SDL_coreaudio.m

/*
  Simple DirectMedia Layer
  Copyright (C) 1997-2024 Sam Lantinga <slouken@libsdl.org>

  This software is provided 'as-is', without any express or implied
  warranty.  In no event will the authors be held liable for any damages
  arising from the use of this software.

  Permission is granted to anyone to use this software for any purpose,
  including commercial applications, and to alter it and redistribute it
  freely, subject to the following restrictions:

  1. The origin of this software must not be misrepresented; you must not
     claim that you wrote the original software. If you use this software
     in a product, an acknowledgment in the product documentation would be
     appreciated but is not required.
  2. Altered source versions must be plainly marked as such, and must not be
     misrepresented as being the original software.
  3. This notice may not be removed or altered from any source distribution.
*/
#include "SDL_internal.h"

#ifdef SDL_AUDIO_DRIVER_COREAUDIO

#include "../SDL_sysaudio.h"
#include "SDL_coreaudio.h"
#include "../../thread/SDL_systhread.h"

#define DEBUG_COREAUDIO 0

#if DEBUG_COREAUDIO
    #define CHECK_RESULT(msg) \
        if (result != noErr) { \
            SDL_Log("COREAUDIO: Got error %d from '%s'!\n", (int)result, msg); \
            return SDL_SetError("CoreAudio error (%s): %d", msg, (int)result); \
        }
#else
    #define CHECK_RESULT(msg) \
        if (result != noErr) { \
            return SDL_SetError("CoreAudio error (%s): %d", msg, (int)result); \
        }
#endif

#ifdef MACOSX_COREAUDIO
// Apparently AudioDeviceID values might not be unique, so we wrap it in an SDL_malloc()'d pointer
//  to make it so. Use FindCoreAudioDeviceByHandle to deal with this redirection, if you need to
//  map from an AudioDeviceID to a SDL handle.
typedef struct SDLCoreAudioHandle
{
    AudioDeviceID devid;
    SDL_bool iscapture;
} SDLCoreAudioHandle;

static SDL_bool TestCoreAudioDeviceHandleCallback(SDL_AudioDevice *device, void *handle)
{
    const SDLCoreAudioHandle *a = (const SDLCoreAudioHandle *) device->handle;
    const SDLCoreAudioHandle *b = (const SDLCoreAudioHandle *) handle;
    return (a->devid == b->devid) && (!!a->iscapture == !!b->iscapture);
}

static SDL_AudioDevice *FindCoreAudioDeviceByHandle(const AudioDeviceID devid, const SDL_bool iscapture)
{
    SDLCoreAudioHandle handle = { devid, iscapture };
    return SDL_FindPhysicalAudioDeviceByCallback(TestCoreAudioDeviceHandleCallback, &handle);
}

static const AudioObjectPropertyAddress devlist_address = {
    kAudioHardwarePropertyDevices,
    kAudioObjectPropertyScopeGlobal,
    kAudioObjectPropertyElementMain
};

static const AudioObjectPropertyAddress default_output_device_address = {
    kAudioHardwarePropertyDefaultOutputDevice,
    kAudioObjectPropertyScopeGlobal,
    kAudioObjectPropertyElementMain
};

static const AudioObjectPropertyAddress default_input_device_address = {
    kAudioHardwarePropertyDefaultInputDevice,
    kAudioObjectPropertyScopeGlobal,
    kAudioObjectPropertyElementMain
};

static const AudioObjectPropertyAddress alive_address = {
    kAudioDevicePropertyDeviceIsAlive,
    kAudioObjectPropertyScopeGlobal,
    kAudioObjectPropertyElementMain
};


static OSStatus DeviceAliveNotification(AudioObjectID devid, UInt32 num_addr, const AudioObjectPropertyAddress *addrs, void *data)
{
    SDL_AudioDevice *device = (SDL_AudioDevice *)data;
    SDL_assert(((const SDLCoreAudioHandle *) device->handle)->devid == devid);

    UInt32 alive = 1;
    UInt32 size = sizeof(alive);
    const OSStatus error = AudioObjectGetPropertyData(devid, addrs, 0, NULL, &size, &alive);

    SDL_bool dead = SDL_FALSE;
    if (error == kAudioHardwareBadDeviceError) {
        dead = SDL_TRUE; // device was unplugged.
    } else if ((error == kAudioHardwareNoError) && (!alive)) {
        dead = SDL_TRUE; // device died in some other way.
    }

    if (dead) {
        #if DEBUG_COREAUDIO
        SDL_Log("COREAUDIO: device '%s' is lost!", device->name);
        #endif
        SDL_AudioDeviceDisconnected(device);
    }

    return noErr;
}

static void COREAUDIO_FreeDeviceHandle(SDL_AudioDevice *device)
{
    SDLCoreAudioHandle *handle = (SDLCoreAudioHandle *) device->handle;
    AudioObjectRemovePropertyListener(handle->devid, &alive_address, DeviceAliveNotification, device);
    SDL_free(handle);
}

