Fragment no longer exists with viewPager 2 and TabLayout

Question

So I have:

Fragment A: here there is a viewPager2 and TabLayout with 2 tabs, Fragment B and C.

Then inside Fragment B, I have a button that goes to a Fragment D outside of the TabLayout , there I want get some data, go back and populate a TextView on Fragment B in the TabLayout in Fragment A.

It works fine until I am on Fragment D and try to get back to Fragment B and get Fragment no longer exists for key f#0

How can I fix this?

Fragment A TabLayout Adapter:

private inner class ScreenSlidePagerAdapter(fa: FragmentActivity) : FragmentStateAdapter(fa) {
    override fun getItemCount(): Int = 2

    override fun createFragment(position: Int): Fragment {
        return if (position == 0) FragmentB()
        else FragmentC()
    }
}

Fragment B:

findNavController().currentBackStackEntry?.savedStateHandle?.getLiveData<String>("key")?.observe(
        viewLifecycleOwner) { result ->
        binding.textView.text = result
    }

Fragment D:

fun onGoBack(data: String){
    findNavController().apply {
                    previousBackStackEntry?.savedStateHandle?.set("key", data)
                    navigateUp()
                }
}

MainActivity onSupportNavigateUp :

override fun onSupportNavigateUp(): Boolean {
    return navController.navigateUp(appBarConfiguration) || super.onSupportNavigateUp()
}

NavGraph:

<navigation xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/navigation"
app:startDestination="@id/fragmentA">

<fragment
    android:id="@+id/fragmentA"
    android:name="com.example.FragmentA"
    android:label="Fragment A" />

<fragment
    android:id="@+id/fragmentB"
    android:name="com.example.FragmentB"
    android:label="FragmentB" />

<fragment
    android:id="@+id/fragmentC"
    android:name="com.example.FragmentC"
    android:label="FragmentC" />

<fragment
    android:id="@+id/fragmentD"
    android:name="com.example.FragmentD"
    android:label="FragmentD" />
</navigation>

Answer 1

You shouldn't add ViewPager page fragments into the NavGraph not only it can introduce memory leaks but also it can introduce IllegalStateException for the navGraph .. Check here for more details.

But what you can do instead to solve this:

Remove the pager B & C fragments from the navigation graph. So the navGraph now contains A & D

Since Fragment B is not a part of the navGraph , Make the navigation that you need from the Fragment B to D through Fragment A:

In Fragment B:

val fragmentA: FragmentA = requireParentFragment() as FragmentA
fragmentA.openFragmentD()

In Fragment A:

fun openFragmentD() {
    val  navHostFragment: NavHostFragment = ...;
    val navController: NavController = navHostFragment.getNavController()
    navController.navigate(FragmentADirections.actionFragmentAToFragmentD())
}

Likewise: As you need to return some data from D back to B

Then receive this data through Fragment A onCreateView()

val data = FragmentAArgs.fromBundle(getArguments())

Now it's easy to either update the ViewPager fragment B with the new data, or you can share it in a shared ViewModel between the fragments.