BoundMessengerService.kt

// MyBoundService.kt
class MyBoundService: Service() {
    val ACTION_DO_WORK = 0x2424
    inner class MyHandler(looper: Looper): Handler(looper) {
        override fun handleMessage(message: Message?) {
            when(message?.what) {
                ACTION_DO_WORK -> doWork()
            }
        }
    }

    private lateinit var messenger: Messenger
    private lateinit var handler: MyHandler

    override fun onCreate() {
        super.onCreate()
        // Create a background thread that eventually will be used to create the Messenger
        val thread = HandlerThread("new-thread").apply {
            start()
            handler = MyServiceHandler(looper)
            messenger = Messenger(handler)
        }
    }
    
    fun doWork() {
        // ...
    }

    override fun onBind(intent: Intent?): IBinder {
        // Support binding to this service
        return messenger.binder
    }
}

// MyActivity.kt
class MyActivity: AppCompatActivity {
    private val serviceConnection = object: ServiceConnection {
        override fun onServiceConnected(name: ComponentName?, binder: IBinder) {
            service = Messenger(binder)
        }

        override fun onServiceDisconnected(name: ComponentName?) {
            service = null
        }
    }

    // ...
    private var service: Messenger? = null
    
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.my_activity)
        // ...
        doWorkBt.setOnClickListener {
            messenger?.obtainMessage().also {
                it.what = MyBoundService.ACTION_DO_WORK
                messenger?.send(it)
            }
        }
        // ...
    }
    override fun onStart() {
        super.onStart()
        val intent = Intent(this, MyBoundService::class.java)
        bindService(intent, serviceConnection, Context.BIND_AUTO_CREATE)  
    }

    override fun onPause() {
        super.onPause()
        val intent = Intent(this, MyBoundService::class.java)
        unbindService(intent)
    }
}