// This only _adds_ new devices. Removal is handled by devices triggering kAudioDevicePropertyDeviceIsAlive property changes.
static void RefreshPhysicalDevices(void)
{
    UInt32 size = 0;
    AudioDeviceID *devs = NULL;
    SDL_bool isstack;

    if (AudioObjectGetPropertyDataSize(kAudioObjectSystemObject, &devlist_address, 0, NULL, &size) != kAudioHardwareNoError) {
        return;
    } else if ((devs = (AudioDeviceID *) SDL_small_alloc(Uint8, size, &isstack)) == NULL) {
        return;
    } else if (AudioObjectGetPropertyData(kAudioObjectSystemObject, &devlist_address, 0, NULL, &size, devs) != kAudioHardwareNoError) {
        SDL_small_free(devs, isstack);
        return;
    }

    const UInt32 total_devices = (UInt32) (size / sizeof(AudioDeviceID));
    for (UInt32 i = 0; i < total_devices; i++) {
        if (FindCoreAudioDeviceByHandle(devs[i], SDL_TRUE) || FindCoreAudioDeviceByHandle(devs[i], SDL_FALSE)) {
            devs[i] = 0;  // The system and SDL both agree it's already here, don't check it again.
        }
    }

    // any non-zero items remaining in `devs` are new devices to be added.
    for (int iscapture = 0; iscapture < 2; iscapture++) {
        const AudioObjectPropertyAddress addr = {
            kAudioDevicePropertyStreamConfiguration,
            iscapture ? kAudioDevicePropertyScopeInput : kAudioDevicePropertyScopeOutput,
            kAudioObjectPropertyElementMain
        };
        const AudioObjectPropertyAddress nameaddr = {
            kAudioObjectPropertyName,
            iscapture ? kAudioDevicePropertyScopeInput : kAudioDevicePropertyScopeOutput,
            kAudioObjectPropertyElementMain
        };
        const AudioObjectPropertyAddress freqaddr = {
            kAudioDevicePropertyNominalSampleRate,
            iscapture ? kAudioDevicePropertyScopeInput : kAudioDevicePropertyScopeOutput,
            kAudioObjectPropertyElementMain
        };

        for (UInt32 i = 0; i < total_devices; i++) {
            const AudioDeviceID dev = devs[i];
            if (!dev) {
                continue;  // already added.
            }

            AudioBufferList *buflist = NULL;
            double sampleRate = 0;

            if (AudioObjectGetPropertyDataSize(dev, &addr, 0, NULL, &size) != noErr) {
                continue;
            } else if ((buflist = (AudioBufferList *)SDL_malloc(size)) == NULL) {
                continue;
            }

            OSStatus result = AudioObjectGetPropertyData(dev, &addr, 0, NULL, &size, buflist);

            SDL_AudioSpec spec;
            SDL_zero(spec);
            if (result == noErr) {
                for (Uint32 j = 0; j < buflist->mNumberBuffers; j++) {
                    spec.channels += buflist->mBuffers[j].mNumberChannels;
                }
            }

            SDL_free(buflist);

            if (spec.channels == 0) {
                continue;
            }

            size = sizeof(sampleRate);
            if (AudioObjectGetPropertyData(dev, &freqaddr, 0, NULL, &size, &sampleRate) == noErr) {
                spec.freq = (int)sampleRate;
            }

            CFStringRef cfstr = NULL;
            size = sizeof(CFStringRef);
            if (AudioObjectGetPropertyData(dev, &nameaddr, 0, NULL, &size, &cfstr) != kAudioHardwareNoError) {
                continue;
            }

            CFIndex len = CFStringGetMaximumSizeForEncoding(CFStringGetLength(cfstr), kCFStringEncodingUTF8);
            char *name = (char *)SDL_malloc(len + 1);
            SDL_bool usable = ((name != NULL) && (CFStringGetCString(cfstr, name, len + 1, kCFStringEncodingUTF8)));

            CFRelease(cfstr);

            if (usable) {
                // Some devices have whitespace at the end...trim it.
                len = (CFIndex) SDL_strlen(name);
                while ((len > 0) && (name[len - 1] == ' ')) {
                    len--;
                }
                usable = (len > 0);
            }

            if (usable) {
                name[len] = '\0';

                #if DEBUG_COREAUDIO
                SDL_Log("COREAUDIO: Found %s device #%d: '%s' (devid %d)\n",
                       ((iscapture) ? "capture" : "output"),
                       (int)i, name, (int)dev);
                #endif
                SDLCoreAudioHandle *newhandle = (SDLCoreAudioHandle *) SDL_calloc(1, sizeof (*newhandle));
                if (newhandle) {
                    newhandle->devid = dev;
                    newhandle->iscapture = iscapture ? SDL_TRUE : SDL_FALSE;
                    SDL_AudioDevice *device = SDL_AddAudioDevice(newhandle->iscapture, name, &spec, newhandle);
                    if (device) {
                        AudioObjectAddPropertyListener(dev, &alive_address, DeviceAliveNotification, device);
                    } else {
                        SDL_free(newhandle);
                    }
                }
            }
            SDL_free(name); // SDL_AddAudioDevice() would have copied the string.
        }
    }

