如何为我的Android TV应用程序实现“立即打牌”？

Question

我很确定我添加了文档https://developer.android.com/training/tv/playback/now-playing中提到的所有代码。我已经把它全部散开了，但是这里是onCreate上的简化：

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main_tv)

        session = MediaSession(this, "MyAPP")
        session.setFlags(MediaSession.FLAG_HANDLES_MEDIA_BUTTONS or MediaSession.FLAG_HANDLES_TRANSPORT_CONTROLS)
        session.setCallback(object : MediaSession.Callback() {
            override fun onPause() {
                super.onPause()
                player?.playWhenReady = false
            }

            override fun onPlay() {
                super.onPlay()
                player?.playWhenReady = true
            }
        })

        session.isActive = true

        val metadataBuilder = MediaMetadata.Builder().apply {
            putString(MediaMetadata.METADATA_KEY_DISPLAY_TITLE, "Title")
            putString(MediaMetadata.METADATA_KEY_DISPLAY_SUBTITLE, "Some subtitle")
            putBitmap(MediaMetadata.METADATA_KEY_DISPLAY_ICON, BitmapFactory.decodeResource(resources, R.mipmap.ic_launcher))

            putString(MediaMetadata.METADATA_KEY_TITLE, "Title again")
            putString(MediaMetadata.METADATA_KEY_ARTIST, "Artist")

        }
        session.setMetadata(metadataBuilder.build())
        val stateBuilder = PlaybackState.Builder().setActions(PlaybackState.ACTION_PAUSE) //just one to test
        session.setPlaybackState(stateBuilder.build())

        //volumeControlStream = AudioManager.STREAM_MUSIC

        //setupList()
    }

当我按下主页时，声音会继续播放，但是没有视频，也没有即时播放卡。

我想念什么？

我已经看过了：Now Playing Card 而这个Android Tv - Now playing card icon How to display Now Playing Card on Android TV，但这些问题/答案都没有帮助我。