         var compositionVideoTrack:AVMutableCompositionTrack!
         let videoTrack = videoAsset.tracks(withMediaType: AVMediaTypeVideo)[0] 
         
-        compositionVideoTrack = self.mixComposition.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: CMPersistentTrackID(kCMPersistentTrackID_Invalid))
+        compositionVideoTrack = mixComposition.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: CMPersistentTrackID(kCMPersistentTrackID_Invalid))
         do {
             try compositionVideoTrack.insertTimeRange(
                 CMTimeRange(start: kCMTimeZero, duration: videoAsset.duration),

        var compositionAudioTrack: AVMutableCompositionTrack!
        let videoTrack = videoAsset.tracks(withMediaType: AVMediaTypeVideo)[0] 
        
        compositionVideoTrack = self.mixComposition.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: 0)
        do {
            try compositionVideoTrack.insertTimeRange(
                CMTimeRange(start: kCMTimeZero, duration: videoAsset.duration),
                of: videoTrack,
                at: self.currentTimeDuration)
        } catch _ {
        }
        compositionVideoTrack.preferredTransform = videoTrack.preferredTransform
        
        compositionAudioTrack = self.mixComposition.addMutableTrack(withMediaType: AVMediaTypeAudio, preferredTrackID: 0)
        do {
            try compositionAudioTrack.insertTimeRange(
                CMTimeRange(start: kCMTimeZero, duration: videoAsset.duration),
                of: videoAsset.tracks(withMediaType: AVMediaTypeAudio)[0] ,
                at: self.currentTimeDuration)
        } catch _ {
        }
        
        self.currentTimeDuration = self.mixComposition.duration
        
        // Add Layer Instruction
        let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: compositionVideoTrack)
        
        // Hide
        layerInstruction.setOpacity(0.0, at: self.currentTimeDuration)
        self.layerInstructions.append(layerInstruction)
        
        return layerInstruction
    }

    public init() {
        
        // AVMutableVideoComposition
        videoComposition.renderSize = CGSize(width: 640, height: 640)
        videoComposition.frameDuration = CMTimeMake(1, 24)
        
    }
    
