Ios 如何在avaudionode swift中播放歌曲特定时间

我制作了一个简单的mp3播放器，它具有播放、暂停和停止特定歌曲的功能。在后台模式下播放的歌曲是指当我将播放器屏幕更改为另一个屏幕时，歌曲会连续播放，但如何在当前时间连续播放歌曲。但实际上，它是从起点重新开始的

代码：

import UIKit
import AVFoundation
import CoreMotion
import MediaPlayer
import Foundation

class ViewController: UIViewController {

var audioEngine = AVAudioEngine()
var playerNode = AVAudioPlayerNode()
let timeShift = AVAudioUnitTimePitch()

override func viewDidLoad() {
    super.viewDidLoad()

    audioEngine.inputNode.reset()
    if playerNode.engine == nil {
        audioEngine.attach(playerNode)
        audioEngine.attach(timeShift)
    }
    audioEngine.connect(playerNode, to: timeShift, format: nil)
    audioEngine.connect(timeShift, to: audioEngine.mainMixerNode, format: nil)
    audioEngine.prepare()
    do {
        try audioEngine.start()
    } catch {
        print("Could not start audio engine")
    }

    do {
        try AVAudioSession.sharedInstance().setCategory(.playback, mode: .default, options: [.mixWithOthers, .allowAirPlay, .defaultToSpeaker])
        try AVAudioSession.sharedInstance().setActive(true, options: .notifyOthersOnDeactivation)
        try AVAudioSession.sharedInstance().setActive(true)
    } catch {
        print(error)
    }

    let urlMain = Bundle.main.url(forResource: "Demo", withExtension: "mp3")!
    let audioAsset = AVURLAsset(url: urlMain, options: nil)
    let audioDuration = audioAsset.duration
    let audioDurationSeconds = CMTimeGetSeconds(audioDuration)
    print(audioDurationSeconds)
    do {
        let audioFile = try AVAudioFile(forReading: urlMain)
        playerNode.scheduleFile(audioFile, at: nil, completionHandler: nil)
        playerNode.play()
    }catch{
        print("No value")
        return
    }
}

@IBAction func pushToSecondVC(_ sender: UIButton) {
    self.dismiss(animated: true, completion: nil)
  }
}