coreaudio: The default SDL audio device now tracks the system output default.

So if you go into System Preferences on a MacBook and toggle between a pair of
connected bluetooth headphones and built-in internal speakers, SDL will
switch the device it is playing sound through, to match this setting, on the
fly.

Likewise if the default output device is a USB thing and is unplugged; as the
default device changes at the system level, SDL will pick this up and carry
on with the new default. This is different from our unplug detection for
specific devices, as in those cases we want to send the app a disconnect
notification, instead of migrating transparently as we now do for default
devices.

Note that this should also work for capture devices; if the device changes,
SDL will start recording from the new default.

Fixes Bugzilla #4851.

src/audio/coreaudio/SDL_coreaudio.h

@@ -47,6 +47,7 @@ struct SDL_PrivateAudioData
 {
     SDL_Thread *thread;
     AudioQueueRef audioQueue;
+    int numAudioBuffers;
     AudioQueueBufferRef *audioBuffer;
     void *buffer;
     UInt32 bufferOffset;
@@ -57,6 +58,7 @@ struct SDL_PrivateAudioData
     SDL_atomic_t shutdown;
 #if MACOSX_COREAUDIO
     AudioDeviceID deviceID;
+    SDL_atomic_t device_change_flag;
 #else
     SDL_bool interrupted;
     CFTypeRef interruption_listener;

src/audio/coreaudio/SDL_coreaudio.m

@@ -34,11 +34,21 @@
 
 #define DEBUG_COREAUDIO 0
 
-#define CHECK_RESULT(msg) \
-    if (result != noErr) { \
-        SDL_SetError("CoreAudio error (%s): %d", msg, (int) result); \
-        return 0; \
-    }
+#if DEBUG_COREAUDIO
+    #define CHECK_RESULT(msg) \
+        if (result != noErr) { \
+            printf("COREAUDIO: Got error %d from '%s'!\n", (int) result, msg); \
+            SDL_SetError("CoreAudio error (%s): %d", msg, (int) result); \
+            return 0; \
+        }
+#else
+    #define CHECK_RESULT(msg) \
+        if (result != noErr) { \
+            SDL_SetError("CoreAudio error (%s): %d", msg, (int) result); \
+            return 0; \
+        }
+#endif
+
 
 #if MACOSX_COREAUDIO
 static const AudioObjectPropertyAddress devlist_address = {
@@ -441,7 +451,7 @@ outputCallback(void *inUserData, AudioQueueRef inAQ, AudioQueueBufferRef inBuffe
     if (!SDL_AtomicGet(&this->enabled) || SDL_AtomicGet(&this->paused)) {
         /* Supply silence if audio is not enabled or paused */
         SDL_memset(inBuffer->mAudioData, this->spec.silence, inBuffer->mAudioDataBytesCapacity);
-    } else if (this->stream ) {
+    } else if (this->stream) {
         UInt32 remaining = inBuffer->mAudioDataBytesCapacity;
         Uint8 *ptr = (Uint8 *) inBuffer->mAudioData;
 
@@ -576,6 +586,18 @@ device_unplugged(AudioObjectID devid, UInt32 num_addr, const AudioObjectProperty
 
     return 0;
 }
+
+/* macOS calls this when the default device changed (if we have a default device open). */
+static OSStatus
+default_device_changed(AudioObjectID inObjectID, UInt32 inNumberAddresses, const AudioObjectPropertyAddress *inAddresses, void *inUserData)
+{
+    SDL_AudioDevice *this = (SDL_AudioDevice *) inUserData;
+    #if DEBUG_COREAUDIO
+    printf("COREAUDIO: default device changed for SDL audio device %p!\n", this);
+    #endif
+    SDL_AtomicSet(&this->hidden->device_change_flag, 1);  /* let the audioqueue thread pick up on this when safe to do so. */
+    return noErr;
+}
 #endif
 