    SDL_small_free(devs, isstack);
}

// this is called when the system's list of available audio devices changes.
static OSStatus DeviceListChangedNotification(AudioObjectID systemObj, UInt32 num_addr, const AudioObjectPropertyAddress *addrs, void *data)
{
    RefreshPhysicalDevices();
    return noErr;
}

static OSStatus DefaultAudioDeviceChangedNotification(const SDL_bool iscapture, AudioObjectID inObjectID, const AudioObjectPropertyAddress *addr)
{
    AudioDeviceID devid;
    UInt32 size = sizeof(devid);
    if (AudioObjectGetPropertyData(inObjectID, addr, 0, NULL, &size, &devid) == noErr) {
        SDL_DefaultAudioDeviceChanged(FindCoreAudioDeviceByHandle(devid, iscapture));
    }
    return noErr;
}

static OSStatus DefaultOutputDeviceChangedNotification(AudioObjectID inObjectID, UInt32 inNumberAddresses, const AudioObjectPropertyAddress *inAddresses, void *inUserData)
{
    #if DEBUG_COREAUDIO
    SDL_Log("COREAUDIO: default output device changed!");
    #endif
    SDL_assert(inNumberAddresses == 1);
    return DefaultAudioDeviceChangedNotification(SDL_FALSE, inObjectID, inAddresses);
}

static OSStatus DefaultInputDeviceChangedNotification(AudioObjectID inObjectID, UInt32 inNumberAddresses, const AudioObjectPropertyAddress *inAddresses, void *inUserData)
{
    #if DEBUG_COREAUDIO
    SDL_Log("COREAUDIO: default input device changed!");
    #endif
    SDL_assert(inNumberAddresses == 1);
    return DefaultAudioDeviceChangedNotification(SDL_TRUE, inObjectID, inAddresses);
}

static void COREAUDIO_DetectDevices(SDL_AudioDevice **default_output, SDL_AudioDevice **default_capture)
{
    RefreshPhysicalDevices();

    AudioObjectAddPropertyListener(kAudioObjectSystemObject, &devlist_address, DeviceListChangedNotification, NULL);

    // Get the Device ID
    UInt32 size;
    AudioDeviceID devid;

    size = sizeof(AudioDeviceID);
    if (AudioObjectGetPropertyData(kAudioObjectSystemObject, &default_output_device_address, 0, NULL, &size, &devid) == noErr) {
        SDL_AudioDevice *device = FindCoreAudioDeviceByHandle(devid, SDL_FALSE);
        if (device) {
            *default_output = device;
        }
    }
    AudioObjectAddPropertyListener(kAudioObjectSystemObject, &default_output_device_address, DefaultOutputDeviceChangedNotification, NULL);

    size = sizeof(AudioDeviceID);
    if (AudioObjectGetPropertyData(kAudioObjectSystemObject, &default_input_device_address, 0, NULL, &size, &devid) == noErr) {
        SDL_AudioDevice *device = FindCoreAudioDeviceByHandle(devid, SDL_TRUE);
        if (device) {
            *default_capture = device;
        }
    }
    AudioObjectAddPropertyListener(kAudioObjectSystemObject, &default_input_device_address, DefaultInputDeviceChangedNotification, NULL);
}

#else  // iOS-specific section follows.

static SDL_bool session_active = SDL_FALSE;

static SDL_bool PauseOneAudioDevice(SDL_AudioDevice *device, void *userdata)
{
    if (device->hidden && device->hidden->audioQueue && !device->hidden->interrupted) {
        AudioQueuePause(device->hidden->audioQueue);
    }
    return SDL_FALSE;  // keep enumerating devices until we've paused them all.
}

static void PauseAudioDevices(void)
{
    (void) SDL_FindPhysicalAudioDeviceByCallback(PauseOneAudioDevice, NULL);
}

static SDL_bool ResumeOneAudioDevice(SDL_AudioDevice *device, void *userdata)
{
    if (device->hidden && device->hidden->audioQueue && !device->hidden->interrupted) {
        AudioQueueStart(device->hidden->audioQueue, NULL);
    }
    return SDL_FALSE;  // keep enumerating devices until we've resumed them all.
}

static void ResumeAudioDevices(void)
{
    (void) SDL_FindPhysicalAudioDeviceByCallback(ResumeOneAudioDevice, NULL);
}

static void InterruptionBegin(SDL_AudioDevice *device)
{
    if (device != NULL && device->hidden->audioQueue != NULL) {
        device->hidden->interrupted = SDL_TRUE;
        AudioQueuePause(device->hidden->audioQueue);
    }
}

static void InterruptionEnd(SDL_AudioDevice *device)
{
    if (device != NULL && device->hidden != NULL && device->hidden->audioQueue != NULL && device->hidden->interrupted && AudioQueueStart(device->hidden->audioQueue, NULL) == AVAudioSessionErrorCodeNone) {
        device->hidden->interrupted = SDL_FALSE;
    }
}

@interface SDLInterruptionListener : NSObject

@property(nonatomic, assign) SDL_AudioDevice *device;

@end

@implementation SDLInterruptionListener

- (void)audioSessionInterruption:(NSNotification *)note
{
    @synchronized(self) {
        NSNumber *type = note.userInfo[AVAudioSessionInterruptionTypeKey];
        if (type.unsignedIntegerValue == AVAudioSessionInterruptionTypeBegan) {
            InterruptionBegin(self.device);
        } else {
            InterruptionEnd(self.device);
        }
    }
}

- (void)applicationBecameActive:(NSNotification *)note
{
    @synchronized(self) {
        InterruptionEnd(self.device);
    }
}

@end

typedef struct
{
    int output;
    int capture;
} CountOpenAudioDevicesData;

static SDL_bool CountOpenAudioDevices(SDL_AudioDevice *device, void *userdata)
{
    CountOpenAudioDevicesData *data = (CountOpenAudioDevicesData *) userdata;
    if (device->hidden != NULL) {  // assume it's open if hidden != NULL
        if (device->iscapture) {
            data->capture++;
        } else {
            data->output++;
        }
    }
    return SDL_FALSE;  // keep enumerating until all devices have been checked.
}

static SDL_bool UpdateAudioSession(SDL_AudioDevice *device, SDL_bool open, SDL_bool allow_playandrecord)
{
    @autoreleasepool {
        AVAudioSession *session = [AVAudioSession sharedInstance];
        NSNotificationCenter *center = [NSNotificationCenter defaultCenter];

