After trying to replicate the issue and doing some testing, I strongly suspect the protected content is an issue. Here's why:
I copied your code, and tested it on my iPod Touch (5th gen, iOS 6.0.1), though instead of coming from a media picker, I just let it loop through all the videos I have on the device (7 of them.) It worked great, and called the completion handler and made proper .mov files in the documents directory of the app sandbox. I moved the .mov files to my Mac and they all played.
These video files had the hasProtectedContent as NO.
So I placed a video file I got from the iTunes store, and confirmed it had the hasProtectedContent as YES. Interestingly, when I try to get the URL from MPMediaItemPropertyAssetURL, I get nil for the protected/iTunes obtained video.
I strongly suspect the media protection is the problem.
Here's the variation of code that I used. I didn't change your conversion code at all, just how the URLs are supplied:
// select all the video files
MPMediaPropertyPredicate *predicate = [MPMediaPropertyPredicate predicateWithValue:[NSNumber numberWithInteger:MPMediaTypeMovie] forProperty:MPMediaItemPropertyMediaType];
MPMediaQuery *query = [[MPMediaQuery alloc] init];
[query addFilterPredicate:predicate];
NSArray *items = [query items];
// now go through them all to export them
NSString* title;
NSURL * url;
AVAssetExportSession *exportSession;
NSString *storePath;
AVAsset *theAsset;
// we fill put the output at this path
NSString *applicationDocumentsDir = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
// loop through the items and export
for (MPMediaItem* item in items)
{
title = [item valueForProperty:MPMediaItemPropertyTitle];
url = [item valueForProperty:MPMediaItemPropertyAssetURL];
NSLog(@"Title: %@, URL: %@",title,url);
theAsset = [AVAsset assetWithURL:url];
if ([theAsset hasProtectedContent]) {
NSLog(@"%@ is protected.",title);
} else {
NSLog(@"%@ is NOT protected.",title);
}
exportSession = [[AVAssetExportSession alloc] initWithAsset:theAsset presetName:AVAssetExportPreset640x480];
storePath = [applicationDocumentsDir stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.mov",title]];
exportSession.outputFileType = AVFileTypeQuickTimeMovie;
exportSession.outputURL = [NSURL fileURLWithPath:storePath];
[exportSession exportAsynchronouslyWithCompletionHandler:^{
NSLog(@"done!");
}];
}
Out of curiosity, are you checking the AVAsset exportable
flag?