如何阅读时间码轨道与AVAsset和斯威夫特3？

Question

我想阅读时间码轨道的时间价值。有一个从苹果公司（见Technical Note 2310）一个优秀的文档，但它是用Objective C.

我已经翻译的核心逻辑夫特3.它的工作原理完全一样ObjC版本，这意味着从一个时间代码轨道上的CMSampleBuffer被读出并转化成一个CMBlockBuffer。当我创建了数据指针CMBlockBufferGetDataPointer（在timecodeFrame() FUNC），这意味着它失败，对原始数据总是给我0帧。所以它归结到一个问题，我怎么正确地处理的原始数据？

import Foundation
import AVFoundation
import CoreMedia

let movie = URL(fileURLWithPath: "videoWithTimecodeTrack.mov")
let asset = AVAsset(url: movie)

asset.loadValuesAsynchronously(forKeys: ["tracks"]) {

    var error: NSError?
    guard asset.statusOfValue(forKey: "tracks", error: &error) == AVKeyValueStatus.loaded
        else { if let error = error { return print(error) } }

    readStartTimecode(asset: asset)
}


func readStartTimecode(ofAsset asset: AVAsset) {

    let timecodeTracks = asset.tracks(withMediaType: AVMediaTypeTimecode)
    guard let timecodeTrack = timecodeTracks.first,
        let assetReader = try? AVAssetReader(asset: asset) else { return }

    let readerOutput = AVAssetReaderTrackOutput(track: timecodeTrack, outputSettings: nil)
    assetReader.add(readerOutput)
    guard assetReader.startReading() else { return }

    while let sampleBuffer = readerOutput.copyNextSampleBuffer() {
        if let frame = timecodeFrame(sampleBuffer: sampleBuffer) {
            print("timecodeFrame: \(frame)")
        }
    }
}

func timecodeFrame(sampleBuffer: CMSampleBuffer) -> UInt32? {

    guard let blockBuffer = CMSampleBufferGetDataBuffer(sampleBuffer),
        let formatDescription = CMSampleBufferGetFormatDescription(sampleBuffer)
        else { return nil }

    var rawData: UnsafeMutablePointer<Int8>? = nil
    var length: Int = 0
    var totalLength: Int = 0

    let status = CMBlockBufferGetDataPointer(blockBuffer, 0, &length, &totalLength, &rawData)
    guard status == kCMBlockBufferNoErr,
        let frameRead = rawData?.pointee
        else { return nil }

    let type = CMFormatDescriptionGetMediaSubType(formatDescription)

    if type == kCMTimeCodeFormatType_TimeCode32 {
        let frame = UInt32(frameRead)
        let bigFrame = CFSwapInt32BigToHost(frame)
        print("kCMTimeCodeFormatType_TimeCode32: \(bigFrame)")
    }
    if type == kCMTimeCodeFormatType_TimeCode64 {
        print("kCMTimeCodeFormatType_TimeCode64")
        // todo
    }
    return nil
}    

编辑：数据指针检索的目标C版本是这样的：

size_t length = 0;
size_t totalLength = 0;
char *rawData = NULL;

CMBlockBufferGetDataPointer(blockBuffer, 0, &length, &totalLength, &rawData);
if (status == kCMBlockBufferNoErr) {
    int32_t *frameNumberRead = (int32_t *)rawData;
    (int)Endian32_Swap(*frameNumberRead)]
}   

Answer 1

该解决方案是不Int8数据转换等UInt32(rawData.pointee)但访问UnsafeMutablePointer<Int8>指针的存储器作为不同类型的（暂时）。这应该是这样的：

if let frames = rawData?.withMemoryRebound(to: UInt32.self, capacity: 1, { CFSwapInt32BigToHost($0.pointee) }) {
    return frames
}

完整的功能是这样的：

func timecodeFrame(sampleBuffer: CMSampleBuffer) -> UInt32? {

    guard let blockBuffer = CMSampleBufferGetDataBuffer(sampleBuffer),
        let formatDescription = CMSampleBufferGetFormatDescription(sampleBuffer)
        else { return nil }

    var rawData: UnsafeMutablePointer<Int8>? = nil
    var length: Int = 0
    var totalLength: Int = 0

    let status = CMBlockBufferGetDataPointer(blockBuffer, 0, &length, &totalLength, &rawData)
    guard status == kCMBlockBufferNoErr else { return nil }

    let type = CMFormatDescriptionGetMediaSubType(formatDescription)

    if type == kCMTimeCodeFormatType_TimeCode32 {
        if let frames = rawData?.withMemoryRebound(to: UInt32.self, capacity: 1, { CFSwapInt32BigToHost($0.pointee) }) {
            return frames
        }
    }
    if type == kCMTimeCodeFormatType_TimeCode64 {
        if let frames = rawData?.withMemoryRebound(to: UInt64.self, capacity: 1, { CFSwapInt64BigToHost($0.pointee) }) {
            return UInt32(frames)
        }
    }
    return nil
}

我希望这是对别人谁想要读取的视频的时间码轨道的起始时间码是有用的。