How to properly use AVAssetExportSession to set metadata for an audio asset?

Question

I have looked at every example I can find for setting meta data using AVAssetExportSession, but even though the audio export works fine ( resultant audio file plays OK ), there's still no meta data exported with the file. I'm using Xcode 4.5, target build iOS 5, test device iOS 6. Please see the code I'm using below, and please let me know what I'm doing wrong.

header

// for metadata export

NSArray *MyMetadata;
AVMutableMetadataItem *common1;
AVMutableMetadataItem *common2;
AVMutableMetadataItem *common3;
AVMutableMetadataItem *common4;

implementation

AVAsset *asset = [AVAsset assetWithURL:audioFileInput];
AVAssetExportSession *exportSession = [AVAssetExportSession exportSessionWithAsset:asset presetName:AVAssetExportPresetPassthrough];

if (!exportSession) {
    return;
}

CMTime startTime = CMTimeMake((int)(floor(fileStartMarker * 100)), 100);
CMTime stopTime = CMTimeMake((int)(ceil(fileEndMarker * 100)), 100);
CMTimeRange exportTimeRange = CMTimeRangeFromTimeToTime(startTime, stopTime);

exportSession.outputURL = audioFileOutput;
exportSession.outputFileType = @"com.apple.coreaudio-format";
exportSession.timeRange = exportTimeRange;

// define meta data for file
// Common metadata
common1 = [[AVMutableMetadataItem alloc] init];    // Title
common1.keySpace = AVMetadataKeySpaceCommon;
common1.key = AVMetadataCommonKeyTitle;
common1.value = @"Title Test Value";

common2 = [[AVMutableMetadataItem alloc] init];    // Description
common2.keySpace = AVMetadataKeySpaceCommon;
common2.key = AVMetadataCommonKeyDescription;
common2.value = @"Description Test Value";

common3 = [[AVMutableMetadataItem alloc] init];   // Creation Date
common3.keySpace = AVMetadataKeySpaceCommon;
common3.key = AVMetadataCommonKeyCreationDate;
common3.value = @"Creation Date Test Value";

common4 = [[AVMutableMetadataItem alloc] init];    // Software
common4.keySpace = AVMetadataKeySpaceCommon;
common4.key = AVMetadataCommonKeySoftware;
common4.value = @"My File Trimmer";

MyMetadata = [[NSArray alloc] initWithObjects:common1, common2, common3, common4, nil];
exportSession.metadata = MyMetadata;

[common1 release];
[common2 release];
[common3 release];
[common4 release];

[exportSession exportAsynchronouslyWithCompletionHandler:^{
    if (exportSession.status == AVAssetExportSessionStatusCompleted) {
        // export done
    }
    else if (exportSession.status == AVAssetExportSessionStatusFailed)     {
        // export failed
    }
}];

[MyMetadata release];

Answer 1

Have you tried setting the identifier?

like:

    let authorMeta = AVMutableMetadataItem()
    authorMeta.identifier = AVMetadataCommonIdentifierAuthor
    authorMeta.key = AVMetadataQuickTimeMetadataKeyAuthor
    authorMeta.keySpace = AVMetadataKeySpaceCommon
    authorMeta.value = "NFer"