CCLee / Blog / Native Module in Android for React-Native

Native Module in Android for React-Native

1.Expo Modules (Kotlin Only)
- 1.1.Module Structure
  - 1.1.1.build.gradle Template
  - 1.1.2.Define Native Module in Kotlin
  - 1.1.3.Create TypeScript Interface and Wrapper for the Module
- 1.2.Key Expo Modules Concepts
  - 1.2.1.AsyncFunction
  - 1.2.2.Type Conversion
  - 1.2.3.Module Configuration
2.Full Implementation of AudioConverer (Not Related to Expo Modules)
- 2.1.Implementation
- 2.2.Code Breakdown and Learning Resources
  - 2.2.1.STEP 1: Extract audio metadata
  - 2.2.2.STEP 2: Create decoder
  - 2.2.3.STEP 3: Decode loop (Producer-Consumer pattern)
  - 2.2.4.STEP 4: WAV header

November 18, 2025

React-Native

1.Expo Modules (Kotlin Only)
- 1.1.Module Structure
  - 1.1.1.build.gradle Template
  - 1.1.2.Define Native Module in Kotlin
  - 1.1.3.Create TypeScript Interface and Wrapper for the Module
- 1.2.Key Expo Modules Concepts
  - 1.2.1.AsyncFunction
  - 1.2.2.Type Conversion
  - 1.2.3.Module Configuration
2.Full Implementation of AudioConverer (Not Related to Expo Modules)
- 2.1.Implementation
- 2.2.Code Breakdown and Learning Resources
  - 2.2.1.STEP 1: Extract audio metadata
  - 2.2.2.STEP 2: Create decoder
  - 2.2.3.STEP 3: Decode loop (Producer-Consumer pattern)
  - 2.2.4.STEP 4: WAV header

1. Expo Modules (Kotlin Only)

1.1. Module Structure

1modules/expo-audio-converter/
2├── android/                          # Android native code
3│   ├── build.gradle                  # Android build config
4│   └── src/main/java/expo/modules/audioconverter/
5│       └── ExpoAudioConverterModule.kt  # Native implementation
6├── ios/                              # iOS native code (if needed)
7│   └── ExpoAudioConverterModule.swift
8├── src/
9│   └── ExpoAudioConverterModule.ts   # TypeScript interface
10├── index.ts                          # Public API
11└── expo-module.config.json           # Module configuration

1.1.1. build.gradle Template

The following build configuration is a boilerplate for android plugin in react-native:

1apply plugin: 'com.android.library'
2
3group = 'expo.modules.audioconverter'
4version = '0.6.3'
5
6def expoModulesCorePlugin = new File(project(":expo-modules-core").projectDir.absolutePath, "ExpoModulesCorePlugin.gradle")
7apply from: expoModulesCorePlugin
8applyKotlinExpoModulesCorePlugin()
9useCoreDependencies()
10useExpoPublishing()
11
12// If you want to use the managed Android SDK versions from expo-modules-core, set this to true.
13// The Android SDK versions will be bumped from time to time in SDK releases and may introduce breaking changes in your module code.
14// Most of the time, you may like to manage the Android SDK versions yourself.
15def useManagedAndroidSdkVersions = false
16if (useManagedAndroidSdkVersions) {
17  useDefaultAndroidSdkVersions()
18} else {
19  buildscript {
20    // Simple helper that allows the root project to override versions declared by this library.
21    ext.safeExtGet = { prop, fallback ->
22      rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
23    }
24  }
25  project.android {
26    compileSdkVersion safeExtGet("compileSdkVersion", 34)
27    defaultConfig {
28      minSdkVersion safeExtGet("minSdkVersion", 21)
29      targetSdkVersion safeExtGet("targetSdkVersion", 34)
30    }
31  }
32}
33
34android {
35  namespace "expo.modules.audioconverter"
36  defaultConfig {
37    versionCode 1
38    versionName "0.6.3"
39  }
40  lintOptions {
41    abortOnError false
42  }
43}
44

1.1.2. Define Native Module in Kotlin

1package expo.modules.audioconverter
2
3import android.media.MediaCodec
4import android.media.MediaExtractor
5import android.media.MediaFormat
6import expo.modules.kotlin.modules.Module
7import expo.modules.kotlin.modules.ModuleDefinition
8import java.io.File
9import java.io.FileOutputStream
10import java.io.RandomAccessFile
11
12class ExpoAudioConverterModule : Module() {
13    override fun definition() = ModuleDefinition {
14        Name("ExpoAudioConverter")
15
16        AsyncFunction("convertToWav") { inputPath: String, outputPath: String ->
17            try {
18                // Remove file:// prefix if present
19                val cleanInputPath = inputPath.removePrefix("file://")
20                val cleanOutputPath = outputPath.removePrefix("file://")
21                
22                val inputFile = File(cleanInputPath)
23                val outputFile = File(cleanOutputPath)
24                
25                if (!inputFile.exists()) {
26                    throw Error("Input file does not exist: $cleanInputPath")
27                }
28                
29                val converter = AudioConverter()
30                converter.convertToWav(inputFile, outputFile)
31                
32                if (!outputFile.exists()) {
33                    throw Error("Output file was not created: $cleanOutputPath")
34                }
35                
36                mapOf(
37                    "outputPath" to outputFile.absolutePath
38                )
39            } catch (e: Exception) {
40                throw Error("Conversion failed: ${e.message}")
41            }
42        }
43    } 
44}

