Quiero fusionar el AVAsset-arrayVideos en un solo video y guárdelo en el carrete de la cámara. Raywenderlich.com tiene un grantutorial donde dos videos se fusionan en uno. Creé el siguiente código, sin embargo, el video que obtengo después de exportar al rollo de cámara incluye solo el primer y el último video de la matriz (excluyendo el resto de los videos en el medio dearrayVideos) ¿Me estoy perdiendo de algo?

var arrayVideos = [AVAsset]() //Videos Array    
var atTimeM: CMTime = CMTimeMake(0, 0)
var lastAsset: AVAsset!
var layerInstructionsArray = [AVVideoCompositionLayerInstruction]()
var completeTrackDuration: CMTime = CMTimeMake(0, 1)
var videoSize: CGSize = CGSize(width: 0.0, height: 0.0)

func mergeVideoArray(){

    let mixComposition = AVMutableComposition()
    for videoAsset in arrayVideos{
        let videoTrack = mixComposition.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: Int32(kCMPersistentTrackID_Invalid))
        do {
            if videoAsset == arrayVideos.first{
                atTimeM = kCMTimeZero
            } else{
                atTimeM = lastAsset!.duration
            }
            try videoTrack.insertTimeRange(CMTimeRangeMake(kCMTimeZero, videoAsset.duration), of: videoAsset.tracks(withMediaType: AVMediaTypeVideo)[0], at: atTimeM)  
            videoSize = videoTrack.naturalSize
        } catch let error as NSError {
            print("error: \(error)")
        }
        completeTrackDuration = CMTimeAdd(completeTrackDuration, videoAsset.duration)
        let videoInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: videoTrack)
        if videoAsset != arrayVideos.last{
            videoInstruction.setOpacity(0.0, at: videoAsset.duration)
        }
        layerInstructionsArray.append(videoInstruction)
        lastAsset = videoAsset            
    }

    let mainInstruction = AVMutableVideoCompositionInstruction()
    mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, completeTrackDuration)
    mainInstruction.layerInstructions = layerInstructionsArray        

    let mainComposition = AVMutableVideoComposition()
    mainComposition.instructions = [mainInstruction]
    mainComposition.frameDuration = CMTimeMake(1, 30)
    mainComposition.renderSize = CGSize(width: videoSize.width, height: videoSize.height)

    let documentDirectory = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0]
    let dateFormatter = DateFormatter()
    dateFormatter.dateStyle = .long
    dateFormatter.timeStyle = .short
    let date = dateFormatter.string(from: NSDate() as Date)
    let savePath = (documentDirectory as NSString).appendingPathComponent("mergeVideo-\(date).mov")
    let url = NSURL(fileURLWithPath: savePath)

    let exporter = AVAssetExportSession(asset: mixComposition, presetName: AVAssetExportPresetHighestQuality)
    exporter!.outputURL = url as URL
    exporter!.outputFileType = AVFileTypeQuickTimeMovie
    exporter!.shouldOptimizeForNetworkUse = true
    exporter!.videoComposition = mainComposition
    exporter!.exportAsynchronously {

        PHPhotoLibrary.shared().performChanges({
            PHAssetChangeRequest.creationRequestForAssetFromVideo(atFileURL: exporter!.outputURL!)
        }) { saved, error in
            if saved {
                let alertController = UIAlertController(title: "Your video was successfully saved", message: nil, preferredStyle: .alert)
                let defaultAction = UIAlertAction(title: "OK", style: .default, handler: nil)
                alertController.addAction(defaultAction)
                self.present(alertController, animated: true, completion: nil)
            } else{
                print("video erro: \(error)")

            }
        }
    }
}