commonMetadata array empty for some songs from iPod library - iphone

Question

I'm using a MPMediaPickerControllerto enable the user to pick songs from the device's iPod library. Then I put this songs into an array (in this case playerQueue) and use an AVPlayer instance to play one song after the other:
- (void)setQueueWithItemCollection:(MPMediaItemCollection *)theCollection {
for (MPMediaItem *theMediaItem in theCollection.items) {
NSURL *mediaURL = [theMediaItem valueForProperty:MPMediaItemPropertyAssetURL];
AVPlayerItem *thePlayerItem = [AVPlayerItem playerItemWithURL:mediaURL];
// Don't add protected tracks to queue as they cannot be played
if (!thePlayerItem.asset.hasProtectedContent && thePlayerItem.asset.isPlayable) [self.playerQueue addObject:thePlayerItem];
// Further implementation
}
}
Now I get the song title using this code snippet (with AVPlayer *musicPlayer):
AVPlayerItem *currentItem = self.musicPlayer.currentItem;
AVAsset *asset = currentItem.asset;
// Further implementation
NSLog(#"DEBUG: Meta data: %#", asset.commonMetadata);
NSArray *titles = [AVMetadataItem metadataItemsFromArray:asset.commonMetadata withKey:AVMetadataCommonKeyTitle keySpace:AVMetadataKeySpaceCommon];
Most times this works without any problem. But there are are also songs for which asset.commonMetadata returns an empty array. However the Music app on my iPhone is able to display the song title (and album artwork etc.) just like iTunes (on the Mac).
The song file is an Apple MPEG 4 Audio file purchased from the iTunes Store.
Why is asset.commonMetadata returning an empty array even though there are meta data available?

Answer 1

I was having this same problem with downloaded m4b audio files. Turns out that AVURLAsset requires the URL string to end in '.m4b' (other audio extensions would work, I suspect). You can't use the URL as is if it is something like http://source.com/download.php?id=12345. Otherwise, the asset.commonMetaData just returns an empty array. I don't know if this is a problem with iTunes assetURLs, but it would be easy to check.

Answer 2

The only solution that worked for me:
Use AudioToolbox.framework to retrieve metadata from such files, i.e. use AudioFileGetProperty function.
It seems like a real bug in AVFoundation…