How to seek within an audio track using avassetreader?

Question

I'm familiar with how to stream audio data from the ipod library using AVAssetReader, but I'm at a loss as to how to seek within the track. e.g. start playback at the halfway point, etc. Starting from the beginning and then sequentially getting successive samples is easy, but surely there must be a way to have random access?

Answer 1

AVAssetReader has a property, timeRange, which determines the time range of the asset from which media data will be read.

@property(nonatomic) CMTimeRange timeRange

The intersection of the value of this property and CMTimeRangeMake(kCMTimeZero, asset.duration) determines the time range of the asset from which media data will be read.

The default value is CMTimeRangeMake(kCMTimeZero, kCMTimePositiveInfinity). You cannot change the value of this property after reading has started.

So, if you want to seek to the middle the track, you'd create a CMTimeRange from asset.duration/2 to asset.duration, and set that as the timeRange on the AVAssetReader.

Answer 2

AVAssetReader is amazingly slow when seeking. If you try to recreate an AVAssetReader to seek while the user is dragging a slider, your app will bring iOS to its knees.

Instead, you should use an AVAssetReader for fast forward only access to video frames, and then also use an AVPlayerItem and AVPlayerItemVideoOutput when the user wants to seek with a slider.

It would be nice if Apple combined AVAssetReader and AVPlayerItem / AVPlayerItemVideoOutput into a new class that was performant and was able to seek quickly.

Be aware that AVPlayerItemVideoOutput will not give back pixel buffers unless there is an AVPlayer attached to the AVPlayerItem. This is obviously a strange implementation detail, but it is what it is.

If you are using AVPlayer and AVPlayerLayer, then you can simply use the seek methods on AVPlayer itself. The above details are only important if you are doing custom rendering with the pixel buffers and/or need to send the pixel buffers to an AVAssetWriter.