отаю над приложением Видео в Swift3 iOS. По сути, мне нужно объединить видео активы и аудиосистемы в один с эффектом затухания и сохранить его в галерее iPhone. Для этого я использую метод ниже:

private func doMerge(arrayVideos:[AVAsset], arrayAudios:[AVAsset], animation:Bool, completion:@escaping Completion) -> Void {

        var insertTime = kCMTimeZero
        var audioInsertTime = kCMTimeZero
        var arrayLayerInstructions:[AVMutableVideoCompositionLayerInstruction] = []
        var outputSize = CGSize.init(width: 0, height: 0)

        // Determine video output size
        for videoAsset in arrayVideos {
            let videoTrack = videoAsset.tracks(withMediaType: AVMediaTypeVideo)[0]

            let assetInfo = orientationFromTransform(transform: videoTrack.preferredTransform)
            var videoSize = videoTrack.naturalSize
            if assetInfo.isPortrait == true {
                videoSize.width = videoTrack.naturalSize.height
                videoSize.height = videoTrack.naturalSize.width
            }
            outputSize = videoSize
        }

        // Init composition
        let mixComposition = AVMutableComposition.init()

        for index in 0..<arrayVideos.count {
            // Get video track
            guard let videoTrack = arrayVideos[index].tracks(withMediaType: AVMediaTypeVideo).first else { continue }

            // Get audio track
            var audioTrack:AVAssetTrack?
            if index < arrayAudios.count {
                if arrayAudios[index].tracks(withMediaType: AVMediaTypeAudio).count > 0 {
                    audioTrack = arrayAudios[index].tracks(withMediaType: AVMediaTypeAudio).first
                }
            }
            // Init video & audio composition track
            let videoCompositionTrack = mixComposition.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: Int32(kCMPersistentTrackID_Invalid))

            let audioCompositionTrack = mixComposition.addMutableTrack(withMediaType: AVMediaTypeAudio, preferredTrackID: Int32(kCMPersistentTrackID_Invalid))

            do {
                let startTime = kCMTimeZero
                let duration = arrayVideos[index].duration

                // Add video track to video composition at specific time
                try videoCompositionTrack.insertTimeRange(CMTimeRangeMake(startTime, duration), of: videoTrack, at: insertTime)

                // Add audio track to audio composition at specific time
                var audioDuration = kCMTimeZero
                if index < arrayAudios.count   {
                    audioDuration = arrayAudios[index].duration
                }

                if let audioTrack = audioTrack {
                    do {
                        try audioCompositionTrack.insertTimeRange(CMTimeRangeMake(startTime, audioDuration), of: audioTrack, at: audioInsertTime)
                    }
                    catch {
                        print(error.localizedDescription)
                    }
                }

                // Add instruction for video track
                let layerInstruction = videoCompositionInstructionForTrack(track: videoCompositionTrack, asset: arrayVideos[index], standardSize: outputSize, atTime: insertTime)

                // Hide video track before changing to new track
                let endTime = CMTimeAdd(insertTime, duration)

                if animation {
                    let timeScale = arrayVideos[index].duration.timescale
                    let durationAnimation = CMTime.init(seconds: 1, preferredTimescale: timeScale)

                    layerInstruction.setOpacityRamp (fromStartOpacity: 1.0, toEndOpacity: 0.0, timeRange: CMTimeRange.init(start: endTime, duration: durationAnimation))
                }
                else {
                    layerInstruction.setOpacity(0, at: endTime)
                }

                arrayLayerInstructions.append(layerInstruction)

                // Increase the insert time
                audioInsertTime = CMTimeAdd(audioInsertTime, audioDuration)
                insertTime = CMTimeAdd(insertTime, duration)
            }
            catch {
                print("Load track error")
            }
        }

        // Main video composition instruction
        let mainInstruction = AVMutableVideoCompositionInstruction()
        mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, insertTime)
        mainInstruction.layerInstructions = arrayLayerInstructions

        // Main video composition
        let mainComposition = AVMutableVideoComposition()
        mainComposition.instructions = [mainInstruction]
        mainComposition.frameDuration = CMTimeMake(1, 30)
        mainComposition.renderSize = outputSize

        // Export to file
        let path = NSTemporaryDirectory().appending("mergedVideo.mp4")
        let exportURL = URL.init(fileURLWithPath: path)

        // Remove file if existed
        FileManager.default.removeItemIfExisted(exportURL)

        // Init exporter
        let exporter = AVAssetExportSession.init(asset: mixComposition, presetName: AVAssetExportPresetHighestQuality)
        exporter?.outputURL = exportURL
        exporter?.outputFileType = AVFileTypeQuickTimeMovie//AVFileType.mp4
        exporter?.shouldOptimizeForNetworkUse = false //true
        exporter?.videoComposition = mainComposition

        // Do export
        exporter?.exportAsynchronously(completionHandler: {
            DispatchQueue.main.async {
                self.exportDidFinish(exporter: exporter, videoURL: exportURL, completion: completion)
            }
        })

    }



fileprivate func exportDidFinish(exporter:AVAssetExportSession?, videoURL:URL, completion:@escaping Completion) -> Void {
        if exporter?.status == AVAssetExportSessionStatus.completed {
            print("Exported file: \(videoURL.absoluteString)")
            completion(videoURL,nil)
        }
        else if exporter?.status == AVAssetExportSessionStatus.failed {
            completion(videoURL,exporter?.error)

            print(exporter?.error as Any)
        }
    }

Проблема: в моем методе exportDidFinish происходит сбой AVAssetExportSessionStatus со следующим сообщением об ошибке:

Домен ошибки = AVFoundationErrorDomain Code = -11800 «Операция не может быть завершена» UserInfo = {NSLocalizedFailureReason = Произошла неизвестная ошибка (-16976), NSLocalizedDescription = Операция не может быть завершена, NSUnderlyingError = 0x1c065fb30 {Код ошибки = NSOSStatusError 16976 "(null)"}}

Может кто-нибудь подсказать мне по этому поводу.