AvassetWriter:开始写入时暂时滞后

Question

我正在开发一个使用AvassetWriter记录视频的应用程序（源媒体是从captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection)输出的样本缓冲区。

我注意到第一次实例化AVAssetWriter，然后致电startWriting()和startSession(atSourceTime: CMTime)，滞后约为30ms。这会导致屏幕上的视频预览暂时悬挂，这也导致视频的前几帧无法正确写入。

奇怪的是，如果我然后重新确定我的AVAssetWriter并再次采取所有相同的步骤，那么从那时起，一切正常。这仅是第一次发生。

虽然也许太长而无法发布，但这是我确定发生错误的功能；

func insert(pixel buffer: CVPixelBuffer, with time: CMTime) {
    // Check for unknown status
    if fileWriter.status == .unknown {
        guard startingVideoTime == nil else {
            print("We've received an error setting the starting video time.")
            return
        }
        startingVideoTime = time
        if fileWriter.startWriting() {
            fileWriter.startSession(atSourceTime: startingVideoTime!)
            isRecording = true
        }
    }
    // Append buffer
    if videoInput.isReadyForMoreMediaData {
        append(pixel: buffer, with: time)
        isRecording = true
    }
}

对于后代，我是从我的captureOutput调用上述功能，仅在用户点击"开始记录"按钮时设置的self.isRecording = true时，我才能进行此功能。

Answer 1

let videoCompressionSettings: [String: Any] = [
  AVVideoCodecKey: AVVideoCodecType.h264,
  AVVideoWidthKey: NSNumber(value: 1080),
  AVVideoHeightKey: NSNumber(value: 1920)
]
let audioCompressionSettings: [String: Any] = [
  AVNumberOfChannelsKey: NSNumber(value: 1),
  AVEncoderAudioQualityForVBRKey: NSNumber(value: 91),
  AVEncoderBitRatePerChannelKey:NSNumber(value: 9600),
  AVEncoderBitRateStrategyKey: AVAudioBitRateStrategy_Variable,
  AVFormatIDKey: NSNumber(value: 1633772320),
  AVSampleRateKey: NSNumber(value: 44100)
]
// My own wrapper for AVAssetWriter
movieWriterManager = MovieWriterManager(videoUrl: recordingVideoURL(), audioUrl: recordingAudioURL(), videoCompressionSettings: videoCompressionSettings, audioCompressionSettings: audioCompressionSettings)
movieWriterManager?.warmup()

您可以尝试通过在AppDelegate的DodFinishLaunchingWithOptions上运行相同的步骤来购买时间，并使用默认的VideoCompressionsettings和AudioCompressionsettings。它实际上不是阻止UI线程，而是仅阻止AvcaptureVideDataOutput的视频输出连接。