Note that in line 14 we register the name of the module (the class), in typescript (only within this plugin project) we will call

1const theInstance = requireNativeModule<ExpoAudioConverterModule>(
2    "ExpoAudioConverter"
3)

to instantiate the registered instance.

Also in expo-module.config.json (go 〈1.1. Module Structure〉 to see where it is located) we record the module that we want to export:

1{
2  "platforms": ["android"],
3  "android": {
4    "modules": ["expo.modules.audioconverter.ExpoAudioConverterModule"]
5  }
6}

On starting the application, Expo will scan for the ExpoAudioConverterModule and bridge our typescript code to this kotlin instance.

Remark. Note that we provide the full classpath in the json, not the filename.

Here Module, ModuleDefinition are all provided by the dependencies injected by useCoreDependencies().
Within the scope (trailing closure) of ModuleDefinition, AsyncFunction is one of the method provided by the class ModuleDefinition via function literal (for detail, refer to my article Function Literals with Receiver).
For synchrouous operations we can implement Function in place of AsyncFunction.

1.1.3. Create TypeScript Interface and Wrapper for the Module

1import { NativeModule, requireNativeModule } from "expo"
2import { Platform } from "react-native"
3
4export interface ConvertToWavResult {
5    outputPath: string
6}
7
8declare class ExpoAudioConverterModule extends NativeModule {
9    convertToWav(inputPath: string, outputPath: string): Promise<ConvertToWavResult>
10}
11
12// This call loads the native module object from the JSI.
13let AudioConverterModule: ExpoAudioConverterModule | null = null
14try {
15    // Only attempt to load on Android
16    if (Platform.OS === "android") {
17        AudioConverterModule = requireNativeModule<ExpoAudioConverterModule>("ExpoAudioConverter")
18    }
19} catch (e) {
20    // Module failed to load
21    console.warn("ExpoAudioConverter native module not available")
22}
23export async function convertToWav(inputPath: string, outputPath: string): Promise<ConvertToWavResult> {
24    if (!AudioConverterModule) {
25        throw new Error("ExpoAudioConverter native module not available")
26    }
27    return await AudioConverterModule.convertToWav(inputPath, outputPath)
28}

So basically we don't export the class ExpoAudioConverter, we simply instantiate it in the typescript file and export the execution convertToWav for use.

1.2. Key Expo Modules Concepts

1.2.1. AsyncFunction

Purpose: Define async native functions callable from JavaScript
Syntax: AsyncFunction("name") { param1: Type, param2: Type -> ... }
Returns: Automatically wrapped in a Promise
Errors: Thrown errors become rejected Promises

1// Native
2AsyncFunction("myFunction") { input: String ->
3    if (input.isEmpty()) throw Error("Input is empty")
4    return "Success"
5}
6
7// JavaScript
8try {
9    const result = await myFunction("")  // Throws error
10} catch (e) {
11    console.error(e.message)  // "Input is empty"
12}

1.2.2. Type Conversion

Expo automatically converts between JavaScript and native types:

JavaScript	Kotlin	Swift
`string`	`String`	`String`
`number`	`Int`, `Double`	`Int`, `Double`
`boolean`	`Boolean`	`Bool`
`object`	`Map<String, Any?>`	`[String: Any?]`
`array`	`List<Any?>`	`[Any?]`
`Promise`	`suspend fun`	`async`

1.2.3. Module Configuration

1// expo-module.config.json
2{
3    "platforms": ["android", "ios"],
4    "android": {
5        "modules": ["expo.modules.audioconverter.ExpoAudioConverterModule"]
6    },
7    "ios": {
8        "modules": ["ExpoAudioConverterModule"]
9    }
10}

