Automatic audio switching between devices in mobile app

Automatic Audio Switching Between Devices

Connected AirPods — audio switches to them. Disconnected — returns to speaker. Connected Bluetooth headset during call — call doesn't interrupt. Sounds like basic OS functionality, but in production without explicit AVAudioSession event handling everything works unpredictably.

How iOS Manages Audio Routing

AVAudioSession — central object. By default iOS switches output device when route changes. Problem — app may not know about it, and current AVAudioPlayer or AVAudioEngine continues on "old" route until next playback operation.

For explicit control — AVAudioSession.routeChangeNotification:

NotificationCenter.default.addObserver(
    self,
    selector: #selector(handleRouteChange(_:)),
    name: AVAudioSession.routeChangeNotification,
    object: nil
)

@objc func handleRouteChange(_ notification: Notification) {
    guard let info = notification.userInfo,
          let reasonValue = info[AVAudioSessionRouteChangeReasonKey] as? UInt,
          let reason = AVAudioSession.RouteChangeReason(rawValue: reasonValue)
    else { return }

    switch reason {
    case .newDeviceAvailable:
        // AirPods connected — switch to them
        resumePlaybackIfNeeded()
    case .oldDeviceUnavailable:
        // Headphones disconnected — pause or switch to speaker
        if let previousRoute = info[AVAudioSessionRouteChangePreviousRouteKey] as? AVAudioSessionRouteDescription {
            let wasHeadphones = previousRoute.outputs.contains {
                $0.portType == .headphones || $0.portType == .bluetoothA2DP
            }
            if wasHeadphones { pausePlayback() }
        }
    case .categoryChange:
        reconfigureEngine()
    default: break
    }
}

oldDeviceUnavailable with pause — standard behavior users expect (Spotify, Apple Music). Without pause audio keeps playing in speaker after accidental headphone disconnection.

AirPods and Automatic Device Switching

AirPods Pro/Max support Automatic Switching — transition between iPhone, iPad, Mac. App can't control this switching but can react to consequences. When AirPods switch between devices app gets routeChangeNotification with reason override or categoryChange.

Subtlety: after route change AVAudioSession.currentRoute updates not instantly. On oldDeviceUnavailable still 50–100ms route shows old device. Need short Task.sleep(nanoseconds: 100_000_000) or check on next runloop cycle.

AVAudioEngine: Rebuilding Graph on Route Change

If app uses AVAudioEngine with effects (equalizer, reverb), route change can reset session. Sign — AVAudioEngine.isRunning returns false after routeChangeNotification.

Correct pattern: subscribe to AVAudioEngineConfigurationChange and reconnect graph:

NotificationCenter.default.addObserver(
    forName: .AVAudioEngineConfigurationChange,
    object: audioEngine, queue: .main
) { [weak self] _ in
    self?.rebuildAudioGraph()
    try? self?.audioEngine.start()
}

rebuildAudioGraph() — disconnect all nodes, change outputNode (now points to new device), reconnect. Without this step AVAudioPlayerNode continues playing but silent — quiet, no errors in logs.

Android: AudioManager and AudioDeviceCallback

On Android route management via AudioManager.AudioDeviceCallback:

val audioManager = getSystemService(Context.AUDIO_SERVICE) as AudioManager

audioManager.registerAudioDeviceCallback(object : AudioDeviceCallback() {
    override fun onAudioDevicesAdded(addedDevices: Array<AudioDeviceInfo>) {
        val bluetooth = addedDevices.firstOrNull {
            it.type == AudioDeviceInfo.TYPE_BLUETOOTH_A2DP ||
            it.type == AudioDeviceInfo.TYPE_BLE_HEADSET
        }
        bluetooth?.let { switchToDevice(it) }
    }

    override fun onAudioDevicesRemoved(removedDevices: Array<AudioDeviceInfo>) {
        pauseIfHeadphonesRemoved(removedDevices)
    }
}, Handler(Looper.getMainLooper()))

AudioManager.setPreferredDevice() (API 28+) allows forcing device choice. Android 12+ added setCommunicationDevice() specifically for calls — don't confuse with regular playback.

Common Mistakes

• Don't call AVAudioSession.setActive(true) after reconfig — get silent playback without error
• Don't handle categoryChange on incoming call — phone call changes session category, after finish need restore it
• Change route on background thread — AVAudioSession operations must be on main thread or explicitly synchronized

Timeline

Basic route change handling (iOS): 3–5 days. Full implementation with AVAudioEngine, Android support, all scenarios (calls, BT headsets, AirPods): 2–3 weeks. Cost calculated individually.