ViewPager2 how to restore fragments when navigating back

Question

Hi folks I have a ViewPager2 with single activity architecture. When I click a button, I swap out the ViewPager2 host fragment with another one using the Jetpack Navigation library.

This calls onDestroyView for the host fragment. When I click back, we are back to onCreateView. How can I return to the ViewPager2 I was looking at, seeing as the host fragment itself is not destroyed?

I believe based on this answer that restoring a ViewPager2 is actually impossible, not sure if this is by design or not. So what is the best practice here, assuming each fragment loads a heavy list, am I supposed to reload all the data every time a user pops the backstack into my viewpager? The only thing I can think of is to have an activity scoped ViewModel which maintains the list of data for each fragment, which sounds ridiculous, imagine if my pages were dynamically generated or I had several recycler views on each fragment....

Here is my attempt, I am trying to do the bare minimum when navigating back, however without assigning the view pager adapter again, I am looking at a blank fragment tab. I don't understand this, the binding has not died, so why is the view pager not capable of restoring my fragment?

OrderTabsFragment.kt

var adapter: TabsPagerAdapter? = null
private var _binding: FragmentOrdersTabsBinding? = null
private val binding get() = _binding!!
private var initted = false

override fun onCreate(savedInstanceState: Bundle?) {
    Timber.d("OrderTabsFragment $initted - onCreate $savedInstanceState")
    super.onCreate(savedInstanceState)

    adapter = TabsPagerAdapter(this, Tabs.values().size)
    adapter?.currentTab = Tabs.valueOf(savedInstanceState?.getString(CURRENT_TAB) ?: Tabs.ACTIVE.name)
}

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View {
    Timber.d("OrderTabsFragment $initted - onCreateView $savedInstanceState, _binding=$_binding")
    if(_binding == null)
        _binding = FragmentOrdersTabsBinding.inflate(inflater, container, false)

    return binding.root
}

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    Timber.d("OrderTabsFragment $initted - onViewCreated $savedInstanceState")

    super.onViewCreated(view, savedInstanceState)

    if(!initted) {
        initted = true

        val viewpager = binding.viewpager
        viewpager.adapter = adapter
        viewpager.isSaveEnabled = false

        binding.tabLayout.addOnTabSelectedListener(object : TabLayout.OnTabSelectedListener {
            override fun onTabSelected(tab: TabLayout.Tab?) {}
            override fun onTabUnselected(tab: TabLayout.Tab?) {}
            override fun onTabReselected(tab: TabLayout.Tab?) {
                if (adapter?.currentTab == Tabs.FILTERED) {
                    showFilterBalloon(tab)
                }
            }
        })
        TabLayoutMediator(binding.tabLayout, viewpager) { tab, position ->
            when (position) {
                0 -> tab.text = getString(R.string.title_active).uppercase(Locale.getDefault())
                1 -> tab.text =
                    getString(R.string.title_scheduled).uppercase(Locale.getDefault())
                2 -> tab.text =
                    getString(R.string.title_complete).uppercase(Locale.getDefault())
            }
        }.attach()
    }
    else{
        val viewpager = binding.viewpager
        viewpager.adapter = adapter //Required otherwise we are looking at a blank fragment tab. The adapter rv was detached and can't be reattached?
        viewpager.isSaveEnabled = false //Required otherwise "Expected the adapter to be 'fresh' while restoring state."
    }
}

override fun onSaveInstanceState(outState: Bundle) {
    super.onSaveInstanceState(outState)
    Timber.d("OrderTabsFragment $initted - onSaveInstanceState")
    outState.putString(CURRENT_TAB, adapter?.currentTab?.name)
}


override fun onDestroy() {
    super.onDestroy()
    Timber.d("OrderTabsFragment $initted - onDestroy")
    binding.viewpager.adapter = null
    _binding = null
    adapter = null
}

enum class Tabs {
    ACTIVE, SCHEDULED, COMPLETE, FILTERED
}

Edit: Here's roughly the same questions coming up in other places 1, 2, 3