        NSString *category = AVAudioSessionCategoryPlayback;
        NSString *mode = AVAudioSessionModeDefault;
        NSUInteger options = AVAudioSessionCategoryOptionMixWithOthers;
        NSError *err = nil;
        const char *hint;

        CountOpenAudioDevicesData data;
        SDL_zero(data);
        (void) SDL_FindPhysicalAudioDeviceByCallback(CountOpenAudioDevices, &data);

        hint = SDL_GetHint(SDL_HINT_AUDIO_CATEGORY);
        if (hint) {
            if (SDL_strcasecmp(hint, "AVAudioSessionCategoryAmbient") == 0) {
                category = AVAudioSessionCategoryAmbient;
            } else if (SDL_strcasecmp(hint, "AVAudioSessionCategorySoloAmbient") == 0) {
                category = AVAudioSessionCategorySoloAmbient;
                options &= ~AVAudioSessionCategoryOptionMixWithOthers;
            } else if (SDL_strcasecmp(hint, "AVAudioSessionCategoryPlayback") == 0 ||
                       SDL_strcasecmp(hint, "playback") == 0) {
                category = AVAudioSessionCategoryPlayback;
                options &= ~AVAudioSessionCategoryOptionMixWithOthers;
            } else if (SDL_strcasecmp(hint, "AVAudioSessionCategoryPlayAndRecord") == 0 ||
                       SDL_strcasecmp(hint, "playandrecord") == 0) {
                if (allow_playandrecord) {
                    category = AVAudioSessionCategoryPlayAndRecord;
                }
            }
        } else if (data.output && data.capture) {
            category = AVAudioSessionCategoryPlayAndRecord;
        } else if (data.capture) {
            category = AVAudioSessionCategoryRecord;
        }

        #ifndef SDL_PLATFORM_TVOS
        if (category == AVAudioSessionCategoryPlayAndRecord) {
            options |= AVAudioSessionCategoryOptionDefaultToSpeaker;
        }
        #endif
        if (category == AVAudioSessionCategoryRecord ||
            category == AVAudioSessionCategoryPlayAndRecord) {
            /* AVAudioSessionCategoryOptionAllowBluetooth isn't available in the SDK for
               Apple TV but is still needed in order to output to Bluetooth devices.
             */
            options |= 0x4; // AVAudioSessionCategoryOptionAllowBluetooth;
        }
        if (category == AVAudioSessionCategoryPlayAndRecord) {
            options |= AVAudioSessionCategoryOptionAllowBluetoothA2DP |
                       AVAudioSessionCategoryOptionAllowAirPlay;
        }
        if (category == AVAudioSessionCategoryPlayback ||
            category == AVAudioSessionCategoryPlayAndRecord) {
            options |= AVAudioSessionCategoryOptionDuckOthers;
        }

        if ([session respondsToSelector:@selector(setCategory:mode:options:error:)]) {
            if (![session.category isEqualToString:category] || session.categoryOptions != options) {
                // Stop the current session so we don't interrupt other application audio
                PauseAudioDevices();
                [session setActive:NO error:nil];
                session_active = SDL_FALSE;

                if (![session setCategory:category mode:mode options:options error:&err]) {
                    NSString *desc = err.description;
                    SDL_SetError("Could not set Audio Session category: %s", desc.UTF8String);
                    return SDL_FALSE;
                }
            }
        } else {
            if (![session.category isEqualToString:category]) {
                // Stop the current session so we don't interrupt other application audio
                PauseAudioDevices();
                [session setActive:NO error:nil];
                session_active = SDL_FALSE;

                if (![session setCategory:category error:&err]) {
                    NSString *desc = err.description;
                    SDL_SetError("Could not set Audio Session category: %s", desc.UTF8String);
                    return SDL_FALSE;
                }
            }
        }

        if ((data.output || data.capture) && !session_active) {
            if (![session setActive:YES error:&err]) {
                if ([err code] == AVAudioSessionErrorCodeResourceNotAvailable &&
                    category == AVAudioSessionCategoryPlayAndRecord) {
                    return UpdateAudioSession(device, open, SDL_FALSE);
                }

                NSString *desc = err.description;
                SDL_SetError("Could not activate Audio Session: %s", desc.UTF8String);
                return SDL_FALSE;
            }
            session_active = SDL_TRUE;
            ResumeAudioDevices();
        } else if (!data.output && !data.capture && session_active) {
            PauseAudioDevices();
            [session setActive:NO error:nil];
            session_active = SDL_FALSE;
        }

        if (open) {
            SDLInterruptionListener *listener = [SDLInterruptionListener new];
            listener.device = device;

            [center addObserver:listener
                       selector:@selector(audioSessionInterruption:)
                           name:AVAudioSessionInterruptionNotification
                         object:session];

            /* An interruption end notification is not guaranteed to be sent if
             we were previously interrupted... resuming if needed when the app
             becomes active seems to be the way to go. */
            // Note: object: below needs to be nil, as otherwise it filters by the object, and session doesn't send foreground / active notifications.
            [center addObserver:listener
                       selector:@selector(applicationBecameActive:)
                           name:UIApplicationDidBecomeActiveNotification
                         object:nil];

