Swift: Retrieve audio file marker list from url?

Question

I just want to get a list of the markers in an audio file. I thought this would be an easy common task that wouldn't be too difficult. However, I can barely find any example code or documentation, so I ended up with this:

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
}

Sadly, this doesn't work: error: memory read failed for 0x0.

I just can't figure out the problem. I checked the url and the size (which are both valid), but it always fails to retrieve the markers. Any help with this would be fantastic!

EDIT: This sort of works, but all the data is completely wrong, and I can't understand how a single audio file can have multiple AudioFileMarkerLists of markers:

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: According to most answers I've seen so far, this should work, but it returns an empty array:

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: I got rid of a bunch of error checking and useful stuff from Ryan's code for anyone wanting to quickly try and find the problem:

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
}

I just hope Apple actually tested AudioFileMarkerList in the first place.

EDIT 4: SOLVED thanks to Rhythmic Fistman and Ryan Francesconi! Final result:

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
}

Answer 1

Looks like you need to use UnsafeBufferPointer to access variable length arrays (like mMarkers). So instead of

out.append(markerList.mMarkers)

which only adds the first element, do this

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

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

Modeled on this answer

Answer 2

EDIT: Simplest solution is to use AudioKit's version of EZAudioFile.markers. Note this is not the same as the original EZAudio framework as I had added this marker code to AudioKit's version only.

import AudioKit
...

if let file = EZAudioFile(url: url) {
    if let markers = file.markers as? [EZAudioFileMarker] {
        for m in markers {
            Swift.print("NAME: \(m.name) FRAME: \(m.framePosition)")
        }
    }
}

If you REALLY want to try in Swift, it would look something like the below. I'm not an expert in this, but as far as I can tell, there is some issue translating the AudioFileMarkerList struct to Swift. This may be solvable, but it seems to me it's best to just use Objective C to accomplish these calls. Here is the almost finished function in Swift. I recommend using AudioKit to accomplish what you need as I have added the marker code to EZAudioFile there. Check: https://github.com/AudioKit/AudioKit/blob/master/AudioKit/Common/Internals/EZAudio/EZAudioFile.m

But for the record here is the Swift code in progress! Note it's hard coded to WAVE files for the moment... Perhaps someone else can finish this?

class func getAudioFileMarkers(_ url: URL) -> [AudioFileMarker]? {
    Swift.print("getAudioFileMarkers() \(url)")

    var err: OSStatus = noErr
    var audioFileID: AudioFileID?

    err = AudioFileOpenURL(url as CFURL,
                           .readPermission,
                           kAudioFileWAVEType,
                           &audioFileID)

    if err != noErr {
        Swift.print("AudioFileOpenURL FAILED, Error: \(err)")
        return nil
    }

    guard audioFileID != nil else {
        return nil
    }

    Swift.print("audioFileID: \(audioFileID)")

    var outSize: UInt32 = 0
    var writable: UInt32 = 0

    err = AudioFileGetPropertyInfo(audioFileID!, kAudioFilePropertyMarkerList, &outSize, &writable)
    if err != noErr {
        Swift.print("AudioFileGetPropertyInfo kAudioFilePropertyMarkerList FAILED, Error: \(err)")
        return nil
    }

    Swift.print("outSize: \(outSize), writable: \(writable)")

    guard outSize != 0 else { return nil }

    let length = NumBytesToNumAudioFileMarkers( Int(outSize) )

    Swift.print("Found \(length) markers")

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

    if length == 0 {
        return nil
    }

    // pull marker list
    err = AudioFileGetProperty(audioFileID!, kAudioFilePropertyMarkerList, &outSize, theData)

    if err != noErr {
        Swift.print("AudioFileGetProperty kAudioFilePropertyMarkerList FAILED, Error: \(err)")
        return nil
    }

    let markerList: AudioFileMarkerList = theData.pointee

    Swift.print("markerList.mMarkers: \(markerList.mMarkers)")
    // this is only showing up as a single AudioFileMarker, not an array of them.
    // I DON'T KNOW WHY. It works in Obj-C. I'm obviously missing something, or there is a problem in translation

    var out = [AudioFileMarker]()

    let mirror = Mirror(reflecting: markerList.mMarkers)
    for m in mirror.children {
        Swift.print( "label: \(m.label) value: \(m.value)" )
    }

    // for now just append the first one.
    // :(
    out.append(markerList.mMarkers)

    // done with this now
    theData.deallocate(capacity: length)

    return out
}