Flutter Soloud Output Capture

Mixer output capture

flutter_soloud taps the master mixer output: everything the engine mixes (all voices, buses, and active global filters) is copied into a native circular buffer and delivered to Dart as a broadcast Stream<Uint8List> of audio chunks. This is a capture of the output the engine produces — it is not an input device. flutter_soloud has no microphone recording; for mic input use another package. Capture runs on all platforms; on web it requires the WebAssembly build (--wasm).

Minimal example

Record 5 seconds of the mix to a raw PCM file:

import 'dart:io';
import 'package:flutter_soloud/flutter_soloud.dart';

Future<void> recordMix(String outputPath) async {
  await SoLoud.instance.init();

  final sink = File(outputPath).openWrite();
  final stream = SoLoud.instance.startMixerOutputStream(
    format: MixerOutputFormat.pcmS16le,
  );
  final sub = stream.listen(sink.add);

  final sound = await SoLoud.instance.loadAsset('assets/music.mp3');
  SoLoud.instance.play(sound);
  await Future<void>.delayed(const Duration(seconds: 5));

  // Stop first: the tail of the buffer is flushed into the stream
  // synchronously on stop, so listeners still attached get the last bytes.
  SoLoud.instance.stopMixerOutputStream();
  await sub.cancel();
  await sink.close();
}

The API shape

All on SoLoud.instance (and mirrored on SoLoudIsolate.instance, see below):

• Stream<Uint8List> startMixerOutputStream({MixerOutputFormat format = MixerOutputFormat.pcmF32le, int sampleRate = -1, int channels = -1, int bufferSizeBytes = 1024 * 1024, int notificationThresholdBytes = 4096, int chunkPCMFrames = -1}) — starts capture and returns a broadcast stream. sampleRate/channels of -1 follow the engine config. Throws SoLoudNotInitializedException if the engine is not initialized. Calling it while already capturing returns the existing stream.
• void stopMixerOutputStream() — stops capture, flushes remaining buffer bytes into the stream synchronously, then closes the stream.
• bool get isMixerOutputStreamRunning — whether capture is active.
• Uint8List getMixerOutputWavHeader() — the finalized 44-byte WAV header; only meaningful with MixerOutputFormat.wav after stopping (returns an empty list otherwise).
• enum MixerOutputFormat — pcmF32le (default, pcmS8, pcmS16le, pcmS32le, opus, vorbis, flac, wav. Helpers: isPcm, bytesPerSample (0 for compressed), bytesPerFrame(channels).

How this differs from what models assume from other packages:

• There is no record()/startRecording() and no AudioRecorder — the API is named after the mixer output. You are recording playback, not a device.
• No onAudio/dataAvailable callback like the Web Audio ScriptProcessorNode/AudioWorklet — the result is an ordinary Dart Stream; you listen to it.
• Unlike just_audio/audioplayers there is no file-target convenience API ("record to path"). You always write the chunks yourself.
• The chunk size is driven by notificationThresholdBytes (compressed formats, and PCM when chunkPCMFrames is -1) or by chunkPCMFrames (PCM only, fixed-size chunks of exactly chunkPCMFrames * channels * bytesPerSample bytes; must be -1 or >= 2048). There is no per-chunk timestamp or duration metadata — just bytes.
• Compressed formats (opus, vorbis, flac) require the plugin to be built with the Xiph libraries; wav and the PCM formats are always available.

Traps

• Not microphone input. If the user asks to record the mic, this API is the wrong tool regardless of how it is named. flutter_soloud has no input capture.
• WAV header is a placeholder until stop. With MixerOutputFormat.wav the stream starts with a 44-byte header whose RIFF/data size fields are zero. After stopMixerOutputStream(), call getMixerOutputWavHeader() and overwrite the first 44 bytes of the file, or players report duration 0 / refuse to play:

  SoLoud.instance.stopMixerOutputStream();
  await sub.cancel();
  await sink.close();
  final header = SoLoud.instance.getMixerOutputWavHeader();
  if (header.length == 44) {
    final raf = File(path).openSync(mode: FileMode.writeOnlyAppend)
      ..setPositionSync(0)
      ..writeFromSync(header);
    await raf.close();
  }
  

• Cancel the subscription after stop, not before. stop() flushes the buffer tail into the stream with synchronous delivery; cancelling first loses the final chunk (which for compressed formats carries the encoder tail).
• chunkPCMFrames is PCM-only. Passing it with opus/vorbis/flac/wav trips an assert in debug builds. Compressed formats always use notificationThresholdBytes.
• Main-isolate stalls create capture gaps. The native side waits for Dart to advance the read position; a 500 ms synchronous block on the listening isolate produces a ~500 ms gap between chunks. Don't do heavy synchronous work (file sync writes on huge chunks, JSON parsing) in the listener; enlarge bufferSizeBytes to absorb bursts, or capture from a worker isolate (below).
• Init ordering. startMixerOutputStream before await SoLoud.instance.init() throws SoLoudNotInitializedException. deinit() auto-stops capture and closes the stream.
• Filters are baked in. Active global filters (e.g. pitchShiftFilter) apply to the captured output — the capture is post-mix, post-filter.

Capturing from a worker isolate

The engine is a C++ singleton that can only be initialized on the main isolate. For heavy consumers (encoding, network streaming), run the capture loop in a spawned isolate via SoLoudIsolate.instance, which exposes startMixerOutputStream, stopMixerOutputStream, isMixerOutputStreamRunning, getMixerOutputWavHeader, plus readSamplesFromFile/readSamplesFromMem. SoLoudIsolate has no init/deinit, no loading, no playback — those stay on the main isolate.

// Main isolate first:
await SoLoud.instance.init();
// ... then inside the spawned isolate (entry point needs
// @pragma('vm:entry-point')):
final stream = SoLoudIsolate.instance.startMixerOutputStream(
  format: MixerOutputFormat.pcmS16le,
  chunkPCMFrames: 2048,
);
final sub = stream.listen((chunk) { /* encode / send */ });
// on shutdown message:
await sub.cancel();
SoLoudIsolate.instance.stopMixerOutputStream();

SoLoudIsolate is marked @experimental; silence lints with // ignore_for_file: experimental_member_use where needed.

More depth

• Demo recording the mix to a file with format picker and WAV header patching: example/lib/mixer_capture/mixer_capture.dart in the flutter_soloud repo.
• Demo of capture in a spawned isolate (including surviving a blocked main isolate): example/lib/mixer_capture/isolate_capture_test.dart.
• Doc page: docs/advanced/mixer_output_capture.mdx (mixer output capture guide).

Keeping this skill current

This skill ships inside the flutter_soloud package, so upgrading flutter_soloud can carry a newer revision of it than the copy installed in the project. To check, run:

dart run flutter_soloud:skills --check

It reports the installed and bundled skill versions and exits non-zero when an update is available. Offer to update with dart run flutter_soloud:skills (which touches only the skills, never pubspec or build files).