[JAVA] How to write React Native bridge ~ Android version ~

I want to use native functions with React Native! Isn't there a time? However, there is no third party I'm looking for from GitHub. It's full of bugs and not maintained. And so on, the native side must also write. A situation like this occurs.

I would like to share a method to bridge native functions to React (js) side, which is useful in such a case.


--Https://github.com/react-native-community/react-native-camera is the subject --Create a module class that uses Android functions directly --Create a package class that implements the ReactPackage interface to handle the created module on the React side.

When using Android's Native function, there are functions without View such as voice and background processing, and functions with View such as camera preview.

Since the method of passing the value to React differs depending on whether there is a View or not, we will consider it separately here.

common part

Implement a class that implements the ReactPackage interface

  1. Register NativeModule that provides functions other than view for use on React side
  2. Register viewManager to use UI Component provided by Native on React side
  3. createJSModules is not currently required


package ...

import ...

public class RNCameraPackage implements ReactPackage {

    public List<NativeModule> createNativeModules(ReactApplicationContext reactApplicationContext) {
        return Arrays.<NativeModule>asList(
                new RCTCameraModule(reactApplicationContext),
                new CameraModule(reactApplicationContext),
                new FaceDetectorModule(reactApplicationContext)

    // Deprecated in RN 0.47
    public List<Class<? extends JavaScriptModule>> createJSModules() {
        return Collections.emptyList();

    public List<ViewManager> createViewManagers(ReactApplicationContext reactApplicationContext) {
        return Arrays.<ViewManager>asList(
                new RCTCameraViewManager(),
                new CameraViewManager()

If you want to bridge Native functions other than UI

Implement a module that inherits ReactContextBaseJavaModule
  1. In the java file on the android / app / src side, create a class that inherits the ReactContextBaseJavaModule class.

  2. Write the Module name

  3. Define constants to be used on React side (without it)

  4. Define the method to be used on React side. At this time, @ReactMethod annotation is added.


public class CameraModule extends ReactContextBaseJavaModule {
    private static final String TAG = "CameraModule";


    public CameraModule(ReactApplicationContext reactContext) {
        mScopedContext = new ScopedContext(reactContext);


    public String getName() {
        return "RNCameraModule";

    public Map<String, Object> getConstants() {
        return Collections.unmodifiableMap(new HashMap<String, Object>() {
                put("Type", getTypeConstants());
                put("FlashMode", getFlashModeConstants());


    public void takePicture(final ReadableMap options, final int viewTag, final Promise promise) {
        final ReactApplicationContext context = getReactApplicationContext();
        final File cacheDirectory = mScopedContext.getCacheDirectory();
        UIManagerModule uiManager = context.getNativeModule(UIManagerModule.class);
        uiManager.addUIBlock(new UIBlock() {
        public void execute(NativeViewHierarchyManager nativeViewHierarchyManager) {
            RNCameraView cameraView = (RNCameraView) nativeViewHierarchyManager.resolveView(viewTag);
                try {
                    if (cameraView.isCameraOpened()) {
                        cameraView.takePicture(options, promise, cacheDirectory);
                    } else {
                        promise.reject("E_CAMERA_UNAVAILABLE", "Camera is not running");
                } catch (Exception e) {
                  promise.reject("E_CAMERA_BAD_VIEWTAG", "takePictureAsync: Expected a Camera component");


When writing ReactMethod, if there is an argument from JS and the type is Object or Array
Object => ReadableMap
Array => ReadableArray

Replace the type with and use the argument. Also, use for all arguments handled by ReactMethod.

If you want to bridge UI-based Native functions

  1. Import com.facebook.react.uimanager.ViewGroupManager
  2. Create a class that inherits ViewGroupManager
  3. Override getName () to determine the Module name
  4. createViewInstanceOverride to describe the UI Component you want to use and create a method that returns that component


package com.example;

import com.facebook.react.bridge.ReadableArray;
import com.facebook.react.common.MapBuilder;
import com.facebook.react.uimanager.ThemedReactContext;
import com.facebook.react.uimanager.ViewGroupManager;

public class CameraViewManager extends ViewGroupManager<RNCameraView> {
    private static final String REACT_CLASS = "RNCamera";

    public void onDropViewInstance(RNCameraView view) {

    // Component name that will be called from JavaScript
    public String getName() {
        return REACT_CLASS;

    // Return the view component instantiated with Activity context
    public TextView createViewInstance(ThemedReactContext reactContext) {
        return new RNCameraView(themedReactContext);

Use Native Module on JS side

  1. Call NativeModule. {Value returned by getName ()}. (Here, export with RNCamera.js so that it can be used as a separate file.


import { NativeModules } from `react-native`;

export default NativeModules.RNCameraModule;

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