Can i monitor a variable changed in another class in kotlin? - android-studio

Question

Im in a fragment1 and i want go to fragment2 if an event occurred in a class called from the fragment1. I have tried a callback of fuction: function in Class call a function in fragment1 to go in fragment but i collect this error:
Process: com.example.ilmiogioco, PID: 7992java.lang.IllegalStateException: Method addObserver must be called on the main thread
at androidx.lifecycle.LifecycleRegistry.enforceMainThreadIfNeeded(LifecycleRegistry.java:317)
at androidx.lifecycle.LifecycleRegistry.addObserver(LifecycleRegistry.java:172)
at androidx.savedstate.SavedStateRegistryController.performRestore(SavedStateRegistryController.java:61)
at androidx.navigation.NavBackStackEntry.<init>(NavBackStackEntry.java:88)
at androidx.navigation.NavBackStackEntry.<init>(NavBackStackEntry.java:73)
at androidx.navigation.NavController.navigate(NavController.java:1138)
at androidx.navigation.NavController.navigate(NavController.java:944)
at androidx.navigation.NavController.navigate(NavController.java:877)
at androidx.navigation.NavController.navigate(NavController.java:863)
at androidx.navigation.NavController.navigate(NavController.java:851)
at com.example.ilmiogioco.FullscreenFragmentSolo.follow(FullscreenFragmentSolo.kt:77)
at com.example.ilmiogioco.Solo.SpaceView.update(SpaceView.kt:276)
at com.example.ilmiogioco.Solo.SpaceView.run(SpaceView.kt:120)
at java.lang.Thread.run(Thread.java:919)
EDIT: I have fullscreenfragmentsolo (fragment1) that want in gameoverfragment (fragment2) if the class spaceview called in fullscreenfragmentsolo collect a lost game. The function follow() is called by spaceview for return in fullscreenfragmentsolo (maybe this is the thread error).
class FullscreenFragmentSolo : Fragment() {
private var spaceView: SpaceView? = null
private lateinit var backgroundMusic: MediaPlayer
private lateinit var window: Window
private var binding: FragmentFullscreenSoloBinding? = null
object size{
var x = 0
var y = 0
}
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
(activity as AppCompatActivity?)!!.supportActionBar!!.hide()
getActivity()?.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);
val soundEffects = SoundEffects(requireContext())
soundEffects.playSound(SoundEffects.backgroundMusic)
val outMetrics = DisplayMetrics()
getActivity()?.getWindow()?.addFlags(WindowManager.LayoutParams.FLAG_LAYOUT_NO_LIMITS);
window = activity?.getWindow()!!
window.attributes.width
window.attributes.height
if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.R) {
val display = activity?.display
display?.getRealMetrics(outMetrics)
} else {
#Suppress("DEPRECATION")
val display = activity?.windowManager?.defaultDisplay
#Suppress("DEPRECATION")
display?.getMetrics(outMetrics)
}
size.y = outMetrics.heightPixels
size.x = outMetrics.widthPixels
backgroundMusic = MediaPlayer.create(requireContext(), R.raw.background_music)
backgroundMusic.isLooping = true
backgroundMusic.start()
val fragmentBinding = FragmentFullscreenSoloBinding.inflate(inflater, container, false)
binding = fragmentBinding
fragmentBinding.root
spaceView = SpaceView(requireContext(), size, this)
return spaceView
}
fun follow(){
findNavController().navigate(R.id.action_fullscreenFragmentSolo_to_gameoverFragment)
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
binding?.soloFragment = this
}
override fun onResume() {
super.onResume()
spaceView?.resume()
}
fun stopFunction() {
spaceView?.stop()
}
override fun onPause() {
super.onPause()
backgroundMusic.release()
spaceView?.pause()
}
override fun onDestroyView() {
super.onDestroyView()
binding = null
}
GameoverFragment:
open class GameoverFragment : Fragment() {
private var binding: GameoverFragmentBinding? = null
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// Inflate the layout for this fragment
val fragmentBinding = GameoverFragmentBinding.inflate(inflater, container, false)
binding = fragmentBinding
return fragmentBinding.root
return view
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
binding?.gameoverFragment = this
}
fun Menu(){
findNavController().navigate(R.id.action_gameoverFragment_to_startFragment)
}
> override fun onDestroyView() {
> super.onDestroyView()
> binding = null }
Can you help me?

Answer 1

This exception is due to your code calling (through navigation) the LifecycleRegistry.addObserver from a thread other than the Main thread. You have to ensure that you call the navigation from the main thread.
Change to this in the follow() function
import android.os.Handler
import android.os.Looper
// ...
fun follow() {
Handler(Looper.getMainLooper()).post {
findNavController().navigate(R.id.action_fullscreenFragmentSolo_to_gameoverFragment)
}
}