mob_audio_capture

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Android only. Meter the device's output mix from inside a Mob app — including audio produced by other apps and by native players (a game's own AudioTrack) that bypass Mob.Audio.

A normal app cannot tap the global output mix with a session-0 Visualizer (privileged — ERROR_NO_INIT on Android, device-verified), so this plugin uses the consent-gated MediaProjection + AudioPlaybackCapture path (Android API 29+).

Because capture requires a per-session system consent dialog and a typed foreground service, it is a test-environment dependency for agent-driven verification ("is the bundled game's audio actually producing signal?"), not a capability you ship in a production app.

⚠️ iOS is not supported — and this capability is not possible on iOS. Apple exposes no API for one app to capture another app's or the system's audio output; it's a hard sandbox/privacy restriction. The iOS build is a permanent :unsupported_on_platform stub, not a pending feature. If you only need to detect your own app's audio (e.g. a game engine running inside the Mob app), don't use this plugin — meter the audio at its source instead; that works on every platform and needs no capture API.

Platform support

PlatformSupport
Android 10+ (API 29)Full output-mix capture, subject to each source app's allowAudioPlaybackCapture (default-on for non-privileged API 29+ apps; VOICE_COMMUNICATION and DRM output never captured).
iOSNot supported — and not possible. No public API exists for an app to capture another app's or the system's output (sandbox/privacy restriction); this is permanent, not a pending feature. Every call returns {:error, :unsupported_on_platform}.

Usage

MobAudioCapture.start(socket)
# → handle_info({:audio_capture, :permission, :granted | :denied}, socket)

# once granted, while audio is playing anywhere on the device:
MobAudioCapture.output_level()
# => {-12.0, -3.4}   # {rms_db, peak_db}, or :silent

MobAudioCapture.stop(socket)

Host setup

Add to the host app's mob.exs plugin list, and declare the capture service in the host AndroidManifest.xml inside <application> (the native build also prints this as a warning):

<service
    android:name="io.mob.audiocapture.AudioCaptureService"
    android:exported="false"
    android:foregroundServiceType="mediaProjection" />

RECORD_AUDIO must be granted at runtime before start/1 (the manifest declaration the plugin contributes is necessary but not sufficient — it's a runtime permission).

Status

Device-verified on both platforms, 2026-07-04.

  • Android (moto g power 2021, Android 11 / API 30) — the full lifecycle over dist RPC: start/1 → MediaProjection consent → {:audio_capture, :permission, :granted}; output_level/0 read :silent at rest, live {rms, peak} (rms ≈ −12…−18 dBFS, peak ≈ −2…−8 dBFS) tracking another app's audio, :silent on pause; stop/1 → {:error, :not_capturing}. See decisions/2026-07-04-android-device-verification.md.
  • iOS (physical iPhone SE, aarch64-apple-ios) — the stub links, loads, and every call returns {:error, :unsupported_on_platform}: the correct, permanent behavior.

MobAudioCapture.DemoScreen ships for manual spot-checks (Start + a live meter). Signed with the shared mob first-party key and released via CI (.github/workflows/release.yml).