+ exportSessionWithAsset:presetName: returning nil on iOS7 - ios

Question

I'm using the following code:
- (AVAssetExportSession *)testRecording
{
AVMutableComposition *composition = [[AVMutableComposition alloc] init];
NSURL *url = [[NSBundle mainBundle] URLForResource:kTestSongName withExtension:#"mp3"];
AVAsset *asset = [AVURLAsset URLAssetWithURL:url options:nil];
CMTimeRange range = CMTimeRangeMake(CMTimeMake(0, 600), asset.duration);
NSError *editError;
BOOL result = [composition insertTimeRange:range ofAsset:asset atTime:kCMTimeZero error:&editError];
if (!result) {
NSLog(#"Error inserting timerange");
}
AVAssetExportSession *exportSession = [AVAssetExportSession
exportSessionWithAsset:composition
presetName:AVAssetExportPresetAppleM4A];
return exportSession;
}
This does what it's supposed to on iOS8 but on iOS7 exportSession is always nil at the end of the method. I tried using assets with different formats (mp3, caf) but the result was the same.
If I play the composition instead of exporting it the sound is fine:
AVPlayerItem * item = [[AVPlayerItem alloc] initWithAsset:composition];
AVPlayer * player = [AVPlayer playerWithPlayerItem:item];
[player play];
Any idea about what might be wrong?

Answer 1

exportSession = [[AVAssetExportSession alloc]
initWithAsset:songAsset
presetName:AVAssetExportPresetAppleM4A];
exportSession.outputFileType = #"com.apple.m4a-audio";
filePath = [[docDir stringByAppendingPathComponent:fileName] stringByAppendingPathExtension:#"m4a.mov"]
exportSession.outputURL = [NSURL fileURLWithPath:filePath];
[exportSession exportAsynchronouslyWithCompletionHandler:^{
if (exportSession.status == AVAssetExportSessionStatusCompleted) {
NSString *newPath = [NSString stringWithFormat:#"%#/%#", documentDirectory, [[[filePath lastPathComponent] substringToIndex:[[filePath lastPathComponent] length]-5] stringByAppendingString:#"a"]];
[[NSFileManager defaultManager] removeItemAtPath:newPath error:nil];
NSError *error = nil;
[[NSFileManager defaultManager] moveItemAtPath:filePath toPath:newPath error:&error];
}
if (exportSession.status == AVAssetExportSessionStatusCancelled) {
}
if (exportSession.status == AVAssetExportSessionStatusExporting) {
}
if (exportSession.status == AVAssetExportSessionStatusFailed) {
NSLog(#"erros:%#",exportSession.error);
}
if (exportSession.status == AVAssetExportSessionStatusUnknown) {
}
if (exportSession.status == AVAssetExportSessionStatusWaiting) {
}
}];
If you want to save in mp3 format, use ffmpeg.