Pass data to Service in Kotlin

Question

I have an app that uses Service class to perform task in foreground.

This service also contains a Handler object to run same function multiple times. I want to change attributes in my activity_main.xml while functions are running in Service. For example when function calculates something in Service the result prints in TextView.

How it would be correct access activity_main's objects to retrieve and change their values and attributes?

Here is what I have:

MainActivity.kt:

class MainActivity : AppCompatActivity() {
    private var notificationManager: NotificationManager? = null
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        buttonStart.setOnClickListener{
            buttonStart.isEnabled = false
            buttonStop.isEnabled = true
            IdListener.startService(this, "Foreground Service is running...")
        }
    }
}

IdListener.kt:

class IdListener : Service() {
    private val CHANNEL_ID = "ForegroundService Kotlin"
    private lateinit var mainHandler: Handler
    private lateinit var mRunnable: Runnable

    companion object {
        fun startService(context: Context, message: String) {
            val startIntent = Intent(context, IdListener::class.java)
            startIntent.putExtra("inputExtra", message)
            ContextCompat.startForegroundService(context, startIntent)
        }
        fun stopService(context: Context) {
            val stopIntent = Intent(context, IdListener::class.java)
            context.stopService(stopIntent)
        }
    }

    override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {

        mainHandler = Handler()
        mRunnable = Runnable { showRandomNumber(tm) }
        mainHandler.postDelayed(mRunnable, 1000)

        val input = intent?.getStringExtra("inputExtra")
        createNotificationChannel()
        val notificationIntent = Intent(this, MainActivity::class.java)
        val pendingIntent = PendingIntent.getActivity(
            this,
            0, notificationIntent, 0
        )
        val notification = NotificationCompat.Builder(this, CHANNEL_ID)
            .setContentTitle("Foreground Service Kotlin Example")
            .setContentText(input)
            .setSmallIcon(R.drawable.ic_notofication)
            .setContentIntent(pendingIntent)
            .build()
        startForeground(1, notification)
        return START_NOT_STICKY
    }
    override fun onBind(intent: Intent): IBinder? {
        return null
    }

    override fun onDestroy() {
        super.onDestroy()
        mainHandler.removeCallbacks(mRunnable)
    }

    private fun createNotificationChannel() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            val serviceChannel = NotificationChannel(CHANNEL_ID, "Foreground Service Channel",
                NotificationManager.IMPORTANCE_DEFAULT)
            val manager = getSystemService(NotificationManager::class.java)
            manager!!.createNotificationChannel(serviceChannel)
        }
    }

    /// function in which I want elements from activity_main.xml to be changed
    fun showRandomNumber(manager: TelephonyManager){
        myTextView.text = "Working..." 
        mainHandler.postDelayed(mRunnable, 1000)
    }

}

Answer 1

Here's how I'd probably handle your case. I don't know exactly what you're doing, but I'm just having the text view show "Working..." when it starts the service until there's an ID available. I haven't tested this and haven't worked with services in a long time, so you might want other input.

object IdServiceData {
    val id = MutableLiveData<String>()
}

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        //...

        myTextView.text = "Working..." // should really use string resource here.
        IdServiceData.id.observe(this) {
            myTextView.text = it.value
        }
    }
}

When an Activity or Fragment observes a LiveData, they automatically stop observing when they are destroyed, so they are not leaked. So your Activity can be destroyed and recreated multiple times while the Service is running and it will keep getting the proper updates.

class IdListener : Service() {
    //...

    private fun broadcastNewId(id: String){
        mainHandler.post {
            IdServiceData.id.value = id
        }
    }
}

If you want better encapsulation, I suppose you could abstract out the MutableLiveData by creating a separate IdServiceDataProvider that has the MutableLiveData and is used by the service, and the IdServiceData would reference the data like this: val id: LiveData<String> = IdServiceDataProvider.id