合成未在 AVMutableComposition 中播放第一个视频

Question

我设置了一个包含两个视频的 AVMutableComposition。 第一个只播放音频，第二个播放音频和视频。 我真的被卡住了，无法弄清楚问题是什么。 尝试了多个视频文件，所以我知道内容是问题所在。

真的没有多少内容可以帮助解决这个问题。

let options = [AVURLAssetPreferPreciseDurationAndTimingKey : true]
        let assetA = AVURLAsset.init(url: Bundle.main.url(forResource: "V2", withExtension: "mp4")!, options:options)
        
        let assetB = AVURLAsset.init(url: Bundle.main.url(forResource: "V1", withExtension: "mp4")!, options:options)
        
        let composition = AVMutableComposition()

        let startTimeA = CMTime(seconds: 0.0, preferredTimescale: CMTimeScale(1.0))
        let startTimeB = CMTime(seconds: 50.0, preferredTimescale: CMTimeScale(1.0))

        let compositionTrackA = composition.addMutableTrack(
            withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid)
        let compositionTrackB = composition.addMutableTrack(
            withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid)
        
        let assetTrackA = assetA.tracks(withMediaType: .video).first

        let assetTrackB = assetB.tracks(withMediaType: .video).first

        // First video
        do {
            // 1
            let timeRangeA = CMTimeRange(start: .zero, duration: assetA.duration)
            // 2
            try compositionTrackA!.insertTimeRange(timeRangeA, of: assetTrackA!, at: startTimeA)

            // 3
            if let audioAssetTrackA = assetA.tracks(withMediaType: .audio).first,
               let compositionAudioTrack = composition.addMutableTrack(
                withMediaType: .audio,
                preferredTrackID: kCMPersistentTrackID_Invalid) {
                try compositionAudioTrack.insertTimeRange(
                    timeRangeA,
                    of: audioAssetTrackA,
                    at: startTimeA)
            }
        } catch {
            // 4
            print(error)
        }

        compositionTrackA!.preferredTransform = assetTrackA!.preferredTransform

        // Second video
        do {
            // 1
            let timeRangeB = CMTimeRange(start: .zero, duration: assetB.duration)
            // 2


            try compositionTrackB!.insertTimeRange(timeRangeB, of: assetTrackB!, at: startTimeB)

            // 3
            if let audioAssetTrackB = assetB.tracks(withMediaType: .audio).first,
               let compositionAudioTrackB = composition.addMutableTrack(
                withMediaType: .audio,
                preferredTrackID: kCMPersistentTrackID_Invalid) {
                try compositionAudioTrackB.insertTimeRange(
                    timeRangeB,
                    of: audioAssetTrackB,
                    at: startTimeB)
            }
        } catch {
            // 4
            print(error)
        }

        compositionTrackB!.preferredTransform = assetTrackB!.preferredTransform

        let avplayeritem = AVPlayerItem(asset: composition)
        player = AVPlayer()
        player.replaceCurrentItem(with: avplayeritem)