文本覆盖不在输出视频Swift中呈现

Question

我一直试图在10秒的视频上叠加文字。但它输出相同的视频而没有叠加。有人能引导我朝着正确的方向前进吗？代码是示例https://www.raywenderlich.com示例

的直接转换

        let mixComposition = AVMutableComposition()
        let videoTrack = mixComposition.addMutableTrack(withMediaType: AVMediaType.video, preferredTrackID: kCMPersistentTrackID_Invalid)

        let urlpath = Bundle.main.path(forResource: "vidoe1", ofType: ".MOV")
        let url = URL(fileURLWithPath: urlpath!)
        let videoasset = AVAsset.init(url: url)

        do {
            try videoTrack?.insertTimeRange(CMTimeRangeMake(kCMTimeZero, videoasset.duration), of: videoasset.tracks(withMediaType: AVMediaType.video).first!, at: kCMTimeZero)
        }
        catch {

        }
        let mainInstruction = AVMutableVideoCompositionInstruction()
        mainInstruction.timeRange =  CMTimeRangeMake(kCMTimeZero, videoasset.duration)

        let videolayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: videoTrack!)
        mainInstruction.layerInstructions = [videolayerInstruction]

        let mainCompositionInst = AVMutableVideoComposition()
        mainCompositionInst.renderSize = CGSize(width: (videoTrack?.naturalSize.width)!, height: (videoTrack?.naturalSize.height)!)
        mainCompositionInst.instructions = [mainInstruction]
        mainCompositionInst.frameDuration = CMTimeMake(1, 30);


        let text = CATextLayer()
        text.font = UIFont(name: "Helvetica-Bold", size: 30.0)
        text.frame = CGRect(x: 0, y: 200, width: (videoTrack?.naturalSize.width)!, height: 100)
        text.string = "hello world"
        text.foregroundColor = UIColor.black.cgColor
        text.alignmentMode = kCAAlignmentCenter

        let overlaylayer = CALayer()
        overlaylayer.addSublayer(text)
        overlaylayer.frame = CGRect(x: 0, y: 0, width: (videoTrack?.naturalSize.width)!, height: (videoTrack?.naturalSize.height)!)
        overlaylayer.masksToBounds = true

        let parentLayer = CALayer()
        let videoLayer = CALayer()


        parentLayer.frame = CGRect(x: 0, y: 0, width: (videoTrack?.naturalSize.width)!, height: (videoTrack?.naturalSize.height)!)
        videoLayer.frame = CGRect(x: 0, y: 0, width: (videoTrack?.naturalSize.width)!, height: (videoTrack?.naturalSize.height)!)
        parentLayer.addSublayer(videoLayer)
        parentLayer.addSublayer(overlaylayer)


        mainCompositionInst.animationTool = AVVideoCompositionCoreAnimationTool.init(postProcessingAsVideoLayer: overlaylayer, in: parentLayer)

        let documentsPath = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0]
        let path = documentsPath + "FinalVidwdw.mov"
        let urlexport = URL(fileURLWithPath: path)

        let exporter = AVAssetExportSession(asset: mixComposition, presetName: AVAssetExportPresetHighestQuality)

        exporter?.outputURL = urlexport
        exporter?.outputFileType = AVFileType.mov
        exporter?.shouldOptimizeForNetworkUse = true
        exporter?.videoComposition = mainCompositionInst
        exporter?.exportAsynchronously(completionHandler: {

            PHPhotoLibrary.shared().performChanges({
                PHAssetChangeRequest.creationRequestForAssetFromVideo(atFileURL: url)
            }) { 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("bad panda")
                }
            }

        }