Why is playing audio through AV Foundation blocking the UI on a slow connection?

I'm using AV Foundation to play an MP3 file loaded over the network, with code that is almost identical to the playback example here: Putting it all Together: Playing a Video File Using AVPlayerLayer, except without attaching a layer for video playback. I was trying to make my app respond to the playback buffer becoming empty on a slow network connection. To do this, I planned to use key-value observing on the AVPlayerItem's playbackBufferEmpty property, but the documentation did not say whether that was possible. I thought it might be possible because the status property can be observed (and is the example above) even though the documentation doesn't say that.

So, in an attempt to create conditions where the buffer would empty, I added code on the server to sleep for two se开发者_运维技巧conds after serving up each 8k chunk of the MP3 file. Much to my surprise, this caused my app's UI (updated using NSTimer) to freeze completely for long periods, despite the fact that it shows almost no CPU usage in the profiler. I tried loading the tracks on another queue with dispatch_async, but that didn't help at all.

Even without the sleep on the server, I've noticed that loading streams using AVPlayerItem keeps the UI from updating for the short time that the stream is being downloaded. I can't see why a slow file download should ever block the responsiveness of the UI. Any idea why this is happening or what I can do about it?

Okay, problem solved. It looks like passing AVURLAssetPreferPreciseDurationAndTimingKey in the options to URLAssetWithURL:options: causes the slowdown. This also only happens when the AVURLAsset's duration property or some other property relating to the stream's timing is accessed from the selector fired by the NSTimer. So if you can avoid polling for timing information, this problem may not affect you, but that's not an option for me. If precise timing is not requested, there's still a delay of around 0.75 seconds to 1 second, but that's all.

Looking back through it, the documentation does warn that precise timing might cause slower performance, but I never imagined 10+ second delays. Why the delay should scale with the loading time of the media is beyond me; it seems like it should only scale with the size of the file. Maybe iOS is doing some kind of heavy polling for new data and/or processing the same bytes over and over.

So now, without "precise timing and duration," the duration of the asset is permanently at 0.0, even when it's fully loaded. I can also answer my original goal of doing KVO on AVPlayerItem.isPlaybackBufferEmpty. It seems KVO would be useless anyway, since the property starts out being NO, changes to YES as soon as I start playback, and continues to be YES even as the media is playing for minutes at a time. The documentation says this about the property:

Indicates whether playback has consumed all buffered media and that playback will stall or end.

So I guess that's not accurate, and, at least in this particular case, the property is not very useful.