            [center addObserver:listener
                       selector:@selector(applicationBecameActive:)
                           name:UIApplicationWillEnterForegroundNotification
                         object:nil];

            device->hidden->interruption_listener = CFBridgingRetain(listener);
        } else {
            SDLInterruptionListener *listener = nil;
            listener = (SDLInterruptionListener *)CFBridgingRelease(device->hidden->interruption_listener);
            [center removeObserver:listener];
            @synchronized(listener) {
                listener.device = NULL;
            }
        }
    }

    return SDL_TRUE;
}
#endif


static int COREAUDIO_PlayDevice(SDL_AudioDevice *device, const Uint8 *buffer, int buffer_size)
{
    AudioQueueBufferRef current_buffer = device->hidden->current_buffer;
    SDL_assert(current_buffer != NULL);  // should have been called from OutputBufferReadyCallback
    SDL_assert(buffer == (Uint8 *) current_buffer->mAudioData);
    current_buffer->mAudioDataByteSize = current_buffer->mAudioDataBytesCapacity;
    device->hidden->current_buffer = NULL;
    AudioQueueEnqueueBuffer(device->hidden->audioQueue, current_buffer, 0, NULL);
    return 0;
}

static Uint8 *COREAUDIO_GetDeviceBuf(SDL_AudioDevice *device, int *buffer_size)
{
    AudioQueueBufferRef current_buffer = device->hidden->current_buffer;
    SDL_assert(current_buffer != NULL);  // should have been called from OutputBufferReadyCallback
    SDL_assert(current_buffer->mAudioData != NULL);
    *buffer_size = (int) current_buffer->mAudioDataBytesCapacity;
    return (Uint8 *) current_buffer->mAudioData;
}

static void OutputBufferReadyCallback(void *inUserData, AudioQueueRef inAQ, AudioQueueBufferRef inBuffer)
{
    SDL_AudioDevice *device = (SDL_AudioDevice *)inUserData;
    SDL_assert(inBuffer != NULL);  // ...right?
    SDL_assert(device->hidden->current_buffer == NULL);  // shouldn't have anything pending
    device->hidden->current_buffer = inBuffer;
    const SDL_bool okay = SDL_OutputAudioThreadIterate(device);
    SDL_assert((device->hidden->current_buffer == NULL) || !okay);  // PlayDevice should have enqueued and cleaned it out, unless we failed or shutdown.

    // buffer is unexpectedly here? We're probably dying, but try to requeue this buffer with silence.
    if (device->hidden->current_buffer) {
        AudioQueueBufferRef current_buffer = device->hidden->current_buffer;
        device->hidden->current_buffer = NULL;
        SDL_memset(current_buffer->mAudioData, device->silence_value, (size_t) current_buffer->mAudioDataBytesCapacity);
        AudioQueueEnqueueBuffer(device->hidden->audioQueue, current_buffer, 0, NULL);
    }
}

static int COREAUDIO_CaptureFromDevice(SDL_AudioDevice *device, void *buffer, int buflen)
{
    AudioQueueBufferRef current_buffer = device->hidden->current_buffer;
    SDL_assert(current_buffer != NULL);  // should have been called from InputBufferReadyCallback
    SDL_assert(current_buffer->mAudioData != NULL);
    SDL_assert(buflen >= (int) current_buffer->mAudioDataByteSize);  // `cpy` makes sure this won't overflow a buffer, but we _will_ drop samples if this assertion fails!
    const int cpy = SDL_min(buflen, (int) current_buffer->mAudioDataByteSize);
    SDL_memcpy(buffer, current_buffer->mAudioData, cpy);
    device->hidden->current_buffer = NULL;
    AudioQueueEnqueueBuffer(device->hidden->audioQueue, current_buffer, 0, NULL);  // requeue for capturing more data later.
    return cpy;
}

static void COREAUDIO_FlushCapture(SDL_AudioDevice *device)
{
    AudioQueueBufferRef current_buffer = device->hidden->current_buffer;
    if (current_buffer != NULL) {  // also gets called at shutdown, when no buffer is available.
        // just requeue the current buffer without reading from it, so it can be refilled with new data later.
        device->hidden->current_buffer = NULL;
        AudioQueueEnqueueBuffer(device->hidden->audioQueue, current_buffer, 0, NULL);
    }
}

static void InputBufferReadyCallback(void *inUserData, AudioQueueRef inAQ, AudioQueueBufferRef inBuffer,
                          const AudioTimeStamp *inStartTime, UInt32 inNumberPacketDescriptions,
                          const AudioStreamPacketDescription *inPacketDescs)
{
    SDL_AudioDevice *device = (SDL_AudioDevice *)inUserData;
    SDL_assert(inAQ == device->hidden->audioQueue);
    SDL_assert(inBuffer != NULL);  // ...right?
    SDL_assert(device->hidden->current_buffer == NULL);  // shouldn't have anything pending
    device->hidden->current_buffer = inBuffer;
    SDL_CaptureAudioThreadIterate(device);

    // buffer is unexpectedly here? We're probably dying, but try to requeue this buffer anyhow.
    if (device->hidden->current_buffer != NULL) {
        SDL_assert(SDL_AtomicGet(&device->shutdown) != 0);
        COREAUDIO_FlushCapture(device);  // just flush it manually, which will requeue it.
    }
}

static void COREAUDIO_CloseDevice(SDL_AudioDevice *device)
{
    if (!device->hidden) {
        return;
    }

