copyNextSampleBuffer hanging for AVAssetReaderTrackOutput - swift

Question

I am trying to create a video compression tool which simply takes in a video file URL, compresses it, and returns the new URL in swift 5 for ios 16.
I have been following a few tutorials online, but they all use functions which have since been deprecated, so I have put together this code here which is a refactoring from various sources:
import Foundation
import AVFoundation
class VideoCompressorModel: ObservableObject {
let bitrate = 2_500_000 // MBPs
func compressFile(urlToCompress: URL, outputURL: URL, completionHandler:#escaping (URL?)->Void) async {
let asset = AVAsset(url: urlToCompress);
//create asset reader
var assetReader: AVAssetReader?
do{
assetReader = try AVAssetReader(asset: asset)
} catch{
assetReader = nil
}
guard let reader = assetReader else{
completionHandler(nil)
return
}
var videoTrack: AVAssetTrack?
var audioTrack: AVAssetTrack?
do {
videoTrack = try await asset.loadTracks(withMediaType: AVMediaType.video).first
audioTrack = try await asset.loadTracks(withMediaType: AVMediaType.audio).first
} catch {
completionHandler(nil)
return
}
guard let videoTrack, let audioTrack else {
completionHandler(nil)
return
}
let videoReaderSettings: [String:Any] = [kCVPixelBufferPixelFormatTypeKey as String:kCVPixelFormatType_32ARGB ]
// ADJUST BIT RATE OF VIDEO HERE
var videoSettings: [String:Any]?
do {
videoSettings = await [
AVVideoCompressionPropertiesKey: [AVVideoAverageBitRateKey:self.bitrate],
AVVideoCodecKey: AVVideoCodecType.h264,
AVVideoHeightKey: try videoTrack.load(.naturalSize).height,
AVVideoWidthKey: try videoTrack.load(.naturalSize).width
]
} catch {
completionHandler(nil)
return
}
guard let videoSettings else {
completionHandler(nil)
return
}
let audioSettings = [
AVSampleRateKey: 44100,
AVFormatIDKey: kAudioFormatLinearPCM
]
let assetReaderVideoOutput = AVAssetReaderTrackOutput(track: videoTrack, outputSettings: videoReaderSettings)
let assetReaderAudioOutput = AVAssetReaderTrackOutput(track: audioTrack, outputSettings: audioSettings)
if reader.canAdd(assetReaderVideoOutput){
reader.add(assetReaderVideoOutput)
}else{
completionHandler(nil)
return
}
if reader.canAdd(assetReaderAudioOutput){
reader.add(assetReaderAudioOutput)
} else{
completionHandler(nil)
return
}
let audioInput = AVAssetWriterInput(mediaType: AVMediaType.audio, outputSettings: nil)
let videoInput = AVAssetWriterInput(mediaType: AVMediaType.video, outputSettings: videoSettings)
do {
videoInput.transform = try await videoTrack.load(.preferredTransform)
} catch{
completionHandler(nil)
return
}
//we need to add samples to the video input
let videoInputQueue = DispatchQueue(label: "videoQueue")
let audioInputQueue = DispatchQueue(label: "audioQueue")
var assetWriter: AVAssetWriter?
do{
assetWriter = try AVAssetWriter(outputURL: outputURL, fileType: AVFileType.mov)
} catch {
assetWriter = nil
}
guard let assetWriter else{
completionHandler(nil)
return
}
assetWriter.shouldOptimizeForNetworkUse = true
assetWriter.add(videoInput)
assetWriter.add(audioInput)
assetWriter.startWriting()
reader.startReading()
assetWriter.startSession(atSourceTime: CMTime.zero)
let closeWriterBothDone:()->Void = {
assetWriter.finishWriting(completionHandler: {
completionHandler((assetWriter.outputURL))
})
reader.cancelReading()
}
print("STARTING ")
let closeWriterAudioDone:()->Void = {
print("FINISHED AUDIO")
videoInput.requestMediaDataWhenReady(on: videoInputQueue) {
while(videoInput.isReadyForMoreMediaData){
let sample = assetReaderVideoOutput.copyNextSampleBuffer()
if (sample != nil){
videoInput.append(sample!)
} else{
videoInput.markAsFinished()
DispatchQueue.main.async {
closeWriterBothDone()
}
break;
}
}
}
}
audioInput.requestMediaDataWhenReady(on: audioInputQueue) {
while(audioInput.isReadyForMoreMediaData){
let sample = assetReaderAudioOutput.copyNextSampleBuffer()
print("hi")
if (sample != nil){
audioInput.append(sample!)
} else{
audioInput.markAsFinished()
DispatchQueue.main.async {
closeWriterAudioDone()
}
break;
}
}
}
}
}
The issue that occurs is that copyNextSampleBuffer for the audio track output hangs after a few calls. I test this simply by having a print("hi") which only gets called a handful of times before the code blocks.
I have tried changing the audio options provided to AVAssetReaderTrackOutput, which I originally had simply as nil, and this only had the effect of increasing the number of times copyNextSampleBuffer gets called before it freezes.
Perhaps there is a better way overall to compress video in newer versions of swift, as this seems somewhat unelegant? If not, is there any bugs in my code that is causing it to hang?