KRecorder

A Kotlin Multiplatform audio recording library for Android and iOS.

Features

• KMP-first — single API for Android and iOS
• Configurable — sample rate, channels, encoding (AAC/PCM/AMR/Opus), output format
• Reactive — StateFlow<RecorderState> with amplitude, duration, status
• Waveform-ready — amplitude normalized to 0.0–1.0 at ~20fps for visualization
• Optional UI — ready-made RecorderView + standalone WaveformView, or bring your own
• Lifecycle-safe — pause/resume/stop/release

Modules

KRecorder/
├── krecorder-core/   ← Recording engine. No UI dependency.
├── krecorder-ui/     ← Optional Compose Multiplatform UI.
└── sample/           ← Demo app showcasing all features.

Module	Description
krecorder-core	Recording interface + platform implementations. Android: MediaRecorder. iOS: AVAudioRecorder.
krecorder-ui	Optional Compose UI: RecorderView (full recorder) + WaveformView (standalone waveform bars).
sample	Runnable demo app with config pickers and live recording.

Demo App

The sample module demonstrates all library features:

• Config pickers — tap chips to change sample rate, channels, encoding, and format live
• Live recording — waveform visualization, timer with red recording indicator
• Pause/Resume/Stop — full lifecycle control
• Dark theme — Material3 dark color scheme

Run the demo

# Android
./gradlew :sample:installDebug

# iOS (via Xcode)
open iosApp/iosApp.xcodeproj

Quick Start

Core only (bring your own UI)

val recorder = KRecorder.create(
    config = RecorderConfig(
        sampleRate = SampleRate.RATE_44100,
        channels = Channels.MONO,
        encoding = AudioEncoding.AAC,
        format = OutputFormat.MP4,
    )
)

// Observe state reactively
recorder.state.collect { state ->
    println("${state.formattedDuration} | Amplitude: ${state.amplitude}")
}

recorder.start()
recorder.pause()
recorder.resume()
recorder.stop()     // file at state.value.outputPath
recorder.release()  // free resources

With built-in UI

val recorder = remember { KRecorder.create() }

RecorderView(
    recorder = recorder,
    onRecordingSaved = { filePath -> },
)

Standalone waveform

WaveformView(
    amplitudes = amplitudes,   // List<Float> of 0.0–1.0 values
    barColor = Color.White,
    barWidth = 3.dp,
    barSpacing = 2.dp,
    height = 200.dp,
)

Configuration

Option	Values	Default
Sample Rate	8000, 16000, 22050, 44100, 48000 Hz	44100
Channels	MONO, STEREO	MONO
Encoding	AAC, PCM_16BIT, AMR_NB, AMR_WB, OPUS	AAC
Format	MP4, THREE_GPP, OGG, WAV	MP4
Output Path	Custom path or null (auto temp file)	null

State

StateFlow<RecorderState> fields:

Field	Type	Description
status	RecorderStatus	IDLE / RECORDING / PAUSED / STOPPED / ERROR
durationMs	Long	Elapsed time in milliseconds
amplitude	Float	Audio level 0.0–1.0, updated ~20fps
outputPath	String?	Output file path
formattedDuration	String	MM:SS.T format

Platform Setup

Android

<uses-permission android:name="android.permission.RECORD_AUDIO" />

iOS

<key>NSMicrophoneUsageDescription</key>
<string>This app needs microphone access to record audio.</string>

Architecture

┌─────────────────────────────────────────┐
│              Your App                    │
│  ┌────────────┐  ┌───────────────────┐  │
│  │krecorder-ui│  │ Your custom UI    │  │
│  │RecorderView│  │ WaveformView      │  │
│  └─────┬──────┘  └────────┬──────────┘  │
│        └────────┬─────────┘             │
│        ┌────────┴─────────┐             │
│        │  krecorder-core  │             │
│        │ KRecorder + Flow │             │
│        ├────────┬─────────┤             │
│        │Android │   iOS   │             │
│        │MediaRec│AVAudioRec             │
│        └────────┴─────────┘             │
└─────────────────────────────────────────┘

Tech Stack

• Kotlin 2.3.20 / Compose Multiplatform 1.10.3
• Kotlinx Coroutines 1.10.2 / Gradle 8.14.3
• Android: MediaRecorder / iOS: AVAudioRecorder

License

Apache 2.0