如何在Swift 3中为视频添加每秒变化的文本水印？

import AVFoundation

func addTextWatermarkToVideo(inputURL: URL, outputURL: URL, watermarkText: String) {
    let videoAsset = AVURLAsset(url: inputURL)
    let composition = AVMutableComposition()
    
    guard let videoTrack = composition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid),
          let audioTrack = composition.addMutableTrack(withMediaType: .audio, preferredTrackID: kCMPersistentTrackID_Invalid),
          let assetVideoTrack = videoAsset.tracks(withMediaType: .video).first,
          let assetAudioTrack = videoAsset.tracks(withMediaType: .audio).first else {
        return
    }
    
    do {
        try videoTrack.insertTimeRange(CMTimeRangeMake(start: .zero, duration: videoAsset.duration), of: assetVideoTrack, at: .zero)
        try audioTrack.insertTimeRange(CMTimeRangeMake(start: .zero, duration: videoAsset.duration), of: assetAudioTrack, at: .zero)
    } catch {
        return
    }
    
    let videoSize = assetVideoTrack.naturalSize
    
    let videoComposition = AVMutableVideoComposition()
    videoComposition.renderSize = videoSize
    videoComposition.frameDuration = CMTimeMake(value: 1, timescale: 30)
    
    let instruction = AVMutableVideoCompositionInstruction()
    instruction.timeRange = CMTimeRangeMake(start: .zero, duration: videoAsset.duration)
    
    let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: videoTrack)
    instruction.layerInstructions = [layerInstruction]
    
    let textLayer = CATextLayer()
    textLayer.string = watermarkText
    textLayer.font = UIFont.systemFont(ofSize: 24)
    textLayer.fontSize = 24
    textLayer.foregroundColor = UIColor.white.cgColor
    textLayer.alignmentMode = .center
    textLayer.frame = CGRect(x: 0, y: videoSize.height - 100, width: videoSize.width, height: 100)
    
    let overlayLayer = CALayer()
    overlayLayer.addSublayer(textLayer)
    overlayLayer.frame = CGRect(x: 0, y: 0, width: videoSize.width, height: videoSize.height)
    
    let parentLayer = CALayer()
    let videoLayer = CALayer()
    parentLayer.frame = CGRect(x: 0, y: 0, width: videoSize.width, height: videoSize.height)
    videoLayer.frame = CGRect(x: 0, y: 0, width: videoSize.width, height: videoSize.height)
    parentLayer.addSublayer(videoLayer)
    parentLayer.addSublayer(overlayLayer)
    
    videoComposition.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videoLayer, in: parentLayer)
    
    let exportSession = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetHighestQuality)
    exportSession?.outputURL = outputURL
    exportSession?.outputFileType = .mp4
    exportSession?.videoComposition = videoComposition
    
    exportSession?.exportAsynchronously(completionHandler: {
        if exportSession?.status == .completed {
            // 导出成功
        } else {
            // 导出失败
        }
    })
}

let inputURL = URL(fileURLWithPath: "path_to_input_video")
let outputURL = URL(fileURLWithPath: "path_to_output_video")
let watermarkText = "Watermark Text"

addTextWatermarkToVideo(inputURL: inputURL, outputURL: outputURL, watermarkText: watermarkText)