How to Play Audio Using AVAudioEngine and AVAudioPlayerNode in Swift?

Question

I'm trying to play audio using AVAudioEngine and AVAudioPlayerNode. However, in "engine.connect...", I get this:

"PlaySound[21867:535102] [AudioHAL_Client] AudioHardware.cpp:666:AudioObjectGetPropertyData: AudioObjectGetPropertyData: no object with given ID 0"

The code after "engine.connect..." seems to run because it prints "buffer?.format", but there's no sound.

Could someone let me know what I'm missing? Here's my testing code.

let audioNode = AVAudioPlayerNode()
let path = Bundle.main.path(forResource: "Sounds/Test.mp3", ofType: nil)
let url = URL(fileURLWithPath: path!)
let file = try! AVAudioFile(forReading: url)
let buffer = AVAudioPCMBuffer(pcmFormat: file.processingFormat, frameCapacity: AVAudioFrameCount(file.length))
try! file.read(into:buffer!)
let engine = AVAudioEngine()
engine.attach(audioNode)
engine.connect(audioNode, to: engine.mainMixerNode, format: buffer?.format)
// PlaySound[21867:535102] [AudioHAL_Client] AudioHardware.cpp:666:AudioObjectGetPropertyData:  AudioObjectGetPropertyData: no object with given ID 0
engine.prepare()
try! engine.start()
audioNode.play()
audioNode.scheduleBuffer(buffer!, at: nil, options: .loops, completionHandler: nil)
debugPrint(buffer?.format)
// Optional(<AVAudioFormat 0x60000212d1d0:  1 ch,  44100 Hz, Float32>)

In engine.connect(), I also tried engine.outputNode.outputFormat(forBus: 0) instead of buffer?.format. I got no luck.

Answer 1

It works if I put engine outside like this:

class ViewController: NSViewController {
    let engine = AVAudioEngine()
    override func viewDidLoad() {