Swift：从 url 检索音频文件标记列表？

我只想获取音频文件中的标记列表。我认为这将是一项简单的常见任务，不会太困难。但是，我几乎找不到任何示例代码或文档，所以我最终得到了以下结果：

private func getMarkers(_ url: CFURL) -> AudioFileMarkerList {

  var file: AudioFileID?
  var size: UInt32 = 0
  var markers = AudioFileMarkerList()

  AudioFileOpenURL(url, .readPermission, kAudioFileWAVEType, &file)
  AudioFileGetPropertyInfo(file!, kAudioFilePropertyMarkerList, &size, nil)
  AudioFileGetProperty(file!, kAudioFilePropertyMarkerList, &size, &markers)

  return markers
}

不幸的是，这不起作用：error: memory read failed for 0x0.

我就是想不出问题所在。我检查了网址和大小（两者都是有效的），但它总是无法检索标记。任何对此的帮助都会很棒！

EDIT:这种方法可行，但所有数据都是完全错误的，我无法理解单个音频文件如何具有多个 AudioFileMarkerLists 标记：

private func getMarkers(_ url: CFURL) -> [AudioFileMarkerList] {

  var file: AudioFileID?
  var size: UInt32 = 0

  AudioFileOpenURL(url, .readPermission, kAudioFileWAVEType, &file)
  AudioFileGetPropertyInfo(file!, kAudioFilePropertyMarkerList, &size, nil)

  let length = NumBytesToNumAudioFileMarkers(Int(size))
  var markers = [AudioFileMarkerList](repeating: AudioFileMarkerList(), count: length)
  AudioFileGetProperty(file!, kAudioFilePropertyMarkerList, &size, &markers)
  return markers
}

EDIT 2:根据到目前为止我看到的大多数答案，这应该可行，但它返回一个空数组：

private func getMarkers(_ url: CFURL) -> [AudioFileMarkerList] {

  var file: AudioFileID?
  var size: UInt32 = 0

  AudioFileOpenURL(url, .readPermission, kAudioFileWAVEType, &file)
  AudioFileGetPropertyInfo(file!, kAudioFilePropertyMarkerList, &size, nil)
  let length = NumBytesToNumAudioFileMarkers(Int(size))

  var markers = [AudioFileMarkerList]()
  markers.reserveCapacity(length)
  AudioFileGetProperty(file!, kAudioFilePropertyMarkerList, &size, &markers)

  return markers

}

EDIT 3:我从 Ryan 的代码中删除了一堆错误检查和有用的内容，供任何想要快速尝试找到问题的人使用：

private func getMarkers(_ url: CFURL) -> [AudioFileMarker]? {

    var file: AudioFileID?
    var size: UInt32 = 0
    var markers: [AudioFileMarker] = []

    AudioFileOpenURL(url, .readPermission, kAudioFileWAVEType, &file)

    AudioFileGetPropertyInfo(file!, kAudioFilePropertyMarkerList, &size, nil)

    let length = NumBytesToNumAudioFileMarkers(Int(size))

    let data = UnsafeMutablePointer<AudioFileMarkerList>.allocate(capacity: length)

    AudioFileGetProperty(file!, kAudioFilePropertyMarkerList, &size, data)

    markers.append(data.pointee.mMarkers)

    data.deallocate(capacity: length)

    return markers
}

我只是希望苹果一开始就测试了 AudioFileMarkerList。

EDIT 4:感谢 Rhythmic Fistman 和 Ryan Francesconi 的帮助解决了！最后结果：

private func getMarkers(_ url: CFURL) -> [AudioFileMarker]? {

  var file: AudioFileID?
  var size: UInt32 = 0
  var markerList: [AudioFileMarker] = []

  AudioFileOpenURL(url, .readPermission, kAudioFileWAVEType, &file)

  AudioFileGetPropertyInfo(file!, kAudioFilePropertyMarkerList, &size, nil)

  let length = NumBytesToNumAudioFileMarkers(Int(size))

  let data = UnsafeMutablePointer<AudioFileMarkerList>.allocate(capacity: length)

  AudioFileGetProperty(file!, kAudioFilePropertyMarkerList, &size, data)

  let markers = UnsafeBufferPointer<AudioFileMarker>(start: &data.pointee.mMarkers, count: length)
  for marker in markers {
    markerList.append(marker)
  }

  data.deallocate(capacity: length)

  return markerList
}

看起来你需要使用UnsafeBufferPointer访问可变长度数组（例如mMarkers）。所以而不是

out.append(markerList.mMarkers)

只添加第一个元素，执行此操作

let markersBuffer = UnsafeBufferPointer<AudioFileMarker>(start: &data.pointee.mMarkers,
                                               count: Int(data.pointee.mNumberMarkers))

for marker in markersBuffer {
    markers.append(marker)
}

仿照这个答案 https://stackoverflow.com/a/27061639/22147