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avfoundation/coreaudio: `AudioDeviceID` doesn't exist on iOS

10. avfoundation/coreaudio: AudioDeviceID doesn't exist on iOS​

What happened: enabling mpv's avfoundation audio output (which meson had auto-enabled once it detected the relevant Apple frameworks were present) failed with several undeclared-type errors centered on AudioDeviceID/AudioStreamID.

Investigation, done by reading mpv's actual current source (uploaded directly for this purpose, not relying on search snippets or the error message alone):

  • audio/out/ao_avfoundation.m already has three separate #if TARGET_OS_IPHONE blocks setting up an AVAudioSession — clear, deliberate upstream iOS support. But one call was left unguarded: [p->renderer setAudioOutputDeviceUniqueID:...], which Apple's own headers mark API_UNAVAILABLE(ios, ...). This looked like a genuine oversight in upstream mpv (a missing guard, not an intentional exclusion), since the surrounding code clearly already handles iOS.
  • The compile errors, though, actually originated in shared utility files (ao_coreaudio_utils.c/.h, ao_coreaudio_chmap.c/.h) under a combined #if HAVE_COREAUDIO || HAVE_AVFOUNDATION guard. We verified, function by function, which of the guarded declarations actually take an AudioDeviceID/AudioStreamID (real CoreAudio HAL types with no iOS equivalent) versus which are device-independent (AudioChannelLayout-based channel-map helpers that ao_avfoundation.m genuinely calls). The guard had simply never been split to distinguish these — everything sharing one condition meant enabling avfoundation dragged in HAL-only code it never uses.

Fix: a 6-patch series (buildscripts/patches/mpv/0001 through 0006), applied automatically by buildscripts/include/apply-mpv-patches.sh (called from download.sh right after mpv is cloned):

  1. Guard the one unguarded setAudioOutputDeviceUniqueID: call behind #if !TARGET_OS_IPHONE. 2–5. Narrow the shared-utility guards so only the genuinely AudioDeviceID-dependent declarations/definitions require HAVE_COREAUDIO, leaving the device-independent ones (ca_get_acl, ca_find_standard_layout, ca_log_layout) available under the original HAVE_COREAUDIO || HAVE_AVFOUNDATION condition.
  2. A follow-up fix (see next entry) for a guard we initially missed.

Each patch was test-applied against a completely fresh mpv checkout (not assumed to apply cleanly) and checked with a small Python script that verifies #if/#endif balance across every file touched, since an unbalanced patch can look fine in a diff while silently breaking compilation in a confusing way.

Result: mpv.sh now enables -Davfoundation=enabled (previously force-disabled entirely as the first, simpler fix), giving iOS builds mpv's more modern AVSampleBufferAudioRenderer-based audio output alongside audiounit, including capabilities like spatial audio support that audiounit alone doesn't provide. coreaudio itself remains disabled — its full HAL device enumeration/selection genuinely has no iOS equivalent, unlike avfoundation's narrower, already-mostly-iOS-compatible surface.

Lesson: a compile error pointing at "undeclared type" doesn't always mean the surrounding feature is impossible on the target platform — it can mean a guard condition was written too broadly, bundling device-independent and device-dependent code together. Worth checking which code a failing feature actually calls before concluding the whole feature is unsupportable.