**Merging Multiple Audio Files in iOS**

AudioMerge.swift

/**
provide an array of audio files urls and get them merged
Its important to note that the process is asynchronous and that one would need to show user some sort of a progress indicator 
so that the process does not get interupted
- parameter audioFileUrls: an array of audio file urls
- returns String representing the newly merged file or nil for a failure
*/
func mergeAudioFiles(audioFileUrls: [URL]) -> String? {        
        let composition = AVMutableComposition()
           
        for i in 0 ..< audioFileUrls.count {
            let compositionAudioTrack :AVMutableCompositionTrack = composition.addMutableTrack(withMediaType: AVMediaType.audio, preferredTrackID: CMPersistentTrackID())!
            let filePath = "\(Utilities.getDocumentsDirectory())"+Utilities.getFileName(audioFileUrls[i].path)
            
            let asset = AVURLAsset(url: URL(string: filePath)!)
                        
            let trackContainer = asset.tracks(withMediaType: AVMediaType.audio)
            
            guard trackContainer.count > 0 else{
                return nil
            }
            
            let audioTrack = trackContainer[0]
            let timeRange = CMTimeRange(start: CMTimeMake(0, 600), duration: audioTrack.timeRange.duration)
            try! compositionAudioTrack.insertTimeRange(timeRange, of: audioTrack, at: composition.duration)
        }
        
        let finalUrl = URL(string: "\(getDocumentsDirectory())\(UUID().uuidString)_audio.m4a")
        
        let assetExport = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetAppleM4A)
        assetExport?.outputFileType = AVFileType.m4a
        assetExport?.outputURL = finalUrl
        assetExport?.exportAsynchronously(completionHandler:{
            self.hud.dismiss()
                switch assetExport!.status
                {
                case AVAssetExportSessionStatus.failed:
                    print("AUDIO_MERGE -> failed \(String(describing: assetExport!.error!))")
                case AVAssetExportSessionStatus.cancelled:
                    print("AUDIO_MERGE -> cancelled \(String(describing: assetExport!.error))")
                case AVAssetExportSessionStatus.unknown:
                    print("AUDIO_MERGE -> unknown\(String(describing: assetExport!.error))")
                case AVAssetExportSessionStatus.waiting:
                    print("AUDIO_MERGE -> waiting\(String(describing: assetExport!.error))")
                case AVAssetExportSessionStatus.exporting:
                    print("AUDIO_MERGE -> exporting\(String(describing: assetExport!.error) )")
                default:
                    print("Audio Concatenation Complete")
                    print("Old audio :: \(getFileName(audioFileUrls[0]!.path))")
                    print("New audio :: \(getFileName(finalUrl!.path))")
                    print("Merged    :: \(getFileName(finalUrl!.path))")
                   }
        })
        
        return finalUrl!.path
}
   
/**
Get the set document directory for the application
*/
func getDocumentsDirectory() -> URL {
        let paths = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
        let documentsDirectory = paths[0]
        return documentsDirectory
}

/**
get filename from a given file url or path
- parameter fileUrl: file path to be extracted
- returns: filename : String
*/
func getFileName(_ fileUrl : String) -> String{
        return URL(string: fileUrl)!.lastPathComponent
}