 static void
@@ -586,8 +608,9 @@ COREAUDIO_CloseDevice(_THIS)
 /* !!! FIXME: what does iOS do when a bluetooth audio device vanishes? Headphones unplugged? */
 /* !!! FIXME: (we only do a "default" device on iOS right now...can we do more?) */
 #if MACOSX_COREAUDIO
-    /* Fire a callback if the device stops being "alive" (disconnected, etc). */
-    AudioObjectRemovePropertyListener(this->hidden->deviceID, &alive_address, device_unplugged, this);
+    if (this->handle != NULL) {  /* we don't register this listener for default devices. */
+        AudioObjectRemovePropertyListener(this->hidden->deviceID, &alive_address, device_unplugged, this);
+    }
 #endif
 
     if (iscapture) {
@@ -676,6 +699,26 @@ prepare_device(_THIS, void *handle, int iscapture)
     this->hidden->deviceID = devid;
     return 1;
 }
+
+static int
+assign_device_to_audioqueue(_THIS)
+{
+    const AudioObjectPropertyAddress prop = {
+        kAudioDevicePropertyDeviceUID,
+        this->iscapture ? kAudioDevicePropertyScopeInput : kAudioDevicePropertyScopeOutput,
+        kAudioObjectPropertyElementMaster
+    };
+
+    OSStatus result;
+    CFStringRef devuid;
+    UInt32 devuidsize = sizeof (devuid);
+    result = AudioObjectGetPropertyData(this->hidden->deviceID, &prop, 0, NULL, &devuidsize, &devuid);
+    CHECK_RESULT("AudioObjectGetPropertyData (kAudioDevicePropertyDeviceUID)");
+    result = AudioQueueSetProperty(this->hidden->audioQueue, kAudioQueueProperty_CurrentDevice, &devuid, devuidsize);
+    CHECK_RESULT("AudioQueueSetProperty (kAudioQueueProperty_CurrentDevice)");
+
+    return 1;
+}
 #endif
 
 static int
@@ -696,26 +739,21 @@ prepare_audioqueue(_THIS)
         CHECK_RESULT("AudioQueueNewOutput");
     }
 
-#if MACOSX_COREAUDIO
-{
-    const AudioObjectPropertyAddress prop = {
-        kAudioDevicePropertyDeviceUID,
-        iscapture ? kAudioDevicePropertyScopeInput : kAudioDevicePropertyScopeOutput,
-        kAudioObjectPropertyElementMaster
-    };
-    CFStringRef devuid;
-    UInt32 devuidsize = sizeof (devuid);
-    result = AudioObjectGetPropertyData(this->hidden->deviceID, &prop, 0, NULL, &devuidsize, &devuid);
-    CHECK_RESULT("AudioObjectGetPropertyData (kAudioDevicePropertyDeviceUID)");
-    result = AudioQueueSetProperty(this->hidden->audioQueue, kAudioQueueProperty_CurrentDevice, &devuid, devuidsize);
-    CHECK_RESULT("AudioQueueSetProperty (kAudioQueueProperty_CurrentDevice)");
+    #if MACOSX_COREAUDIO
+    if (!assign_device_to_audioqueue(this)) {
+        return 0;
+    }
 
-    /* !!! FIXME: what does iOS do when a bluetooth audio device vanishes? Headphones unplugged? */
-    /* !!! FIXME: (we only do a "default" device on iOS right now...can we do more?) */
-    /* Fire a callback if the device stops being "alive" (disconnected, etc). */
-    AudioObjectAddPropertyListener(this->hidden->deviceID, &alive_address, device_unplugged, this);
-}
-#endif
+    /* only listen for unplugging on specific devices, not the default device, as that should
+       switch to a different device (or hang out silently if there _is_ no other device). */
+    if (this->handle != NULL) {
+        /* !!! FIXME: what does iOS do when a bluetooth audio device vanishes? Headphones unplugged? */
+        /* !!! FIXME: (we only do a "default" device on iOS right now...can we do more?) */
+        /* Fire a callback if the device stops being "alive" (disconnected, etc). */
+        /* If this fails, oh well, we won't notice a device had an extraordinary event take place. */
+        AudioObjectAddPropertyListener(this->hidden->deviceID, &alive_address, device_unplugged, this);
+    }
+    #endif
 
     /* Calculate the final parameters for this audio specification */
     SDL_CalculateAudioSpec(&this->spec);
@@ -779,6 +817,7 @@ prepare_audioqueue(_THIS)
         numAudioBuffers = ((int)SDL_ceil(MINIMUM_AUDIO_BUFFER_TIME_MS / msecs) * 2);
     }
 
