Swift 3 : How to export video with text using AVVideoComposition

I am trying to useAVVideoComposition to add some text on top of a video and save the video. This is the code I use:

I Create an AVMutableComposition and AVVideoComposition

var mutableComp =          AVMutableComposition()
var mutableVidComp =       AVMutableVideoComposition()
var compositionSize :      CGSize?

func configureAsset(){

    let options =               [AVURLAssetPreferPreciseDurationAndTimingKey : "true"]
    let videoAsset =             AVURLAsset(url: Bundle.main.url(forResource: "Car", withExtension: "mp4")! , options : options)
    let videoAssetSourceTrack =  videoAsset.tracks(withMediaType: AVMediaTypeVideo).first! as AVAssetTrack

    compositionSize = videoAssetSourceTrack.naturalSize

    let mutableVidTrack =       mutableComp.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: kCMPersistentTrackID_Invalid)
    let trackRange =            CMTimeRangeMake(kCMTimeZero, videoAsset.duration)

    do {
        try mutableVidTrack.insertTimeRange( trackRange, of: videoAssetSourceTrack, at: kCMTimeZero)

        mutableVidTrack.preferredTransform = videoAssetSourceTrack.preferredTransform

    }catch { print(error) }

    snapshot =       mutableComp
    mutableVidComp = AVMutableVideoComposition(propertiesOf: videoAsset)
 }

II Setup the layers

  func applyVideoEffectsToComposition()   {

    // 1 - Set up the text layer
    let subTitle1Text =            CATextLayer()
    subTitle1Text.font =           "Helvetica-Bold" as CFTypeRef
    subTitle1Text.frame =           CGRect(x: self.view.frame.midX - 60 , y: self.view.frame.midY - 50, width: 120, height: 100)
    subTitle1Text.string =         "Bench"
    subTitle1Text.foregroundColor = UIColor.black.cgColor
    subTitle1Text.alignmentMode =   kCAAlignmentCenter

    // 2 - The usual overlay
    let overlayLayer = CALayer()
    overlayLayer.addSublayer(subTitle1Text)
    overlayLayer.frame = CGRect(x: 0, y: 0, width: compositionSize!.width, height: compositionSize!.height)
    overlayLayer.masksToBounds = true


    // 3 - set up the parent layer
    let parentLayer =   CALayer()
    let videoLayer =    CALayer()
    parentLayer.frame = CGRect(x: 0, y: 0, width: compositionSize!.width, height: compositionSize!.height)
    videoLayer.frame =  CGRect(x: 0, y: 0, width: compositionSize!.width, height: compositionSize!.height)

    parentLayer.addSublayer(videoLayer)
    parentLayer.addSublayer(overlayLayer)

    mutableVidComp.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videoLayer, in: parentLayer)

 }

III . Save video with AVMutbaleVideoComposition

func saveAsset (){

    func deleteFile(_ filePath:URL) {

        guard FileManager.default.fileExists(atPath: filePath.path) else { return }

        do {
            try    FileManager.default.removeItem(atPath: filePath.path) }
        catch {fatalError("Unable to delete file: \(error) : \(#function).")} }


    let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0] as URL
    let filePath =           documentsDirectory.appendingPathComponent("rendered-audio.mp4")
    deleteFile(filePath)

    if let exportSession = AVAssetExportSession(asset: mutableComp , presetName: AVAssetExportPresetHighestQuality){

        exportSession.videoComposition = mutableVidComp

        //  exportSession.canPerformMultiplePassesOverSourceMediaData = true
        exportSession.outputURL =                   filePath
        exportSession.shouldOptimizeForNetworkUse = true
        exportSession.timeRange =                   CMTimeRangeMake(kCMTimeZero, mutableComp.duration)
        exportSession.outputFileType =              AVFileTypeQuickTimeMovie



        exportSession.exportAsynchronously {
            print("finished: \(filePath) :  \(exportSession.status.rawValue) ")

            if exportSession.status.rawValue == 4 {

                print("Export failed -> Reason: \(exportSession.error!.localizedDescription))")
                print(exportSession.error!)

            }

        }

    }

}

Then I run all three methods in the viewDidLoad method for a quick test. The problem is that when I run the app ,the result of the export is the original video without the title on it.

What am I missing here?

UPDATE

I notice that adding a subTitle1Text.backgroundColor property in part II of the code makes a colored CGRect corresponding to subTitle1Text.frame appear on top of the video when exported.

(See Image)

When this code is modified for playback using AVSynchronizedLayer the desired layer can be seen on top of the video with text on it. So perhaps this is a bug in AVFoundation itself.

I suppose I am only left with the option of using a customVideoCompositorClass. The problem with that is that it takes a lot of time to render the video . Here is an example that uses AVVideoCompositing