Show or hide AppBar and Toolbar - depending on the fragment

Issue

I use one activity and few fragments. I want to hide toolbar only on splash screen. I wrote this:

class MainActivity : AppCompatActivity() {
lateinit var auth: FirebaseAuth

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    auth = Firebase.auth

    val currentUser = auth.currentUser

    updateUI(currentUser)
    setSupportActionBar(toolbar)

    findNavController(R.id.nav_container).addOnDestinationChangedListener { _, destination, _ ->
        when (destination.id) {
            R.id.splashScreenFragment -> {
                supportActionBar?.hide()
                appBarLayoutz.visibility = View.GONE
            }
            else -> {
                supportActionBar?.show()
                appBarLayoutz.visibility = View.VISIBLE
            }
        }
    }
}

private fun updateUI(currentUser: FirebaseUser?) {
    if (currentUser != null) {
        findNavController(R.id.nav_container).navigate(R.id.action_splashScreenFragment_to_mainPageFragment)
    } else {
        CoroutineScope(Dispatchers.Default).launch {
            delay(500)
            findNavController(R.id.nav_container).navigate(R.id.action_splashScreenFragment_to_loginFragment)
        }
    }
}

}

but it returns android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views. error. Full logs:

2020-10-04 18:36:56.949 23070-23235/pl.rybson.musicquiz E/AndroidRuntime: FATAL EXCEPTION: DefaultDispatcher-worker-1
Process: pl.rybson.musicquiz, PID: 23070
android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
    at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:8613)
    at android.view.ViewRootImpl.requestLayout(ViewRootImpl.java:1528)
    at android.view.View.requestLayout(View.java:24634)
    at android.view.View.setFlags(View.java:15312)
    at android.view.View.setVisibility(View.java:10919)
    at com.google.android.material.appbar.AppBarLayout.setVisibility(AppBarLayout.java:417)
    at pl.rybson.musicquiz.ui.MainActivity$onCreate$1.onDestinationChanged(MainActivity.kt:43)
    at androidx.navigation.NavController.dispatchOnDestinationChanged(NavController.java:498)
    at androidx.navigation.NavController.navigate(NavController.java:1097)
    at androidx.navigation.NavController.navigate(NavController.java:935)
    at androidx.navigation.NavController.navigate(NavController.java:868)
    at androidx.navigation.NavController.navigate(NavController.java:854)
    at androidx.navigation.NavController.navigate(NavController.java:842)
    at pl.rybson.musicquiz.ui.MainActivity$updateUI$1.invokeSuspend(MainActivity.kt:57)
    at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
    at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:56)
    at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:571)
    at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:738)
    at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:678)
    at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:665)

2020-10-04 18:36:56.959 23070-23235/pl.rybson.musicquiz I/Process: Sending signal. PID: 23070 SIG: 9

Du you have idea how to fix it?

Solution

Replace CoroutineScope(Dispatchers.Default).launch with CoroutineScope(Dispatchers.Main).launch - you're calling navigate() on a background thread, which causes your onDestinationChanged to be called on a background thread, which causes your error.

By using the Dispatchers.Main dispatcher, your navigate() call will happen on the main thread. Note that coroutines already handle not blocking the main thread when you call delay, so it absolutely fine to use Dispatchers.Main there.

Answered By - ianhanniballake