2.1. Implementation

1class AudioConverter {
2    fun convertToWav(inputFile: File, outputFile: File) {
3        val extractor = MediaExtractor()
4        var format: MediaFormat? = null
5        
6        try {
7            extractor.setDataSource(inputFile.absolutePath)
8            
9            // Find the audio track
10            for (i in 0 until extractor.trackCount) {
11                format = extractor.getTrackFormat(i)
12                if (format.getString(MediaFormat.KEY_MIME)?.startsWith("audio/") == true) {
13                    extractor.selectTrack(i)
14                    break
15                }
16            }
17
18            if (format == null) {
19                throw IllegalArgumentException("No audio track found")
20            }
21
22            // Get audio properties
23            val sampleRate = format.getInteger(MediaFormat.KEY_SAMPLE_RATE)
24            println("sampleRate: $sampleRate")
25            val channelCount = format.getInteger(MediaFormat.KEY_CHANNEL_COUNT)
26            println("channelCount: $channelCount")
27            // Create decoder
28            val mime = format.getString(MediaFormat.KEY_MIME)
29            val decoder = MediaCodec.createDecoderByType(mime!!)
30            decoder.configure(format, null, null, 0)
31            decoder.start()
32
33            // Prepare output stream and write WAV header
34            val outputStream = FileOutputStream(outputFile)
35            writeWavHeader(outputStream, 0, sampleRate, channelCount)
36            
37            // Start decoding
38            val info = MediaCodec.BufferInfo()
39            var totalBytes = 0
40            
41            while (true) {
42                val inputBufferId = decoder.dequeueInputBuffer(10000)
43                if (inputBufferId >= 0) {
44                    val inputBuffer = decoder.getInputBuffer(inputBufferId)!!
45                    val sampleSize = extractor.readSampleData(inputBuffer, 0)
46                    
47                    if (sampleSize < 0) {
48                        decoder.queueInputBuffer(inputBufferId, 0, 0, 0, MediaCodec.BUFFER_FLAG_END_OF_STREAM)
49                    } else {
50                        decoder.queueInputBuffer(inputBufferId, 0, sampleSize, extractor.sampleTime, 0)
51                        extractor.advance()
52                    }
53                }
54
55                val outputBufferId = decoder.dequeueOutputBuffer(info, 10000)
56                if (outputBufferId >= 0) {
57                    val outputBuffer = decoder.getOutputBuffer(outputBufferId)!!
58                    val chunk = ByteArray(info.size)
59                    outputBuffer.get(chunk)
60                    outputBuffer.clear()
61                    
62                    if (chunk.isNotEmpty()) {
63                        outputStream.write(chunk)
64                        totalBytes += chunk.size
65                    }
66                    
67                    decoder.releaseOutputBuffer(outputBufferId, false)
68                    
69                    if (info.flags and MediaCodec.BUFFER_FLAG_END_OF_STREAM != 0) {
70                        break
71                    }
72                }
73            }
74
75            // Clean up
76            outputStream.close()
77            decoder.stop()
78            decoder.release()
79            extractor.release()
80
81            // Update WAV header with final size
82            updateWavHeader(outputFile, totalBytes)
83
84        } catch (e: Exception) {
85            e.printStackTrace()
86        }
87    }
88
89    private fun writeWavHeader(
90        outputStream: FileOutputStream,
91        totalAudioLen: Int,
92        sampleRate: Int,
93        channels: Int
94    ) {
95        val totalDataLen = totalAudioLen + 36
96        val byteRate = sampleRate * channels * 2 // 16 bits per sample
97
98        val header = ByteArray(44).apply {
99            // RIFF header
100            this[0] = 'R'.code.toByte()
101            this[1] = 'I'.code.toByte()
102            this[2] = 'F'.code.toByte()
103            this[3] = 'F'.code.toByte()
104            // Total file size - 8
105            this[4] = (totalDataLen and 0xff).toByte()
106            this[5] = (totalDataLen shr 8 and 0xff).toByte()
107            this[6] = (totalDataLen shr 16 and 0xff).toByte()
108            this[7] = (totalDataLen shr 24 and 0xff).toByte()
109            // WAVE
110            this[8] = 'W'.code.toByte()
111            this[9] = 'A'.code.toByte()
112            this[10] = 'V'.code.toByte()
113            this[11] = 'E'.code.toByte()
114            // fmt chunk
115            this[12] = 'f'.code.toByte()
116            this[13] = 'm'.code.toByte()
117            this[14] = 't'.code.toByte()
118            this[15] = ' '.code.toByte()
119            // fmt chunk size
120            this[16] = 16
121            this[17] = 0
122            this[18] = 0
123            this[19] = 0
124            // Audio format (PCM)
125            this[20] = 1
126            this[21] = 0
127            // Number of channels
128            this[22] = channels.toByte()
129            this[23] = 0
130            // Sample rate
131            this[24] = (sampleRate and 0xff).toByte()
132            this[25] = (sampleRate shr 8 and 0xff).toByte()
133            this[26] = (sampleRate shr 16 and 0xff).toByte()
134            this[27] = (sampleRate shr 24 and 0xff).toByte()
135            // Byte rate
136            this[28] = (byteRate and 0xff).toByte()
137            this[29] = (byteRate shr 8 and 0xff).toByte()
138            this[30] = (byteRate shr 16 and 0xff).toByte()
139            this[31] = (byteRate shr 24 and 0xff).toByte()
140            // Block align
141            this[32] = (channels * 2).toByte()
142            this[33] = 0
143            // Bits per sample
144            this[34] = 16
145            this[35] = 0
146            // data chunk
147            this[36] = 'd'.code.toByte()
148            this[37] = 'a'.code.toByte()
149            this[38] = 't'.code.toByte()
150            this[39] = 'a'.code.toByte()
151            // Data size
152            this[40] = (totalAudioLen and 0xff).toByte()
153            this[41] = (totalAudioLen shr 8 and 0xff).toByte()
154            this[42] = (totalAudioLen shr 16 and 0xff).toByte()
155            this[43] = (totalAudioLen shr 24 and 0xff).toByte()
156        }
157        
158        outputStream.write(header, 0, 44)
159    }
160
161    private fun updateWavHeader(file: File, totalAudioLen: Int) {
162        RandomAccessFile(file, "rw").use { raf ->
163            // Update file size
164            raf.seek(4)
165            val totalDataLen = totalAudioLen + 36
166            raf.write((totalDataLen and 0xff).toByte().toInt())
167            raf.write((totalDataLen shr 8 and 0xff).toByte().toInt())
168            raf.write((totalDataLen shr 16 and 0xff).toByte().toInt())
169            raf.write((totalDataLen shr 24 and 0xff).toByte().toInt())
170            
171            // Update data size
172            raf.seek(40)
173            raf.write((totalAudioLen and 0xff).toByte().toInt())
174            raf.write((totalAudioLen shr 8 and 0xff).toByte().toInt())
175            raf.write((totalAudioLen shr 16 and 0xff).toByte().toInt())
176            raf.write((totalAudioLen shr 24 and 0xff).toByte().toInt())
177        }
178    }
179}

