AudioQueue PropertyListener IsRunning only callback once - iphone

Question

OSStatus err = AudioQueueNewOutput(&audioDescription, AudioPlayerAQOutputCallback, ( void* )self, nil, nil, 0, &audioQueue);
if( err != noErr )
NSLog(#"Couldn't open AudioFile.");
err = AudioQueueAddPropertyListener(audioQueue, kAudioQueueProperty_IsRunning, isRunningProc, self);
if( err != noErr )
NSLog(#"Couldn't register for playback state changes.");
this callback function only be called once after AudioQueueStart(audioQueue, NULL);
what ever i call AudioQueuePause(audioQueue);
or audio reach to end.
static void isRunningProc(void * inUserData,
AudioQueueRef inAQ,
AudioQueuePropertyID inID)
what i have missed?

Answer 1

I did a short test on this:
It does indeed seem like the callback it is not called either for pause or for start, when you are resuming a pause.
But this is not something you cannot solve. You started the song somehow. This will trigger the property listener. Equally if the song stops. Or you stop it. You may have to trigger the property listener yourself somehow using something like this in your play routine:
if (bytesRead == 0) {
//This will trigger the property listener
AudioQueueStop(inAQ, false);
}
else {
AudioQueueEnqueueBuffer(inAQ, inBuffer, 0, NULL);
}
As AudioQueue is concerned, as long as you keep feeding it with audio buffers to play, it is still playing. (I also tested not feeding any buffers at all, which did not trigger a stop, so you have to call stop explicitly to trigger the property listener.)
This means you already know whether your song is playing or not. To pause or un-pause is requested by clicking your button. If the song is not playing, don't do anything. If the song is playing, call AudioQueuePause and set a flag that you have paused your music. Remember to check the error code. (See (1) below). If the flag says that you have paused the music, call AudioQueueStart, and clear the flag indicating if you have paused or not. Again check the error code.
(1) Why check the error code?
First, although unlikely, an error may occur because it is a blue moon.
However, my concern is with multiple threads. AudioQueue obviously runs on a separate thread than your GUI. That means if you test a flag whether music is playing or not, this state cannot fully be trusted because it might have changed since you tested the status. Another thread might have snuck in between your test and your action based on that test.
Say you check that the song is already playing. (It is.)
Then you ask the song to pause, but the song is really stopped since it reached the end in the meantime before you got to ask the song to pause.
Then you ask to pause the song. But it is already stopped.
What happens then? I don't really know. It might not even be a problem in this situation, but things like this is worth considering. It needs testing, or at least a consulting with the documentation.
How about another scenario? What if the song is stopped and you ask to start it again. I would think that is a worse scenario, but it might not be a problem. Again consider those cases and check the documentation, or even test yourself.