为什么在快速合并两个视频或更改背景音乐后视频变成黑屏？

在我的 ios 应用程序中，我想合并两个视频并更改背景音乐。我尝试过，对于普通视频来说效果很好。但是，当我选择任何延时视频，然后尝试合并或更改背景音乐时，视频变成全黑屏。

对于我的应用程序，我使用 swift 4.2 和 xcode-10。我也在 swift 4 和 swift 5 中尝试过，都返回相同的结果。

这是我的代码：

class Export: NSObject {

    let defaultSize = CGSize(width: 1920, height: 1920)
    typealias Completion = (URL?, Error?) -> Void

    func mergeVideos(arrayVideos:[URL], exportURL: URL, completion:@escaping Completion) -> Void {

        var errors: Error!
        var insertTime = kCMTimeZero
        var arrayLayerInstructions:[AVMutableVideoCompositionLayerInstruction] = []
        var outputSize = CGSize(width: 0, height: 0)

        // Determine video output size
        for url in arrayVideos {

            let videoAsset = AVAsset(url: url)
            let videoTrack = videoAsset.tracks(withMediaType: AVMediaType.video)[0]

            var videoSize = videoTrack.naturalSize.applying(videoTrack.preferredTransform)

            videoSize.width = fabs(videoSize.width)
            videoSize.height = fabs(videoSize.height)

            if outputSize.height == 0 || videoSize.height > outputSize.height {
                outputSize.height = videoSize.height
            }

            if outputSize.width == 0 || videoSize.width > outputSize.width {
                outputSize.width = videoSize.width
            }
        }

        // Silence sound (in case of video has no sound track)
        guard let silenceURL = Bundle.main.url(forResource: "silence", withExtension: "mp3") else { completion(nil, errors); return }
        let silenceAsset = AVAsset(url:silenceURL)
        let silenceSoundTrack = silenceAsset.tracks(withMediaType: AVMediaType.audio).first

        // Init composition
        let mixComposition = AVMutableComposition.init()

        for url in arrayVideos {

            let videoAsset = AVAsset(url: url)
            // Get video track
            guard let videoTrack = videoAsset.tracks(withMediaType: AVMediaType.video).first else {

                print("video asset track not found")
                continue
            }

            // Get audio track
            var audioTrack:AVAssetTrack?
            if videoAsset.tracks(withMediaType: AVMediaType.audio).count > 0 {
                audioTrack = videoAsset.tracks(withMediaType: AVMediaType.audio).first
            }
            else {
                audioTrack = silenceSoundTrack
            }

            // Init video & audio composition track
            guard let videoCompositionTrack = mixComposition.addMutableTrack(withMediaType: AVMediaType.video, preferredTrackID: Int32(kCMPersistentTrackID_Invalid)) else { completion(nil, errors); return }

            guard let audioCompositionTrack = mixComposition.addMutableTrack(withMediaType: AVMediaType.audio, preferredTrackID: Int32(kCMPersistentTrackID_Invalid)) else { completion(nil, errors); return }

            do {
                let startTime = kCMTimeZero
                let duration = videoAsset.duration

                // Add video track to video composition at specific time
                try videoCompositionTrack.insertTimeRange(CMTimeRangeMake(startTime, duration),
                                                           of: videoTrack,
                                                           at: insertTime)

                // Add audio track to audio composition at specific time
                if let audioTrack = audioTrack {
                    try audioCompositionTrack.insertTimeRange(CMTimeRangeMake(startTime, duration),
                                                               of: audioTrack,
                                                               at: insertTime)
                }

                // Add instruction for video track
                let layerInstruction = videoCompositionInstructionForTrack(track: videoCompositionTrack, asset: videoAsset, standardSize: outputSize,  atTime: insertTime)

                // Hide video track before changing to new track
                let endTime = CMTimeAdd(insertTime, duration)
                layerInstruction.setOpacity(0, at: endTime)
                arrayLayerInstructions.append(layerInstruction)

                // Increase the insert time
                insertTime = CMTimeAdd(insertTime, duration)
            }
            catch {
                print("Load track error")
            }
        }

        // Main video composition instruction
        let mainInstruction = AVMutableVideoCompositionInstruction()
        mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, insertTime)
        mainInstruction.layerInstructions = arrayLayerInstructions

        // Main video composition
        let mainComposition = AVMutableVideoComposition() 
        mainComposition.instructions = [mainInstruction]
        mainComposition.frameDuration = CMTimeMake(1, 30)
        mainComposition.renderSize = outputSize

