Audio player duration changes while playing

Question

When I take the duration of an audio file before playing it with:

audioPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:url error:&error];
...
[audioPlayer prepareToPlay];
duration = audioPlayer.duration;

I get a duration of for example 16.71s. Then when taking samples every 0.2s while playing, the duration changes. It changes every 1.2 to 1.6 seconds to: 16.71s, 17.02s, 17.23s, 17.33s, 17.38s, 17.43s, 17.38s, 17.29s, 17.31s, then stays at 17.51s for the last 2.5 seconds. So it goes up and down.

I sample in a method that updates a slider position, and also shows the elapsed time and total duration. You can imagine it looks really weird to see the duration (which is int-truncated) go from 16 to 17 while playing. Additionally, the slider position will be off with all this drifting.

Does anyone have an idea why this is?

Now that we're talking about duration: Does anyone know why audio player.duration can return values that are about twice the actual duration when [audioPlayer prepareToPlay] is omitted?

Answer 1

The duration returned by avaudioplayer's duration method is a best estimate of the total duration of the file based on how much of the file has been decoded so far. That's why the duration continues to get refined as you check it over time.

In order to get a better time, I use AVAsset's duration method. It explains what is going on a bit better here:

https://developer.apple.com/library/mac/documentation/AVFoundation/Reference/AVAsset_Class/Reference/Reference.html#//apple_ref/occ/instp/AVAsset/duration

If you specify providesPreciseDurationAndTiming = YES, then AVAsset will decode the file if needed to determine its duration with accuracy. If the decode time is too long for your use, you can disable it.

In my situation, I use the AVURLAsset subclass:

            AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:localURL options:[NSDictionary dictionaryWithObjectsAndKeys: [NSNumber numberWithBool:YES], AVURLAssetPreferPreciseDurationAndTimingKey, nil]];
            float t = CMTimeGetSeconds(asset.duration);

Answer 2

@AndyKorth's answer is the best! Here it is in Swift

guard let audioPlayer = audioPlayer, let url = audioPlayer.url else { return }

let assetOpts = [AVURLAssetPreferPreciseDurationAndTimingKey: true]
let asset = AVURLAsset(url: url, options: assetOpts)

let assetDuration: Float64 = CMTimeGetSeconds(asset.duration)

// compare both
print("assetDuration: ", assetDuration)

print("audioPlayerDuration: ", Float(audioPlayer.duration))