2.2. Code Breakdown and Learning Resources

2.2.1. STEP 1: Extract audio metadata

Reference: https://developer.android.com/reference/android/media/MediaExtractor

1val extractor = MediaExtractor()
2extractor.setDataSource(inputFile.absolutePath)
3val format = extractor.getTrackFormat(0)  // Get audio format info

2.2.2. STEP 2: Create decoder

Reference: https://developer.android.com/reference/android/media/MediaCodec

1val decoder = MediaCodec.createDecoderByType(mime)  // AAC, MP3, etc.
2decoder.configure(format, null, null, 0)
3decoder.start()

2.2.3. STEP 3: Decode loop (Producer-Consumer pattern)

1while (true) {
2    // Producer: Feed compressed data
3    val inputBufferId = decoder.dequeueInputBuffer(10000)
4    extractor.readSampleData(inputBuffer, 0)
5    decoder.queueInputBuffer(...)
6    
7    // Consumer: Get decoded PCM
8    val outputBufferId = decoder.dequeueOutputBuffer(info, 10000)
9    outputStream.write(pcmData)
10}

2.2.4. STEP 4: WAV header

Reference: http://soundfile.sapp.org/doc/WaveFormat/

1writeWavHeader(outputStream, totalBytes, sampleRate, channels)

Contents

Contents

1. Expo Modules (Kotlin Only)

1.1. Module Structure

1.1.1. build.gradle Template

1.1.2. Define Native Module in Kotlin

1.1.3. Create TypeScript Interface and Wrapper for the Module

1.2. Key Expo Modules Concepts

1.2.1. AsyncFunction

1.2.2. Type Conversion

1.2.3. Module Configuration

2.1. Implementation

2.2. Code Breakdown and Learning Resources

2.2.1. STEP 1: Extract audio metadata

2.2.2. STEP 2: Create decoder

2.2.3. STEP 3: Decode loop (Producer-Consumer pattern)

2.2.4. STEP 4: WAV header

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Contents

Contents

1. Expo Modules (Kotlin Only)

1.1. Module Structure

1.1.1. build.gradle Template

1.1.2. Define Native Module in Kotlin

1.1.3. Create TypeScript Interface and Wrapper for the Module

1.2. Key Expo Modules Concepts

1.2.1. AsyncFunction

1.2.2. Type Conversion

1.2.3. Module Configuration

2. Full Implementation of AudioConverer (Not Related to Expo Modules)

2.1. Implementation

2.2. Code Breakdown and Learning Resources

2.2.1. STEP 1: Extract audio metadata

2.2.2. STEP 2: Create decoder

2.2.3. STEP 3: Decode loop (Producer-Consumer pattern)

2.2.4. STEP 4: WAV header

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