        // Init exporter
        guard let exporter = AVAssetExportSession.init(asset: mixComposition, presetName: AVAssetExportPresetHighestQuality) else {

            errors = "exporter initialization failed" as? Error
            completion(nil, errors)
            return
        }
        exporter.outputURL = exportURL
        exporter.outputFileType = AVFileType.mov
        exporter.shouldOptimizeForNetworkUse = true
        exporter.videoComposition = mainComposition

        // Do export
        exporter.exportAsynchronously(completionHandler: {


        })

    }
}

// MARK:- Private methods
extension Export {
    fileprivate func orientationFromTransform(transform: CGAffineTransform) -> (orientation: UIImageOrientation, isPortrait: Bool) {
        var assetOrientation = UIImageOrientation.up
        var isPortrait = false
        if transform.a == 0 && transform.b == 1.0 && transform.c == -1.0 && transform.d == 0 {
            assetOrientation = .right
            isPortrait = true
        } else if transform.a == 0 && transform.b == -1.0 && transform.c == 1.0 && transform.d == 0 {
            assetOrientation = .left
            isPortrait = true
        } else if transform.a == 1.0 && transform.b == 0 && transform.c == 0 && transform.d == 1.0 {
            assetOrientation = .up
        } else if transform.a == -1.0 && transform.b == 0 && transform.c == 0 && transform.d == -1.0 {
            assetOrientation = .down
        }
        return (assetOrientation, isPortrait)
    }

    fileprivate func videoCompositionInstructionForTrack(track: AVCompositionTrack, asset: AVAsset, standardSize:CGSize, atTime: CMTime) -> AVMutableVideoCompositionLayerInstruction {

        let instruction = AVMutableVideoCompositionLayerInstruction(assetTrack: track)
        let assetTrack = asset.tracks(withMediaType: AVMediaType.video)[0]
        let assetSize = assetTrack.naturalSize

        let transform = assetTrack.preferredTransform
        let assetInfo = orientationFromTransform(transform: transform)

        let aspectFillRatio:CGFloat = 1

        if assetInfo.isPortrait {

            let scaleFactor = CGAffineTransform(scaleX: aspectFillRatio, y: aspectFillRatio)
            let posX = standardSize.width/2 - (assetSize.height * aspectFillRatio)/2
            let posY = standardSize.height/2 - (assetSize.width * aspectFillRatio)/2
            let moveFactor = CGAffineTransform(translationX: posX, y: posY)

            instruction.setTransform(assetTrack.preferredTransform.concatenating(scaleFactor).concatenating(moveFactor), at: atTime)

        } else {
            let scaleFactor = CGAffineTransform(scaleX: aspectFillRatio, y: aspectFillRatio)
            let posX = standardSize.width/2 - (assetSize.width * aspectFillRatio)/2
            let posY = standardSize.height/2 - (assetSize.height * aspectFillRatio)/2
            let moveFactor = CGAffineTransform(translationX: posX, y: posY)
            var concat = assetTrack.preferredTransform.concatenating(scaleFactor).concatenating(moveFactor)

            if assetInfo.orientation == .down {
                let fixUpsideDown = CGAffineTransform(rotationAngle: CGFloat(Double.pi))
                concat = fixUpsideDown.concatenating(scaleFactor).concatenating(moveFactor)
            }

            instruction.setTransform(concat, at: atTime)
        }
        return instruction
    }
}

我预计延时视频会像普通视频一样工作并且不会出现黑屏

// 主视频合成指令

将此代码替换为以下代码

    let instruction = AVMutableVideoCompositionInstruction()
    instruction.timeRange = CMTimeRangeMake(start: CMTime.zero, duration: mutableComposition.duration)

    let videotrack = mutableComposition.tracks(withMediaType: AVMediaType.video)[0] as AVAssetTrack
    let layerinstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: videotrack)
    
    let rgb = CGColorSpaceCreateDeviceRGB()
    let myColor : [CGFloat] = [1.0, 1.0, 1.0, 1.0] //white
    let ref = CGColor(colorSpace: rgb, components: myColor)
    instruction.backgroundColor = ref

    
    instruction.layerInstructions = NSArray(object: layerinstruction) as [AnyObject] as! [AVVideoCompositionLayerInstruction]
    videoComposition.instructions = [instruction]