Invalid File Output AVAssetExport - ios

Question

When i run my app and click the save button to save the two files mixed together my app crashes saying invalid file output. I dont see why this error shows up because the two files being mixed are mp3 and output file is mp3.
Code:
#IBAction func mixButton (sender:AnyObject!) {
let oldAsset = self.player.currentItem.asset
let type = AVMediaTypeAudio
let audioFile = NSBundle.mainBundle().URLForResource("file1", withExtension: "mp3")
let asset1 = AVURLAsset(URL: audioFile, options: nil)
let arr2 = asset1.tracksWithMediaType(type)
let track2 = arr2.last as AVAssetTrack
let duration : CMTime = track2.timeRange.duration
let comp = AVMutableComposition()
let comptrack = comp.addMutableTrackWithMediaType(type,
preferredTrackID: Int32(kCMPersistentTrackID_Invalid))
comptrack.insertTimeRange(CMTimeRangeMake(CMTimeMakeWithSeconds(0,600), CMTimeMakeWithSeconds(5,600)), ofTrack:track2, atTime:CMTimeMakeWithSeconds(0,600), error:nil)
comptrack.insertTimeRange(CMTimeRangeMake(CMTimeSubtract(duration, CMTimeMakeWithSeconds(5,600)), CMTimeMakeWithSeconds(5,600)), ofTrack:track2, atTime:CMTimeMakeWithSeconds(5,600), error:nil)
let type3 = AVMediaTypeAudio
let s = NSBundle.mainBundle().URLForResource("file2", withExtension:"mp3")
let asset = AVURLAsset(URL:s, options:nil)
let arr3 = asset.tracksWithMediaType(type3)
let track3 = arr3.last as AVAssetTrack
let comptrack3 = comp.addMutableTrackWithMediaType(type3, preferredTrackID:Int32(kCMPersistentTrackID_Invalid))
comptrack3.insertTimeRange(CMTimeRangeMake(CMTimeMakeWithSeconds(0,600), CMTimeMakeWithSeconds(10,600)), ofTrack:track3, atTime:CMTimeMakeWithSeconds(0,600), error:nil)
let params = AVMutableAudioMixInputParameters(track:comptrack3)
params.setVolume(1, atTime:CMTimeMakeWithSeconds(0,600))
params.setVolumeRampFromStartVolume(1, toEndVolume:0, timeRange:CMTimeRangeMake(CMTimeMakeWithSeconds(7,600), CMTimeMakeWithSeconds(3,600)))
let mix = AVMutableAudioMix()
mix.inputParameters = [params]
let item = AVPlayerItem(asset:comp)
item.audioMix = mix
mixedFile = comp //global variable for mixed file
}
}
#IBAction func saveButton(sender: AnyObject) {
let documentsPath = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] as String
let savedFileTest = documentsPath + "/myfile.mp3"
if (NSFileManager.defaultManager().fileExistsAtPath(savedFileTest)) {
NSFileManager.defaultManager().removeItemAtPath(savedFileTest, error: nil)
}
let url = NSURL.fileURLWithPath(savedFileTest)
let exporter = AVAssetExportSession(asset: mixedFile, presetName: AVAssetExportPresetHighestQuality)
exporter.outputURL = url
exporter.outputFileType = AVFileTypeMPEGLayer3
exporter.exportAsynchronouslyWithCompletionHandler({
switch exporter.status{
case AVAssetExportSessionStatus.Failed:
println("failed \(exporter.error)")
case AVAssetExportSessionStatus.Cancelled:
println("cancelled \(exporter.error)")
default:
println("complete")
}

Answer 1

The output file is not mp3. You can say mp3 but that doesn't make it one. I don't think Apple framework code can save as mp3. It can read it, but due to various licensing issues it can't write it.
Do it as an m4a, like this (I have starred the lines I changed from your original code):
let savedFileTest = documentsPath + "/myfile.m4a" // *
if (NSFileManager.defaultManager().fileExistsAtPath(savedFileTest)) {
NSFileManager.defaultManager().removeItemAtPath(savedFileTest, error: nil)
}
let url = NSURL.fileURLWithPath(savedFileTest)
let exporter = AVAssetExportSession(
asset: mixedFile, presetName: AVAssetExportPresetAppleM4A) // *
exporter.outputURL = url
exporter.outputFileType = AVFileTypeAppleM4A // *
By the way, you're saving the wrong thing (the unmixed comp rather than the mixed item).

Answer 2

You can export it to "AVFileTypeQuickTimeMovie" and rename it to *.mp3.
Init export, here you must set "presentName" argument to "AVAssetExportPresetPassthrough". If not, you will can't export mp3 correct.
AVAssetExportSession *export = [[AVAssetExportSession alloc] initWithAsset:sset presetName:AVAssetExportPresetPassthrough];
Set the ouput type:
exportSession.outputFileType = AVFileTypeQuickTimeMovie;
exportSession.shouldOptimizeForNetworkUse = true;
Export it and set the file name extension to "mp3".