expo

Using native code, especially C++, can be an excellent way to enhance your React Native app by leveraging performance-critical logic or existing C++ libraries. This guide will walk you through setting up a C++ library in a React Native project using a development build.

Requirements

Before you start, make sure you have the following:

• React Native CLI installed (since the managed Expo Go app does not support custom native code).
• Development Build configuration for Expo, if you’re using Expo.
• A basic understanding of JNI (Java Native Interface) if you're working with Android, or bridging concepts if you're working with iOS.

Step 1: Set Up Your React Native Project

First, create a new React Native project (if you haven’t already):

npx react-native init MyApp
cd MyApp

For Expo projects, you would need to create a development build. Check Expo’s Development Build Documentation for details.

Step 2: Add Your C++ Library

1. Create a New Folder for Native Code: Inside your project, add a folder for the C++ files, e.g., cpp/.

2. Add Your C++ Code: Inside the cpp/ folder, create a C++ file (e.g., MyLibrary.cpp). This file will contain the native code you want to use in React Native.

   // cpp/MyLibrary.cpp
   #include 
   
   extern "C"
   JNIEXPORT jint JNICALL
   Java_com_myapp_MyModule_addNumbers(JNIEnv* env, jobject obj, jint a, jint b) {
      return a + b;
   }

Step 3: Configure Android to Use the C++ Library

1. Update build.gradle: Add NDK support in your Android project if not enabled. Open android/app/build.gradle and add the following under defaultConfig:

   externalNativeBuild {
      cmake {
          cppFlags "-std=c++17"
      }
   }
   ndk {
      abiFilters "armeabi-v7a", "arm64-v8a", "x86", "x86_64" // Customize as needed
   }

2. Configure CMake: Create a CMakeLists.txt file in your project’s root directory or the cpp/ directory.

   # CMakeLists.txt
   cmake_minimum_required(VERSION 3.4.1)
   
   add_library( # Sets the name of the library.
               mylibrary
   
               # Sets the library as a shared library.
               SHARED
   
               # Provides the relative path to your source file(s).
               cpp/MyLibrary.cpp )
   
   find_library( # Sets the path to the log library.
                log-lib
                log )
   
   target_link_libraries( # Links your native library with the log library.
                         mylibrary
                         ${log-lib} )

3. Update Android Native Code Bridge: To expose this method to JavaScript, create a native module. Add a new file, MyModule.java, in android/app/src/main/java/com/myapp/:

   // android/app/src/main/java/com/myapp/MyModule.java
   package com.myapp;
   
   import androidx.annotation.NonNull;
   import com.facebook.react.bridge.ReactContextBaseJavaModule;
   import com.facebook.react.bridge.ReactMethod;
   import com.facebook.react.bridge.Promise;
   
   public class MyModule extends ReactContextBaseJavaModule {
      static {
          System.loadLibrary("mylibrary"); // Loads the C++ library
      }
   
      @NonNull
      @Override
      public String getName() {
          return "MyModule";
      }
   
      @ReactMethod
      public void addNumbers(int a, int b, Promise promise) {
          promise.resolve(addNumbersJNI(a, b));
      }
   
      public native int addNumbersJNI(int a, int b);
   }

   Register the module in MainApplication.java under getPackages to ensure React Native can use it.

Step 4: Write the JavaScript Bridge

1. In your React Native code, create a JavaScript file to wrap the native module:

   // MyModule.js
   import { NativeModules } from 'react-native';
   const { MyModule } = NativeModules;
   
   export const addNumbers = (a, b) => MyModule.addNumbers(a, b);

2. Now, you can use addNumbers in your React Native code:

   import React, { useState } from 'react';
   import { Button, Text, View } from 'react-native';
   import { addNumbers } from './MyModule';
   
   const App = () => {
      const [result, setResult] = useState(null);
   
      const handleAddNumbers = async () => {
          const sum = await addNumbers(5, 10);
          setResult(sum);
      };
   
      return (
          
              
              {result && Result: {result}}
          
      );
   };
   
   export default App;

Step 5: Build and Run Your App

Finally, you need to rebuild the Android project to link the native code:

cd android
./gradlew clean
cd ..
npx react-native run-android

If using Expo with development builds, make sure your build reflects these changes. For more details, refer to Expo’s Development Builds documentation.

---

With this setup, you’ve successfully integrated a C++ library in your React Native project, enabling you to call native C++ functions directly from JavaScript. This opens up possibilities for using optimized C++ code, accessing hardware-accelerated libraries, or reusing existing C++ code in your React Native app.