How do I get a function in another thread to start the next function in the main thread after it is done? - multithreading

Question

Here is, what I'm trying to do:
A Switch is turned on, starting a service in another thread (works fine so far)
When this service is successful, it should then start another function within the main thread
I don't mind whether the function is called directly by the service or the service is returning a "success"-value to the main thread, what then starts the next function from there.
Here is, what the important parts of the code looks like:
Main thread:
class SendNotif : AppCompatActivity() {
val context = this
private lateinit var Switch: Switch
// Start LocationService when the switch is on
Switch.setOnCheckedChangeListener { buttonView, isChecked ->
if (isChecked) {
Toast.makeText(context, "Starting LocationService", Toast.LENGTH_SHORT).show()
Intent(applicationContext, LocationService::class.java).apply {
action = LocationService.ACTION_START
startService(this)
}
} else {
Toast.makeText(context, "Stopping LocationService", Toast.LENGTH_SHORT).show()
Intent(applicationContext, LocationService::class.java).apply {
action = LocationService.ACTION_STOP
startService(this)
}
}
}
}
fun InitiateMessage() {
// This is the function, that is supposed to start after the LocationService
}
}
This is the LocationService. After being successful, the function InitiateMessage() should start.
class LocationService: Service() {
private val serviceScope = CoroutineScope(SupervisorJob() + Dispatchers.IO)
private lateinit var locationClient: LocationClient
var lat = 0.0F
var long = 0.0F
override fun onBind(p0: Intent?): IBinder? {
return null
}
override fun onCreate() {
super.onCreate()
locationClient = DefaultLocationClient(
applicationContext,
LocationServices.getFusedLocationProviderClient(applicationContext)
)
}
// Start or stop the service
override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
when(intent?.action) {
ACTION_START -> start()
ACTION_STOP -> stop()
}
return super.onStartCommand(intent, flags, startId)
}
private fun start() {
// Starting notification
val notification = NotificationCompat.Builder(this, "location")
.setContentTitle("Tracking location...")
.setContentText("Location: null")
.setSmallIcon(R.drawable.ic_launcher_background)
// Can't swipe this notification away
.setOngoing(true)
val notificationManager = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
// Starting the location updates
locationClient
// Every 10 seconds
.getLocationUpdates(10000L)
.catch { e -> e.printStackTrace() }
.onEach { location ->
lat = location.latitude.toString().toFloat() // .takeLast(3) // taking only the last 3 digits
long = location.longitude.toString().toFloat() // .takeLast(3)
val updatedNotification = notification.setContentText(
"Location: ($lat, $long)"
)
// notificationManager.notify(1, updatedNotification.build())
// Geofence
MyGeofence(lat, long)
}
.launchIn(serviceScope)
// startForeground(1, notification.build())
}
private fun stop() {
// Stopping the notification
stopForeground(true)
// Stopping the location service
stopSelf()
}
override fun onDestroy() {
super.onDestroy()
serviceScope.cancel()
}
companion object {
const val ACTION_START = "ACTION_START"
const val ACTION_STOP = "ACTION_STOP"
}
fun MyGeofence(lat : Float, long : Float){
val context = this
var db = DataBaseHandler(context)
var data = db.readData()
// Setting the accuracy of the geofence
val acc = 2
val safelat : Double = data.get(0).LocLat.toFloat().round(acc)
val safelong = data.get(0).LocLong.toFloat().round(acc) // .take(acc).take(acc)
val h = Handler(context.mainLooper)
if(safelat == lat.toFloat().round(acc) && safelong == long.toFloat().round(acc)){
h.post(Runnable { Toast.makeText(context, "You have reached your safe refuge! " + lat.toFloat().round(acc) + " " + long.toFloat().round(acc), Toast.LENGTH_LONG).show() })
// ToDo: Right hereafter the function InitiateMessage() should start
}
else{
h.post(Runnable { Toast.makeText(context, "You are still in great danger! " + lat.toFloat().round(acc) + " " + long.toFloat().round(acc), Toast.LENGTH_LONG).show() })
}
}
fun Float.round(decimals: Int): Double {
var multiplier = 1.0
repeat(decimals) { multiplier *= 10 }
return round(this * multiplier) / multiplier
}
}
So far, I tried it with a Looper, which did not work.
java.lang.RuntimeException: Can't create handler inside thread Thread[DefaultDispatcher-worker-1,5,main] that has not called Looper.prepare()
But I guess the far easier way would be a returned value by the service. How do I implement this, and how do I start the next function through this returned value?

Answer 1

I solved my problem with an observe-function and a companion object, that is a MutableLiveData.
The companion object is placed inside the main thread:
companion object {
// var iamsafe: Boolean = false
val iamsafe: MutableLiveData<Boolean> by lazy {
MutableLiveData<Boolean>()
}
}
The observe-function is placed within onCreate:
val safeObserver = Observer<Boolean> { newState ->
Toast.makeText(context, "Initiating message to my mate.", Toast.LENGTH_SHORT).show()
InitiateMessage()
}
iamsafe.observe(this, safeObserver)
The companion is changed in the second thread like this:
SendNotif.iamsafe.postValue (true)