pixelBufferAdaptor.pixelBufferPool总是为零

Question

我正在使用this SO answer的代码段

但是总是得到pixelBufferAdaptor.pixelBufferPool == nil，我该如何解决？

```

func writeImagesAsMovie(allImages: [UIImage], videoPath: String, videoSize: CGSize, videoFPS: Int32) {
    // Create AVAssetWriter to write video
    guard let assetWriter = createAssetWriter(path: videoPath, size: videoSize) else {
        print("Error converting images to video: AVAssetWriter not created")
        return
    }

    // If here, AVAssetWriter exists so create AVAssetWriterInputPixelBufferAdaptor
    let writerInput = assetWriter.inputs.filter { $0.mediaType == AVMediaType.video }.first!
    let sourceBufferAttributes: [String: AnyObject] = [
        kCVPixelBufferPixelFormatTypeKey as String: Int(kCVPixelFormatType_32ARGB) as AnyObject,
        kCVPixelBufferWidthKey as String: videoSize.width as AnyObject,
        kCVPixelBufferHeightKey as String: videoSize.height as AnyObject
    ]
    let pixelBufferAdaptor = AVAssetWriterInputPixelBufferAdaptor(assetWriterInput: writerInput, sourcePixelBufferAttributes: sourceBufferAttributes)


    if assetWriter.canAdd(writerInput) {
        assetWriter.add(writerInput)
    } else {
        print("Cannot add asset writer")
    }

    // Start writing session
    assetWriter.startWriting()
    assetWriter.startSession(atSourceTime: kCMTimeZero)
    if pixelBufferAdaptor.pixelBufferPool == nil {
        print("Error converting images to video: pixelBufferPool nil after starting session")
        return
    }
}

func createAssetWriter(path: String, size: CGSize) -> AVAssetWriter? {
    // Convert <path> to NSURL object
    let pathURL = URL.init(fileURLWithPath: path)

    // Return new asset writer or nil
    do {
        // Create asset writer
        guard let newWriter = try? AVAssetWriter(outputURL: pathURL, fileType: AVFileType.mp4) else {
            fatalError("AVAssetWriter error")
        }

        // Define settings for video input
        let videoSettings: [String: AnyObject] = [
            AVVideoCodecKey: AVVideoCodecType.h264 as AnyObject,
            AVVideoWidthKey: size.width as AnyObject,
            AVVideoHeightKey: size.height as AnyObject
        ]

        // Add video input to writer
        let assetWriterVideoInput = AVAssetWriterInput(mediaType: AVMediaType.video, outputSettings: videoSettings)
        newWriter.add(assetWriterVideoInput)

        // Return writer
        print("Created asset writer for \(size.width)x\(size.height) video")
        return newWriter
    } catch {
        print("Error creating asset writer: \(error)")
        return nil
    }
}


stream.writeImagesAsMovie(allImages: stream.imageArray, videoPath: "video.mp4",
                          videoSize: stream.imageArray[0].size, videoFPS: 10)

```