Audio Mixing in iOS using AVFoundation doesnt work

Audio Mixing in iOS using AVFoundation doesnt work

I am trying to stitch a bunch of videos together and then add some music
over the video in iOS. The audio is added using AVMutableAudioMix. However
when the video is finally exported the audio mix is missing. Here is how
the code looks like :
- (void)mergeVideos:(NSArray *)videos{
AVMutableComposition *mixComposition = [[AVMutableComposition alloc]
init];
AVMutableCompositionTrack *videoTrack = [mixComposition
addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID:kCMPersistentTrackID_Invalid];
AVMutableCompositionTrack *audioTrack = [mixComposition
addMutableTrackWithMediaType:AVMediaTypeAudio
preferredTrackID:kCMPersistentTrackID_Invalid];
CMTime currentTime = kCMTimeZero;
for (AVAsset *asset in videos) {
// 2 - Video track
[videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero,
asset.duration)
ofTrack:[[asset
tracksWithMediaType:AVMediaTypeVideo]
objectAtIndex:0] atTime:currentTime
error:nil];
[audioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero,
asset.duration)
ofTrack:[[asset
tracksWithMediaType:AVMediaTypeAudio]
objectAtIndex:0] atTime:currentTime
error:nil];
currentTime = CMTimeAdd(currentTime, asset.duration);
}
// 4 - Get path
NSArray *paths =
NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,
NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *myPathDocs = [documentsDirectory
stringByAppendingPathComponent:
[NSString
stringWithFormat:@"mergeVideo-%d.mp4",arc4random()
% 1000]];
NSURL *url = [NSURL fileURLWithPath:myPathDocs];
// 5 - Create exporter
NSArray *compatiblePresets = [AVAssetExportSession
exportPresetsCompatibleWithAsset:mixComposition];
NSString *quality = AVAssetExportPresetHighestQuality;
//load the audio
AVMutableAudioMix *audioMix = nil;
NSURL *audioURL = [self loadAudioFile];// gives a url for a .caf file
from the bundle
if (audioURL) {
AVURLAsset *audioAsset = [AVURLAsset assetWithURL:audioURL];
AVAssetTrack *aTrack = (AVAssetTrack *)[[audioAsset
tracksWithMediaType:AVMediaTypeAudio] firstObject];
AVMutableAudioMixInputParameters *trackMix =
[AVMutableAudioMixInputParameters
audioMixInputParametersWithTrack:aTrack];
[trackMix setVolumeRampFromStartVolume:0 toEndVolume:1 timeRange:
CMTimeRangeMake(CMTimeMakeWithSeconds(0, 1),
CMTimeMakeWithSeconds(3, 1))];
[trackMix setVolume:1.0 atTime:kCMTimeZero];
AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];
audioMix.inputParameters = [NSArray arrayWithObject:trackMix];
}
AVAssetExportSession *exporter = [[AVAssetExportSession alloc]
initWithAsset:mixComposition
presetName:quality];
if (audioMix) {
exporter.audioMix = audioMix;
}
exporter.outputURL=url;
exporter.outputFileType = AVFileTypeMPEG4;
exporter.shouldOptimizeForNetworkUse = YES;
[exporter exportAsynchronouslyWithCompletionHandler:^{
dispatch_async(dispatch_get_main_queue(), ^{
[self exportDidFinish:exporter];
});
}];
}
There is no error while executing. Just that the music contained in the
.caf file doesnt get mixed to the exported file. Any idea what is going
on?