS/Dart bindings
mobile_checkNativeCompat Verify existing packages support New Architecture
mobile_checkBuildHealth Verify the project builds after adding native code

Common Pitfalls

  • Missing prebuild - after adding an Expo module, run npx expo prebuild --clean to regenerate native projects.
  • Forgetting both platforms - always implement Swift AND Kotlin; crashing on one platform is a common oversight.
  • Thread safety - native modules run on the native thread; UI updates need DispatchQueue.main (iOS) or runOnUiThread (Android).
  • Memory leaks - clean up listeners and observers in OnDestroy / deinit.
  • Expo Go incompatibility - custom native modules require a dev build, not Expo Go.
  • Missing config - expo-module.config.json must list your module class names for auto-linking.

See Also

  • mobile-config-plugins - modify native project config without ejecting
  • mobile-sdk-upgrade - ensure native modules remain compatible after upgrades
  • mobile-debugging - debug native module crashes with Xcode/Android Studio