guard let assetExport = AVAssetExportSession(asset: mixComposition, presetName: AVAssetExportPresetHighestQuality) else { return nil }
        assetExport.outputFileType = AVFileType.mov
        assetExport.outputURL = mergedAudioVideoUrl!
        assetExport.videoComposition = mainComposition//
        assetExport.exportAsynchronously { () -> Void in