问如何使用Swift在后台播放音频？

EN

import UIKit
import AVFoundation
import MediaPlayer
import GoogleMobileAds


    class ViewController: UIViewController, GADInterstitialDelegate {
    
        @IBOutlet weak var exitMapButton: UIButton!
        @IBOutlet weak var radarMap: UIWebView!
        var interstitial: GADInterstitial!
        func createAndLoadInterstitial() -> GADInterstitial {
            var interstitial = GADInterstitial(adUnitID: "adUnitID-XXXX")
            interstitial.delegate = self
            interstitial.loadRequest(GADRequest())
            return interstitial
        }
        
        func getAd(){
            if (self.interstitial.isReady)
            {
                self.interstitial.presentFromRootViewController(self)
                self.interstitial = self.createAndLoadInterstitial()
            }
        }
        @IBOutlet weak var ataturkButton: UIButton!
        @IBOutlet weak var sabihaButton: UIButton!
        @IBOutlet weak var esenbogaButton: UIButton!
        @IBOutlet weak var weatherButton: UIButton!
        @IBOutlet weak var statusLabel: UILabel!
        @IBOutlet weak var playButton: UIButton!
        @IBOutlet weak var webViewButton: UIButton!
        var googleBannerView: GADBannerView!
override func viewDidLoad() {
            super.viewDidLoad()
        }
class PlayerAv {
            var audioLink: String?
            var player: AVPlayer
            init(link: String) {
                self.audioLink = link
                self.player = AVPlayer(URL: NSURL(string: link))
            }
        }
        var myPlayer = PlayerAv(link: "http://somewebsite.com/abc.pls")
        var setTowerState = ""
        
        @IBAction func sliderValueChanged(sender: UISlider) {
            var currentValue = Float(sender.value)
            println(currentValue)
            myPlayer.player.volume = currentValue
        }
        @IBAction func getWeatherWindow(sender: AnyObject) {
            UIApplication.sharedApplication().openURL(NSURL(string: "http://somewebpage.com")!)
            println("Directed to weather page")
        }
        @IBAction func changeToAtaturk() {
            myPlayer.player.pause()
            myPlayer = PlayerAv(link: "http://somewebsite.com/abc.pls")
            myPlayer.audioLink == ""
            println("\(myPlayer.audioLink!)--a")
            playButton.setTitle("Pause", forState: UIControlState.Normal)
            myPlayer.player.play()
            setTowerState = "ataturk"
            statusLabel.text = "Status: Playing, LTBA"
        }
        @IBAction func changeToEsenboga() {
            myPlayer.player.pause()
            myPlayer = PlayerAv(link: "http://somewebsite.com/def.pls")
            println("\(myPlayer.audioLink!)--a")
            playButton.setTitle("Pause", forState: UIControlState.Normal)
            myPlayer.player.play()
            setTowerState = "esenboga"
            statusLabel.text = "Status: Playing, LTAC"
        }
        @IBAction func changeToSabiha() {
            myPlayer.player.pause()
            myPlayer = PlayerAv(link: "http://somewebsite.com/efg.pls")
            println("\(myPlayer.audioLink!)--a")
            playButton.setTitle("Pause", forState: UIControlState.Normal)
            myPlayer.player.play()
            setTowerState = "sabiha"
            statusLabel.text = "Status: Playing, LTFJ"
        }
        override func didReceiveMemoryWarning() {
            super.didReceiveMemoryWarning()
            // Dispose of any resources that can be recreated.
        }
        @IBAction func playButtonPressed(sender: AnyObject) {
            toggle()
        }
        func toggle() {
            if playButton.titleLabel?.text == "Play" {
                playRadio()
                println("Playing")
                statusLabel.text = "Status: Playing"
            } else {
                pauseRadio()
                println("Paused")
                statusLabel.text = "Status: Paused"
            }
        }
        func playRadio() {
            myPlayer.player.play()
            playButton.setTitle("Pause", forState: UIControlState.Normal)   
        }
        func pauseRadio() {
            myPlayer.player.pause()
            playButton.setTitle("Play", forState: UIControlState.Normal)
        }
    }