PlayerActivity.kt

class PlayerActivity : AppCompatActivity() {

    private lateinit var mMediaBrowserCompat: MediaBrowserCompat
    private val connectionCallback: MediaBrowserCompat.ConnectionCallback = object : MediaBrowserCompat.ConnectionCallback() {
        override fun onConnected() {

            // The browser connected to the session successfully, use the token to create the controller
            super.onConnected()
            mMediaBrowserCompat.sessionToken.also { token ->
                val mediaController = MediaControllerCompat(this@PlayerActivity, token)
                MediaControllerCompat.setMediaController(this@PlayerActivity, mediaController)
            }
            playPauseBuild()
            Log.d("onConnected", "Controller Connected")
        }

        override fun onConnectionFailed() {
            super.onConnectionFailed()
            Log.d("onConnectionFailed", "Connection Failed")

        }

    }
    private val mControllerCallback = object : MediaControllerCompat.Callback() {
    }

    fun playPauseBuild() {
        val mediaController = MediaControllerCompat.getMediaController(this@PlayerActivity)
        btn.setOnClickListener {
            val state = mediaController.playbackState.state
            // if it is not playing then what are you waiting for ? PLAY ! 
            if (state == PlaybackStateCompat.STATE_PAUSED ||
                    state == PlaybackStateCompat.STATE_STOPPED ||
                    state == PlaybackStateCompat.STATE_NONE
            ) {

                mediaController.transportControls.playFromUri(Uri.parse("asset:///heart_attack.mp3"), null)
                btn.text = "Pause"
            }
            // you are playing ? knock it off ! 
            else if (state == PlaybackStateCompat.STATE_PLAYING ||
                    state == PlaybackStateCompat.STATE_BUFFERING ||
                    state == PlaybackStateCompat.STATE_CONNECTING
            ) {
                mediaController.transportControls.pause()
                btn.text = "Play"
            }
        }
        mediaController.registerCallback(mControllerCallback)

    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_player)
        val componentName = ComponentName(this, MusicService::class.java)
        // initialize the browser
        mMediaBrowserCompat = MediaBrowserCompat(
                this, componentName, //Identifier for the service
                connectionCallback,
                null
        )

    }

    override fun onStart() {
        super.onStart()
        // connect the controllers again to the session 
        // without this connect() you won't be able to start the service neither control it with the controller
        mMediaBrowserCompat.connect()
    }

    override fun onStop() {
        super.onStop()
        // Release the resources
        val controllerCompat = MediaControllerCompat.getMediaController(this)
        controllerCompat?.unregisterCallback(mControllerCallback)
        mMediaBrowserCompat.disconnect()
    }
    

}