    // dispose of the audio queue before waiting on the thread, or it might stall for a long time!
    if (device->hidden->audioQueue) {
        AudioQueueFlush(device->hidden->audioQueue);
        AudioQueueStop(device->hidden->audioQueue, 0);
        AudioQueueDispose(device->hidden->audioQueue, 0);
    }

    if (device->hidden->thread) {
        SDL_assert(SDL_AtomicGet(&device->shutdown) != 0); // should have been set by SDL_audio.c
        SDL_WaitThread(device->hidden->thread, NULL);
    }

    #ifndef MACOSX_COREAUDIO
    UpdateAudioSession(device, SDL_FALSE, SDL_TRUE);
    #endif

    if (device->hidden->ready_semaphore) {
        SDL_DestroySemaphore(device->hidden->ready_semaphore);
    }

    // AudioQueueDispose() frees the actual buffer objects.
    SDL_free(device->hidden->audioBuffer);
    SDL_free(device->hidden->thread_error);
    SDL_free(device->hidden);
}

#ifdef MACOSX_COREAUDIO
static int PrepareDevice(SDL_AudioDevice *device)
{
    SDL_assert(device->handle != NULL);  // this meant "system default" in SDL2, but doesn't anymore

    const SDLCoreAudioHandle *handle = (const SDLCoreAudioHandle *) device->handle;
    const AudioDeviceID devid = handle->devid;
    OSStatus result = noErr;
    UInt32 size = 0;

    AudioObjectPropertyAddress addr = {
        0,
        kAudioObjectPropertyScopeGlobal,
        kAudioObjectPropertyElementMain
    };

    UInt32 alive = 0;
    size = sizeof(alive);
    addr.mSelector = kAudioDevicePropertyDeviceIsAlive;
    addr.mScope = device->iscapture ? kAudioDevicePropertyScopeInput : kAudioDevicePropertyScopeOutput;
    result = AudioObjectGetPropertyData(devid, &addr, 0, NULL, &size, &alive);
    CHECK_RESULT("AudioDeviceGetProperty (kAudioDevicePropertyDeviceIsAlive)");
    if (!alive) {
        return SDL_SetError("CoreAudio: requested device exists, but isn't alive.");
    }

    // some devices don't support this property, so errors are fine here.
    pid_t pid = 0;
    size = sizeof(pid);
    addr.mSelector = kAudioDevicePropertyHogMode;
    result = AudioObjectGetPropertyData(devid, &addr, 0, NULL, &size, &pid);
    if ((result == noErr) && (pid != -1)) {
        return SDL_SetError("CoreAudio: requested device is being hogged.");
    }

    device->hidden->deviceID = devid;

    return 0;
}

static int AssignDeviceToAudioQueue(SDL_AudioDevice *device)
{
    const AudioObjectPropertyAddress prop = {
        kAudioDevicePropertyDeviceUID,
        device->iscapture ? kAudioDevicePropertyScopeInput : kAudioDevicePropertyScopeOutput,
        kAudioObjectPropertyElementMain
    };

    OSStatus result;
    CFStringRef devuid;
    UInt32 devuidsize = sizeof(devuid);
    result = AudioObjectGetPropertyData(device->hidden->deviceID, &prop, 0, NULL, &devuidsize, &devuid);
    CHECK_RESULT("AudioObjectGetPropertyData (kAudioDevicePropertyDeviceUID)");
    result = AudioQueueSetProperty(device->hidden->audioQueue, kAudioQueueProperty_CurrentDevice, &devuid, devuidsize);
    CHECK_RESULT("AudioQueueSetProperty (kAudioQueueProperty_CurrentDevice)");

    // !!! FIXME: do we need to CFRelease(devuid)?

    return 0;
}
#endif

static int PrepareAudioQueue(SDL_AudioDevice *device)
{
    const AudioStreamBasicDescription *strdesc = &device->hidden->strdesc;
    const SDL_bool iscapture = device->iscapture;
    OSStatus result;

    SDL_assert(CFRunLoopGetCurrent() != NULL);

    if (iscapture) {
        result = AudioQueueNewInput(strdesc, InputBufferReadyCallback, device, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode, 0, &device->hidden->audioQueue);
        CHECK_RESULT("AudioQueueNewInput");
    } else {
        result = AudioQueueNewOutput(strdesc, OutputBufferReadyCallback, device, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode, 0, &device->hidden->audioQueue);
        CHECK_RESULT("AudioQueueNewOutput");
    }

    #ifdef MACOSX_COREAUDIO
    if (AssignDeviceToAudioQueue(device) < 0) {
        return -1;
    }
    #endif

    SDL_UpdatedAudioDeviceFormat(device);  // make sure this is correct.

    // Set the channel layout for the audio queue
    AudioChannelLayout layout;
    SDL_zero(layout);
    switch (device->spec.channels) {
    case 1:
        layout.mChannelLayoutTag = kAudioChannelLayoutTag_Mono;
        break;
    case 2:
        layout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;
        break;
    case 3:
        layout.mChannelLayoutTag = kAudioChannelLayoutTag_DVD_4;
        break;
    case 4:
        layout.mChannelLayoutTag = kAudioChannelLayoutTag_Quadraphonic;
        break;
    case 5:
        layout.mChannelLayoutTag = kAudioChannelLayoutTag_MPEG_5_0_A;
        break;
    case 6:
        layout.mChannelLayoutTag = kAudioChannelLayoutTag_MPEG_5_1_A;
        break;
    case 7:
        // FIXME: Need to move channel[4] (BC) to channel[6]
        layout.mChannelLayoutTag = kAudioChannelLayoutTag_MPEG_6_1_A;
        break;
    case 8:
        layout.mChannelLayoutTag = kAudioChannelLayoutTag_MPEG_7_1_A;
        break;
    }
    if (layout.mChannelLayoutTag != 0) {
        result = AudioQueueSetProperty(device->hidden->audioQueue, kAudioQueueProperty_ChannelLayout, &layout, sizeof(layout));
        CHECK_RESULT("AudioQueueSetProperty(kAudioQueueProperty_ChannelLayout)");
    }

