How to write a custom udta atom with AVAssetWriter?

Question

I'm using AVAssetWriter on iOS 9.3 SDK to write an AAC packaged in MP4 container. Pretty standard stuff. It is working well.

However, now I need to add an atom contained in the udta atom (called mine for this example), so I have done this:

AVAssetWriter * writer =....  

NSData * valueData = [NSJSONSerialization dataWithJSONObject:obj options:0 error:nil];

AVMutableMetadataItem * item = [AVMutableMetadataItem metadataItem];  
item.keySpace = AVMetadataKeySpaceQuickTimeUserData; //udta  
item.key = @"mine";  
item.value = valueData;

writer.metadata = @[item];  

[writer startWriting];  
...  

And then in the end, the udta atom doesn't appear at all in the output file. What is going wrong here?

Answer 1

Finally found the issue. If you create your asset writer in the pure MP4 ( AVFileTypeMPEG4 ) mode, then custom metadata seems to be dropped on the floor silently. The solution is to use the QuickTime container file type:

AVAssetWriter * writer = [AVAssetWriter assetWriterWithURL:url fileType:AVFileTypeQuickTimeMovie error:nil];