+    this->hidden->numAudioBuffers = numAudioBuffers;
     this->hidden->audioBuffer = SDL_calloc(1, sizeof (AudioQueueBufferRef) * numAudioBuffers);
     if (this->hidden->audioBuffer == NULL) {
         SDL_OutOfMemory();
@@ -794,6 +833,7 @@ prepare_audioqueue(_THIS)
         CHECK_RESULT("AudioQueueAllocateBuffer");
         SDL_memset(this->hidden->audioBuffer[i]->mAudioData, this->spec.silence, this->hidden->audioBuffer[i]->mAudioDataBytesCapacity);
         this->hidden->audioBuffer[i]->mAudioDataByteSize = this->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(this->hidden->audioQueue, this->hidden->audioBuffer[i], 0, NULL);
         CHECK_RESULT("AudioQueueEnqueueBuffer");
     }
@@ -809,6 +849,20 @@ static int
 audioqueue_thread(void *arg)
 {
     SDL_AudioDevice *this = (SDL_AudioDevice *) arg;
+
+    #if MACOSX_COREAUDIO
+    const AudioObjectPropertyAddress default_device_address = {
+        this->iscapture ? kAudioHardwarePropertyDefaultInputDevice : kAudioHardwarePropertyDefaultOutputDevice,
+        kAudioObjectPropertyScopeGlobal,
+        kAudioObjectPropertyElementMaster
+    };
+
+    if (this->handle == NULL) {  /* opened the default device? Register to know if the user picks a new default. */
+        /* we don't care if this fails; we just won't change to new default devices, but we still otherwise function in this case. */
+        AudioObjectAddPropertyListener(kAudioObjectSystemObject, &default_device_address, default_device_changed, this);
+    }
+    #endif
+
     const int rc = prepare_audioqueue(this);
     if (!rc) {
         this->hidden->thread_error = SDL_strdup(SDL_GetError());
@@ -820,8 +874,36 @@ audioqueue_thread(void *arg)
 
     /* init was successful, alert parent thread and start running... */
     SDL_SemPost(this->hidden->ready_semaphore);
+
     while (!SDL_AtomicGet(&this->hidden->shutdown)) {
         CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0.10, 1);
+
+        #if MACOSX_COREAUDIO
+        if ((this->handle == NULL) && SDL_AtomicGet(&this->hidden->device_change_flag)) {
+            SDL_AtomicSet(&this->hidden->device_change_flag, 0);
+
+            #if DEBUG_COREAUDIO
+            printf("COREAUDIO: audioqueue_thread is trying to switch to new default device!\n");
+            #endif
+
+            /* if any of this fails, there's not much to do but wait to see if the user gives up
+               and quits (flagging the audioqueue for shutdown), or toggles to some other system
+               output device (in which case we'll try again). */
+            const AudioDeviceID prev_devid = this->hidden->deviceID;
+            if (prepare_device(this, this->handle, this->iscapture) && (prev_devid != this->hidden->deviceID)) {
+                AudioQueueStop(this->hidden->audioQueue, 1);
+                if (assign_device_to_audioqueue(this)) {
+                    int i;
+                    for (i = 0; i < this->hidden->numAudioBuffers; i++) {
+                        SDL_memset(this->hidden->audioBuffer[i]->mAudioData, this->spec.silence, this->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? */
+                        AudioQueueEnqueueBuffer(this->hidden->audioQueue, this->hidden->audioBuffer[i], 0, NULL);
+                    }
+                    AudioQueueStart(this->hidden->audioQueue, NULL);
+                }
+            }
+        }
+        #endif
     }
 
     if (!this->iscapture) {  /* Drain off any pending playback. */
@@ -829,6 +911,13 @@ audioqueue_thread(void *arg)
         CFRunLoopRunInMode(kCFRunLoopDefaultMode, secs, 0);
     }
 
+    #if MACOSX_COREAUDIO
+    if (this->handle == NULL) {
+        /* we don't care if this fails; we just won't change to new default devices, but we still otherwise function in this case. */
+        AudioObjectRemovePropertyListener(kAudioObjectSystemObject, &default_device_address, default_device_changed, this);
+    }
+    #endif
+
     return 0;
 }