    // Make sure we can feed the device a minimum amount of time
    double MINIMUM_AUDIO_BUFFER_TIME_MS = 15.0;
    #ifdef SDL_PLATFORM_IOS
    if (SDL_floor(NSFoundationVersionNumber) <= NSFoundationVersionNumber_iOS_7_1) {
        // Older iOS hardware, use 40 ms as a minimum time
        MINIMUM_AUDIO_BUFFER_TIME_MS = 40.0;
    }
    #endif

    int numAudioBuffers = 2;
    const double msecs = (device->sample_frames / ((double)device->spec.freq)) * 1000.0;
    if (msecs < MINIMUM_AUDIO_BUFFER_TIME_MS) { // use more buffers if we have a VERY small sample set.
        numAudioBuffers = ((int)SDL_ceil(MINIMUM_AUDIO_BUFFER_TIME_MS / msecs) * 2);
    }

    device->hidden->numAudioBuffers = numAudioBuffers;
    device->hidden->audioBuffer = SDL_calloc(numAudioBuffers, sizeof(AudioQueueBufferRef));
    if (device->hidden->audioBuffer == NULL) {
        return -1;
    }

    #if DEBUG_COREAUDIO
    SDL_Log("COREAUDIO: numAudioBuffers == %d\n", numAudioBuffers);
    #endif

    for (int i = 0; i < numAudioBuffers; i++) {
        result = AudioQueueAllocateBuffer(device->hidden->audioQueue, device->buffer_size, &device->hidden->audioBuffer[i]);
        CHECK_RESULT("AudioQueueAllocateBuffer");
        SDL_memset(device->hidden->audioBuffer[i]->mAudioData, device->silence_value, device->hidden->audioBuffer[i]->mAudioDataBytesCapacity);
        device->hidden->audioBuffer[i]->mAudioDataByteSize = device->hidden->audioBuffer[i]->mAudioDataBytesCapacity;
        // !!! FIXME: should we use AudioQueueEnqueueBufferWithParameters and specify all frames be "trimmed" so these are immediately ready to refill with SDL callback data?
        result = AudioQueueEnqueueBuffer(device->hidden->audioQueue, device->hidden->audioBuffer[i], 0, NULL);
        CHECK_RESULT("AudioQueueEnqueueBuffer");
    }

    result = AudioQueueStart(device->hidden->audioQueue, NULL);
    CHECK_RESULT("AudioQueueStart");

    return 0;  // We're running!
}

static int AudioQueueThreadEntry(void *arg)
{
    SDL_AudioDevice *device = (SDL_AudioDevice *)arg;

    if (device->iscapture) {
        SDL_CaptureAudioThreadSetup(device);
    } else {
        SDL_OutputAudioThreadSetup(device);
    }

    if (PrepareAudioQueue(device) < 0) {
        device->hidden->thread_error = SDL_strdup(SDL_GetError());
        SDL_PostSemaphore(device->hidden->ready_semaphore);
        return 0;
    }

    // init was successful, alert parent thread and start running...
    SDL_PostSemaphore(device->hidden->ready_semaphore);

    // This would be WaitDevice/WaitCaptureDevice in the normal SDL audio thread, but we get *BufferReadyCallback calls here to know when to iterate.
    while (!SDL_AtomicGet(&device->shutdown)) {
        CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0.10, 1);
    }

    if (device->iscapture) {
        SDL_CaptureAudioThreadShutdown(device);
    } else {
        // Drain off any pending playback.
        const CFTimeInterval secs = (((CFTimeInterval)device->sample_frames) / ((CFTimeInterval)device->spec.freq)) * 2.0;
        CFRunLoopRunInMode(kCFRunLoopDefaultMode, secs, 0);
        SDL_OutputAudioThreadShutdown(device);
    }

    return 0;
}

static int COREAUDIO_OpenDevice(SDL_AudioDevice *device)
{
    // Initialize all variables that we clean on shutdown
    device->hidden = (struct SDL_PrivateAudioData *)SDL_calloc(1, sizeof(*device->hidden));
    if (device->hidden == NULL) {
        return -1;
    }

    #ifndef MACOSX_COREAUDIO
    if (!UpdateAudioSession(device, SDL_TRUE, SDL_TRUE)) {
        return -1;
    }

    // Stop CoreAudio from doing expensive audio rate conversion
    @autoreleasepool {
        AVAudioSession *session = [AVAudioSession sharedInstance];
        [session setPreferredSampleRate:device->spec.freq error:nil];
        device->spec.freq = (int)session.sampleRate;
        #ifdef SDL_PLATFORM_TVOS
        if (device->iscapture) {
            [session setPreferredInputNumberOfChannels:device->spec.channels error:nil];
            device->spec.channels = (int)session.preferredInputNumberOfChannels;
        } else {
            [session setPreferredOutputNumberOfChannels:device->spec.channels error:nil];
            device->spec.channels = (int)session.preferredOutputNumberOfChannels;
        }
        #else
        // Calling setPreferredOutputNumberOfChannels seems to break audio output on iOS
        #endif /* SDL_PLATFORM_TVOS */
    }
    #endif

