Replace the silence-emitting render callback from commit 1 with
real playback that drives AudioHandler::fill_buffer and downmixes
its 48 kHz stereo f32 output to the i16 mono buffer VPIO expects.
Pipeline per render callback (mirrors the cpal-output + sdl_output
contracts so the platform-neutral playback path is preserved):
1. Lock the shared Arc<Mutex<AudioHandler>>, ask fill_buffer to
populate a stereo-f32 scratch slice of length 2*num_frames.
AudioHandler runs Opus decode + per-client jitter buffer + mix
internally. Same primitive every other platform calls.
2. If output_muted is true, zero the i16 output buffer and return.
We still ran fill_buffer in step 1 so the jitter buffer drains
while muted — preventing unbounded growth — which matches the
cpal/SDL backend contract.
3. Downmix stereo -> mono with master gain:
mono_f32 = (l + r) * 0.5 * gain
i16_out = (mono_f32.clamp(-1.0, 1.0) * i16::MAX) as i16
The 0.5 average preserves total signal energy with 3 dB
headroom against sum-of-correlated-peaks clipping. Multiply by
gain after the downmix saves one mul per sample. Hard-clip on
the i16 cast is acceptable because the upstream stereo signal
is already in [-1.0, 1.0] from the f32 mix; only gain >1.0
creates clipping pressure and that path is identical to every
other backend's i16 conversion.
Closure ownership:
* scratch_stereo: Vec<f32> moved into the FnMut closure. First
callback grows it to 2*num_frames; subsequent callbacks reuse
the backing allocation. The audio thread never hits the
allocator on steady-state callbacks.
* handler_for_render / output_gain_for_render / output_muted_for_render
are Arc clones taken before the closure literal.
Public API change: AudioEngine -> IosVoiceUnit::start parameters
that were previously underscored (commit 1 placeholder) are now
all consumed by the wiring. Signature is unchanged, just the
binder names lose the leading underscore. engine.rs call-site
is unaffected.
Build verify on Mac (target aarch64-apple-ios): cargo check
clean in 0.33s, no errors, no warnings.
Build counter 63 -> 64 — About dialog shows v1.0.0-rc.8+64.
EdisonJwa
e7c3ffa6d2