如何为AVMetadataFormatiTunesMetadata创建AVMutableMetadataItem

Question

I am trying to update the track number of files using AVMutableMetadataItem's. I have been having some success except for AVMetadataFormatiTunesMetadata. I cannot seem to figure out what I need to set as the value to get this to work. I have tried to create an archive of a NSArray as follows

[NSKeyedArchiver archivedDataWithRootObject:[NSArray arrayWithObjects:
    [NSNumber numberWithUnsignedInt:13],
    [NSNumber numberWithUnsignedInt:32],
    nil]];

But I end up with a track and trackOf numbers that are way off. What exactly am I supposed to be passing in as the value for an AVMutableMetadataItem with a key of AVMetadataiTunesMetadataKeyTrackNumber?

Answer 1

It took me a very long time to figure that one out. Thanks to Apple's Core Audio documentation, which says absolutely nothing about how to deal with it. Or any of the other keys for that matter. I had to examine an MP4 file with track information before I understood.

Answer

You need to assign it with an NSData containing the track information.

The data must consists of four 16-bit big endian values, whereas the 2nd is the track number and the 3rd is the total tracks in collection . 1st and 4th should be zero.

So basically you need to do this

int16_t trackNumber = 1; // track number
int16_t tracksInCollection = 12; // total number of tracks in collection
int16_t data[4] = { 0, trackNumber, tracksInCollection, 0 };

metadataItem.keySpace = AVMetadataKeySpaceiTunes;
metadataItem.key = AVMetadataiTunesMetadataKeyTrackNumber;
metadataItem.value = [NSData dataWithBytes:data length:sizeof(data)];

Notice: The same approach is applied for the AVMetadataiTunesMetadataKeyDiscNumber key.

A remarks on endianness

If you don't want to worry about byte-order, you can "borrow" a methods from the Berkeley sockets API. Or it might be a macro. Anyhow, it works like this:

bigendianval = htons(val);

or

int16_t trackNumber = htons(myTrackNumberVariable);

htons ( Host to network short ) will convert your 16-bit numbers to big endian - regardless of endianness of your own system. IP-networks are also big-endian, and therefore htons is reusable here.

Answer 2

Similarly, for disk number:

AVMutableMetadataItem *metaItem = [AVMutableMetadataItem metadataItem];
metaItem.key=@1684632427;

int16_t diskNumber = htons([dict[@"discNumber"]intValue]);
int16_t disksInCollection = htons([dict[@"discCount"]intValue]);
int16_t data[3] = {0, diskNumber, disksInCollection};
metaItem.value=[NSData dataWithBytes:data length:sizeof(data)];
metaItem.keySpace=AVMetadataKeySpaceiTunes;