我有一个AVPlayer设置，但我似乎不能得到视频音频继续播放时，手机被锁定或在主屏幕上。我已经编辑了info.plist并启用了音频背景模式，但我认为需要添加一些代码。请你能帮我。谢谢
下面是我的代码：

import UIKit
import AVFoundation
import AVKit

class ViewController: UIViewController {

    @IBAction func WarmUp(sender: UIButton)
    {
        WarmUpVideo()
    }

    func WarmUpVideo()
    {
        let filePath = NSBundle.mainBundle().pathForResource("132", ofType: "MOV")
        let videoURL = NSURL(fileURLWithPath: filePath!)

        let player = AVPlayer(URL: videoURL)
        let playerViewController = AVPlayerViewController()

        playerViewController.player = player

        self.presentViewController(playerViewController, animated: true) { () -> Void in playerViewController.player!.play()
        }

    }

    func playExternalVideo()
    {

}

    @IBAction func CoolDown(sender: UIButton)
    {
        CoolDownVideo()
    }

    func CoolDownVideo()
    {
        let filePath = NSBundle.mainBundle().pathForResource("132", ofType: "mp4")
        let videoURL = NSURL(fileURLWithPath: filePath!)

        let player = AVPlayer(URL: videoURL)
        let playerViewController = AVPlayerViewController()

        playerViewController.player = player

        self.presentViewController(playerViewController, animated: true) { () -> Void in playerViewController.player!.play()
        }

    }

}