動画再生アプリをバックグラウンドにしたときにPicture in Pictureで再生したい

概要

ios14からpicture in pictureがバックグラウンドでも再生できるということで試してみたんですがバックグラウンドに回したときに再生されません。どうしたら良いのでしょうか？
こちらのドキュメントを参考に試していました。
Adopting Picture in Picture in a Custom Player

コード

swift
1import UIKit
2import AVFoundation
3import AVKit
4
5class PlayerView: UIView {
6    override class var layerClass: AnyClass {
7        return AVPlayerLayer.self
8    }
9    var playerLayer: AVPlayerLayer! {
10        return layer as? AVPlayerLayer
11    }
12}
13
14class ViewController: UIViewController {
15    @IBOutlet weak var playButton: UIButton!
16    
17    var playerController = AVPlayerViewController()
18    var player = AVPlayer()
19    var pictureInPictureController: AVPictureInPictureController!
20    let playerView = PlayerView()
21    
22    override func viewDidLoad() {
23        super.viewDidLoad()
24
25        let startImage = AVPictureInPictureController.pictureInPictureButtonStartImage
26        let stopImage = AVPictureInPictureController.pictureInPictureButtonStopImage
27        
28        playButton.setImage(startImage, for: .normal)
29        playButton.setImage(stopImage, for: .selected)
30        
31        setupPictureInPicture()
32    }
33    
34    func setupPictureInPicture() {
35        if AVPictureInPictureController.isPictureInPictureSupported() {
36            try!AVAudioSession.sharedInstance().setActive(true)
37            
38            playerView.playerLayer.player = player
39            pictureInPictureController = AVPictureInPictureController(playerLayer: playerView.playerLayer)
40            
41        } else {
42            fatalError("device not support")
43        }
44    }
45
46    
47    @IBAction func playButtonTappedAction(_ sender: Any) {
48        guard let url = Bundle.main.url(forResource: "sample", withExtension: "mp4") else {
49            return
50        }
51        let item = AVPlayerItem(url: url)
52        player.replaceCurrentItem(with: item)
53        playerController.player = player
54        self.present(playerController, animated: true) {
55            self.player.play()
56        }
57    }
58}
59