my dispatch_async on q1, that calls a UI object on main queue, which adds a callback on q1, crashes

Question

It sounds confusing but it looks like this

AVPlayer *capturedPlayer = _player;
dispatch_async(_subtitlesQueue, ^{
    // Parse the requested subtitle track and create a subtitle time observer
    subripString = [NSString stringWithContentsOfFile:filePath encoding:NSUTF8StringEncoding error:nil];
    subripEntries = [SubRipParser parse:subripString];
    if (!subripEntries.count)
        return;
    dispatch_async(dispatch_get_main_queue(), ^{
        _subtitlesTimeObserver = [capturedPlayer addPeriodicTimeObserverForInterval:CMTimeMake(1, 5)
                                                                              queue:_subtitlesQueue
                                                                         usingBlock:^(CMTime time){}];
    });
});

The above piece of code is called when a button is clicked. It crashes. I'm new to GCD and the whole queue thing so perhaps I'm misunderstanding, but shouldn't the above work?

If I change the call on the main queue to a synchronous then it works. The crash happens from the subtitleQueue on a call to AVPlayer's makePeriodicCall (or the like).

The async call also works if I add the periodic time observer to the main queue instead of custom serial queue. However, the docs say that adding on a different queue should be ok.

Question 2) And while I'm here, I also have a question about the part that "captures" the AVPlayer. Is capturing the variable like that safe enough or do I have to use __weak and make sure it's not NULL within the block? My situation is such that the controller that contains the AVPlayer is a singleton, so it exists throughout the lifetime of the application. I think this makes not using the __weak modifier ok. Am I correct in thinking this?

Cheers, and thanks for any help!

EDIT: The exception is a EXC_BAD_ACCESS code 2, so something which shouldn't be accessed is. It happens on a separate thread that is running the _subtitlesQueue. And it happens on a call to [AVPlayerPeriodicCaller _effectiveRateChanged]

I also printed out the values for the capturedPlayer and _subtitlesQueue (pointer values) before the outer dispatch_async is called on the _subtitlesQueue, before the inner dispatch_async is called on the main queue and inside the dispatch_async on the main queue before the addPeriodicTimeObserver is called. They are all the same.

EDIT2: If I add a synchronized block around the periodic time observer creation on the subtitleQueue then things work...

@synchronized(_subtitlesQueue) {
    _subtitlesTimeObserver = [capturedPlayer addPeriodicTimeObserverForInterval:CMTimeMake(1, 5)
                                                                          queue:_subtitlesQueue
                                                                     usingBlock:subtitleTimeObservedBlock];
}

All

is _subtitlesQueue a local variable? instance variable? When is it created and released? — newacct
_subtitlesQueue is a member variable created when the view controller is initialised. — aliak
Oh and it's release is supposed to be handled by ARC (as of ios6) — aliak
Can you provide a small but self-contained example program that exhibits the problem? — jlstrecker

erikprice erikprice · Accepted Answer · 2013-08-29T15:50:58

There seems to be a bug that causes EXC_BAD_ACCESS in -[AVPlayerPeriodicCaller _effectiveRateChanged] when you add a periodic observer to a playing AVPlayer. The workaround that I'm using is:

BOOL playing = player.rate > 0.0f;
if (playing)
{
    [player pause];
}

[player addPeriodicTimeObserverForTimeInterval:myTime queue:mySerialQueue usingBlock:myBlock];

if (playing)
{
    [player play];
}

As you pointed out, another workaround is to pass NULL instead of a serial queue, since that has the effect of enqueueing the blocks on the main thread dispatch queue.

my dispatch_async on q1, that calls a UI object on main queue, which adds a callback on q1, crashes

1 Answers