要解决AVAssetWriter的第一帧要么是空白的，要么是黑色的问题，可以通过以下代码示例来处理:

1. 首先，创建一个AVAssetWriter对象和一个AVAssetWriterInput对象，并配置它们以写入视频数据:

guard let videoOutputURL = // 设置输出URL
    let assetWriter = try? AVAssetWriter(url: videoOutputURL, fileType: AVFileType.mov),
    let assetWriterInput = AVAssetWriterInput(mediaType: AVMediaType.video, outputSettings: videoSettings) else {
        return
}

// 配置AVAssetWriterInput
assetWriterInput.expectsMediaDataInRealTime = true

// 添加AVAssetWriterInput到AVAssetWriter
assetWriter.add(assetWriterInput)

2. 在开始写入数据之前，使用AVAssetWriterInputPixelBufferAdaptor来创建一个CVPixelBufferPool和一个CVPixelBuffer:

let pixelBufferAdaptor = AVAssetWriterInputPixelBufferAdaptor(assetWriterInput: assetWriterInput, sourcePixelBufferAttributes: nil)

// 创建一个CVPixelBufferPool
let pixelBufferAttributes: [String: Any] = [
    kCVPixelBufferPixelFormatTypeKey as String: kCVPixelFormatType_32ARGB,
    kCVPixelBufferWidthKey as String: videoSize.width,
    kCVPixelBufferHeightKey as String: videoSize.height
]
var pixelBufferPool: CVPixelBufferPool?
CVPixelBufferPoolCreate(nil, nil, pixelBufferAttributes as CFDictionary, &pixelBufferPool)

// 创建一个CVPixelBuffer
var pixelBuffer: CVPixelBuffer?
CVPixelBufferPoolCreatePixelBuffer(nil, pixelBufferPool!, &pixelBuffer)

3. 在开始写入数据之前，先将第一帧的CVPixelBuffer填充为纯黑色:

if let pixelBuffer = pixelBuffer {
    CVPixelBufferLockBaseAddress(pixelBuffer, [])
    let data = CVPixelBufferGetBaseAddress(pixelBuffer)
    let bytesPerRow = CVPixelBufferGetBytesPerRow(pixelBuffer)
    let width = CVPixelBufferGetWidth(pixelBuffer)
    let height = CVPixelBufferGetHeight(pixelBuffer)
    let context = CGContext(data: data, width: width, height: height, bitsPerComponent: 8, bytesPerRow: bytesPerRow, space: CGColorSpaceCreateDeviceRGB(), bitmapInfo: CGImageAlphaInfo.noneSkipFirst.rawValue)
    context?.setFillColor(UIColor.black.cgColor)
    context?.fill(CGRect(x: 0, y: 0, width: width, height: height))
    CVPixelBufferUnlockBaseAddress(pixelBuffer, [])
}

// 在开始写入数据之前，将第一帧的CVPixelBuffer添加到AVAssetWriterInput
if let pixelBuffer = pixelBuffer {
    pixelBufferAdaptor.append(pixelBuffer, withPresentationTime: kCMTimeZero)
}

4. 现在可以开始将实际的视频帧写入AVAssetWriterInput了。在写入视频帧之前，需要将每个视频帧添加到CVPixelBuffer中，并将其添加到AVAssetWriterInput中:

func writeVideoFrame(sampleBuffer: CMSampleBuffer) {
    if let pixelBuffer = pixelBufferAdaptor.pixelBuffer(forAppendTo: assetWriterInput, withPresentationTime: CMSampleBufferGetPresentationTimeStamp(sampleBuffer)) {
        // 将sampleBuffer的图像数据添加到pixelBuffer中
        // 这一步根据你的具体需求来实现，可以使用CVPixelBuffer的各种方法来处理图像数据
        // 可以使用Core Graphics、Core Image、Metal等框架来处理图像数据
        // ...
        
        // 将pixelBuffer添加到AVAssetWriterInput
        pixelBufferAdaptor.append(pixelBuffer, withPresentationTime: CMSampleBufferGetPresentationTimeStamp(sampleBuffer))
    }
}

通过以上步骤，可以确保AVAssetWriter的第一帧不再是空白的或者黑色的。