    // Setup a AudioStreamBasicDescription with the requested format
    AudioStreamBasicDescription *strdesc = &device->hidden->strdesc;
    strdesc->mFormatID = kAudioFormatLinearPCM;
    strdesc->mFormatFlags = kLinearPCMFormatFlagIsPacked;
    strdesc->mChannelsPerFrame = device->spec.channels;
    strdesc->mSampleRate = device->spec.freq;
    strdesc->mFramesPerPacket = 1;

    const SDL_AudioFormat *closefmts = SDL_ClosestAudioFormats(device->spec.format);
    SDL_AudioFormat test_format;
    while ((test_format = *(closefmts++)) != 0) {
        // CoreAudio handles most of SDL's formats natively.
        switch (test_format) {
        case SDL_AUDIO_U8:
        case SDL_AUDIO_S8:
        case SDL_AUDIO_S16LE:
        case SDL_AUDIO_S16BE:
        case SDL_AUDIO_S32LE:
        case SDL_AUDIO_S32BE:
        case SDL_AUDIO_F32LE:
        case SDL_AUDIO_F32BE:
            break;

        default:
            continue;
        }
        break;
    }

    if (!test_format) { // shouldn't happen, but just in case...
        return SDL_SetError("%s: Unsupported audio format", "coreaudio");
    }
    device->spec.format = test_format;
    strdesc->mBitsPerChannel = SDL_AUDIO_BITSIZE(test_format);
    if (SDL_AUDIO_ISBIGENDIAN(test_format)) {
        strdesc->mFormatFlags |= kLinearPCMFormatFlagIsBigEndian;
    }

    if (SDL_AUDIO_ISFLOAT(test_format)) {
        strdesc->mFormatFlags |= kLinearPCMFormatFlagIsFloat;
    } else if (SDL_AUDIO_ISSIGNED(test_format)) {
        strdesc->mFormatFlags |= kLinearPCMFormatFlagIsSignedInteger;
    }

    strdesc->mBytesPerFrame = strdesc->mChannelsPerFrame * strdesc->mBitsPerChannel / 8;
    strdesc->mBytesPerPacket = strdesc->mBytesPerFrame * strdesc->mFramesPerPacket;

#ifdef MACOSX_COREAUDIO
    if (PrepareDevice(device) < 0) {
        return -1;
    }
#endif

    // This has to init in a new thread so it can get its own CFRunLoop. :/
    device->hidden->ready_semaphore = SDL_CreateSemaphore(0);
    if (!device->hidden->ready_semaphore) {
        return -1; // oh well.
    }

    char threadname[64];
    SDL_GetAudioThreadName(device, threadname, sizeof(threadname));
    device->hidden->thread = SDL_CreateThread(AudioQueueThreadEntry, threadname, device);
    if (!device->hidden->thread) {
        return -1;
    }

    SDL_WaitSemaphore(device->hidden->ready_semaphore);
    SDL_DestroySemaphore(device->hidden->ready_semaphore);
    device->hidden->ready_semaphore = NULL;

    if ((device->hidden->thread != NULL) && (device->hidden->thread_error != NULL)) {
        SDL_WaitThread(device->hidden->thread, NULL);
        device->hidden->thread = NULL;
        return SDL_SetError("%s", device->hidden->thread_error);
    }

    return (device->hidden->thread != NULL) ? 0 : -1;
}

static void COREAUDIO_DeinitializeStart(void)
{
#ifdef MACOSX_COREAUDIO
    AudioObjectRemovePropertyListener(kAudioObjectSystemObject, &devlist_address, DeviceListChangedNotification, NULL);
    AudioObjectRemovePropertyListener(kAudioObjectSystemObject, &default_output_device_address, DefaultOutputDeviceChangedNotification, NULL);
    AudioObjectRemovePropertyListener(kAudioObjectSystemObject, &default_input_device_address, DefaultInputDeviceChangedNotification, NULL);
#endif
}

static SDL_bool COREAUDIO_Init(SDL_AudioDriverImpl *impl)
{
    impl->OpenDevice = COREAUDIO_OpenDevice;
    impl->PlayDevice = COREAUDIO_PlayDevice;
    impl->GetDeviceBuf = COREAUDIO_GetDeviceBuf;
    impl->CaptureFromDevice = COREAUDIO_CaptureFromDevice;
    impl->FlushCapture = COREAUDIO_FlushCapture;
    impl->CloseDevice = COREAUDIO_CloseDevice;
    impl->DeinitializeStart = COREAUDIO_DeinitializeStart;

#ifdef MACOSX_COREAUDIO
    impl->DetectDevices = COREAUDIO_DetectDevices;
    impl->FreeDeviceHandle = COREAUDIO_FreeDeviceHandle;
#else
    impl->OnlyHasDefaultOutputDevice = SDL_TRUE;
    impl->OnlyHasDefaultCaptureDevice = SDL_TRUE;
#endif

    impl->ProvidesOwnCallbackThread = SDL_TRUE;
    impl->HasCaptureSupport = SDL_TRUE;

    return SDL_TRUE;
}

AudioBootStrap COREAUDIO_bootstrap = {
    "coreaudio", "CoreAudio", COREAUDIO_Init, SDL_FALSE
};

#endif // SDL_AUDIO_DRIVER_COREAUDIO