Flutter Soloud Mixing Bus

Mixing buses (sub-mixes) in flutter_soloud

A Bus is a virtual mixer channel: sounds are routed through it, so one volume change or one filter affects everything on the bus. The canonical use is separate Music / SFX / UI buses behind user volume sliders. Unlike audioplayers/just_audio (one player per sound, no sub-mix concept), routing is explicit: you create a bus, play the bus itself on the engine, then play sounds onto it.

Minimal example

import 'package:flutter_soloud/flutter_soloud.dart';

Future<void> main() async {
  await SoLoud.instance.init();

  // 1. Create the bus (synchronous, cheap).
  final sfxBus = SoLoud.instance.createMixingBus(name: 'SFX');

  // 2. REQUIRED: play the bus on the engine. Until this call, the bus
  //    (and everything routed through it) is silent.
  sfxBus.playOnEngine();

  // 3. Route sounds through the bus.
  final explosion = await SoLoud.instance.loadAsset('assets/explosion.mp3');
  sfxBus.play(explosion); // synchronous, returns SoundHandle

  // 4. Collective volume: set it on the bus's own handle, not per-sound.
  SoLoud.instance.setVolume(sfxBus.soundHandle!, 0.5);

  // 5. Shared filter on everything routed through the bus.
  sfxBus.filters.echoFilter.activate();

  // Cleanup: stops all sounds playing through the bus.
  sfxBus.dispose();
}

The API shape

• SoLoud.instance.createMixingBus({String name = ''}) → Bus. Synchronous; registers the bus in the Buses() registry. First bus gets busId == 1; busId == 0 always means "the main engine output".
• Buses() singleton registry: Buses().buses (list of all live buses), Buses().byName('SFX'), Buses().byId(1). Both lookups throw if not found unless you pass orElse.
• bus.playOnEngine({double volume = 1.0, bool paused = false}) → SoundHandle. Makes the bus audible; stores the handle in bus.soundHandle. Calling it again on the same bus stops the earlier instance (only one bus voice at a time).
• Playing onto a bus — two equivalent ways:
  • bus.play(sound, {volume, pan, paused, looping, ...}) and its siblings bus.playClocked, bus.playScheduled, bus.play3d, bus.play3dClocked, bus.play3dScheduled — thin wrappers that forward to the SoLoud method of the same name with busId set.
  • SoLoud.instance.play(sound, busId: sfxBus.busId) — every SoLoud play method takes an int busId = 0 parameter.
• bus.annexSound(handle) — re-parent a live voice (by its SoundHandle) into this bus, e.g. move a sound already playing on the engine onto a filtered bus mid-playback.
• Bus volume: there is no bus.volume property. Use SoLoud.instance.setVolume(bus.soundHandle!, v) / SoLoud.instance.getVolume(bus.soundHandle!). This scales every voice routed through the bus on top of each voice's own volume.
• bus.filters — a FiltersSingle scoped to this bus; filters activated here apply to all routed sounds only (engine-wide filters live on SoLoud.instance.filters). See skill flutter-soloud-filters.
• bus.setChannels({Channels channels = Channels.stereo}) — bus output channel count (Channels.mono/stereo/quad/surround51/dolby71).
• bus.getChannelVolume(channel) — approximate per-channel output level for VU meters. Requires SoLoud.instance.setVisualizationEnabled(true) first; returns 0 for out-of-range channels.
• bus.getActiveVoiceCount() — voices currently playing through the bus; returns 0 if the bus isn't playing on the engine.
• bus.isActive — true once playOnEngine() succeeded.
• bus.dispose() — destroys the bus and stops all sounds routed through it; removes it from Buses().

Everything above (except createMixingBus lookups and dispose) is synchronous and returns SoundHandle directly — do not await them.

Traps

• Silent bus: forgetting playOnEngine() is the #1 failure. Sounds "play" (handles are created, voices count up) but nothing is heard. bus.soundHandle stays null until then, so guard with if (bus.soundHandle != null) before volume/filter-parameter calls.
• Do not invent bus.volume / bus.setVolume(): they don't exist. Volume goes through SoLoud.instance.setVolume(bus.soundHandle!, v). Models coming from audioplayers habitually look for a per-player volume setter — route them to the bus handle instead.
• No await on bus play methods: bus.play(...) returns SoundHandle, not a Future. SoLoud.instance.loadAsset is the async part; playback is synchronous.
• Disposed bus throws: every Bus method throws SoLoudBusDisposedDartException after dispose(). Double-dispose also throws — null out your reference after disposing.
• Create buses after await SoLoud.instance.init(): the constructor calls into the native engine, and playOnEngine() throws SoLoudBackendNotInitedException when the engine isn't up.
• byName/byId throw when missing: they use firstWhere; pass orElse or check Buses().buses first if the bus may not exist.
• Voice limit: bus voices count against the engine's active-voice cap (default 16, raise with SoLoud.instance.setMaxActiveVoiceCount). Hitting it is not an error — the new sound just doesn't play.
• annexSound needs a live handle: annexing a finished/invalid handle is a no-op on the native side; check sound.handles / isActive first.
• Filters need the bus handle for per-instance parameters: e.g. bus.filters.pitchShiftFilter.shift(soundHandle: bus.soundHandle).value = 2.5; after activate(). Plain .shift.value targets the global filter, not the bus voice.

More depth

• references/api.md — full signatures of all six bus play variants, the Buses registry, exceptions, and the AudioManager (music/SFX/voice + ducking) pattern.
• Runnable demo: example/lib/mixing_bus/mixing_bus.dart in this repo — creates buses, plays/annexes sounds, per-bus volume slider, pitch-shift filter toggle, live voice counts.
• Upstream concepts: https://solhsa.com/soloud/mixbus.html

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).