Export

func export(videos: [Video], overlay: UIImage?) {
   let videoComposition = AVMutableVideoComposition()
   videoComposition.frameDuration = CMTimeMake(1, 30)
   videoComposition.renderSize = CGSize(width: 1080, height: 1080)
   let composition = AVMutableComposition()

   var time: Double = 0

   let videoTrack = composition.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: kCMPersistentTrackID_Invalid)
   let audioTrack = composition.addMutableTrack(withMediaType: AVMediaTypeAudio, preferredTrackID: kCMPersistentTrackID_Invalid)

   for video in videos {
     let asset = AVAsset(url: video.url)
     let videoAssetTrack = asset.tracks(withMediaType: AVMediaTypeVideo).first
     let audioAssetTrack = asset.tracks(withMediaType: AVMediaTypeAudio).first
     let atTime = CMTime(seconds: time, preferredTimescale: 30)

     do {
       if let videoAssetTrack = videoAssetTrack {
         try videoTrack.insertTimeRange(CMTimeRangeMake(kCMTimeZero, asset.duration), of: videoAssetTrack, at: atTime)
       }
       if let audioAssetTrack = audioAssetTrack {
         try audioTrack.insertTimeRange(CMTimeRangeMake(kCMTimeZero, asset.duration), of: audioAssetTrack, at: atTime)
       }

     } catch( _) {
       self.delegate?.cameraDidFinishExportingVideo(nil)
       return
     }

     time += asset.duration.seconds
   }

   let outputURL = Utilities.getOutputUrl()
   self.exporter = AVAssetExportSession(asset: composition,
                                        presetName: AVAssetExportPresetHighestQuality)
   if let exporter = self.exporter {
     exporter.outputURL = outputURL
     exporter.shouldOptimizeForNetworkUse = true
     exporter.outputFileType = AVFileTypeMPEG4

     exporter.exportAsynchronously {
       if exporter.status == .completed {
         let video = Video(from: outputURL)
         DispatchQueue.main.async {
           self.delegate?.cameraDidFinishExportingVideo(video)
         }
       } else {
         self.delegate?.cameraDidFinishExportingVideo(nil)
       }
     }
   }
 }