Fragment no longer exists for key FragmentStateAdapter with Viewpager2

Question

I am using ViewPager2 for my project. I need to use nested fragments inside a fragment with viewpager2. it works like charm until I try to navigate between fragments(not nested ones).

After the first time navigating between fragments, the application crash with the error explained below.

the fragment which contains nested fragments OnCreateView method:

View view = inflater.inflate(R.layout.orders_fragment, null);

ViewPager2 viewPager = view.findViewById(R.id.childViewPager);

TabLayout tabs = view.findViewById(R.id.tabs);

SectionsPagerAdapter sectionsPagerAdapter = new SectionsPagerAdapter(getChildFragmentManager(),getLifecycle());

viewPager.setAdapter(sectionsPagerAdapter);

TabLayoutMediator.TabConfigurationStrategy tabConfigurationStrategy = (tab, position) -> {

    String[] order_activity_tabs = getResources().getStringArray(R.array.situations);

    for (int i=0; i<order_activity_tabs.length; i++) {

        if(i==position)
            tab.setText(order_activity_tabs[i]);

    }
};

TabLayoutMediator tabLayoutMediator = new TabLayoutMediator(tabs, viewPager, tabConfigurationStrategy);
tabLayoutMediator.attach();

return view;

When I return the fragment that contains nested fragments it crashes with

 java.lang.IllegalStateException: Fragment no longer exists for key f#0: unique id 4fbe17b8-5e22-4e07-a543-4a79445ad39c
        at androidx.fragment.app.FragmentManagerImpl.getFragment(FragmentManagerImpl.java:365)
        at androidx.viewpager2.adapter.FragmentStateAdapter.restoreState(FragmentStateAdapter.java:549)
        at androidx.viewpager2.widget.ViewPager2.restorePendingState(ViewPager2.java:350)
        at androidx.viewpager2.widget.ViewPager2.dispatchRestoreInstanceState(ViewPager2.java:375)
        at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:4045)
        at android.view.View.restoreHierarchyState(View.java:20253)
        at androidx.fragment.app.Fragment.restoreViewState(Fragment.java:548)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:907)
        at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1303)
        at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:439)
        at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManagerImpl.java:2079)

There are solutions for FragmentStatePagerAdapter but there is no for FragmentStateAdapter. And because I can't override methods in FragmentStateAdapter, implementing this solutions are impossible.

Thank you for reading this. Any help is appreciated.

Answer 1

The accepted answer is not solving the problem. I have solved the issue, by keeping the view as variable outside of the onCreateView method, and only set the adapter and tabs, when the tabCount is 0. Then it saved the state, and fixed the crash. But be careful with reference of the view.

private var _binding: YourFragmentBinding? = null
val binding get() = _binding!!
val bindingSafe get() = _binding

override fun onCreateView(
    inflater: LayoutInflater,
    container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    if (_binding != null)
        return binding.root
    _binding = inflate(inflater, container, false)
    return binding.root
}

override fun onDestroy() {
    super.onDestroy()
    _binding = null
}

 override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    if (binding.yourTabs.tabCount == 0) {
      ...//the rest of the logic you want to do with the UI
    }
}

If you want to do it without viewBinding, just replace the binding with simple View object.

Answer 2

I just had the same issue. The cause is that when the containing fragment is recreated, it's childFragmentManager is as well. And the FragmentStateAdapter looks in the passed fragmentmanager to find fragments it restores state for.

My solution was to have it use the activity's fragmentManager instead of the fragment's, for example use the constructor that takes an activity like

class MyAdapter(f: Fragment) : FragmentStateAdapter(f.activity!!)

or when using the FragmentStateAdapter(FragmentManager, Lifecycle) constructor, make sure to pass parentFragmentManager and not childFragmentManager

Answer 3

There is an issue with viewpager2 detach from recyclerview https://issuetracker.google.com/issues/154751401

As per my knowledge two fixes are there:

1. add this line in your xml viewpager2 component.

   android:saveEnabled="false"
   

But the above one don't restore the current instance, which the fragment will recreate each time it popsBack.

2. The recommended solution is add the below line on your fragment

   override fun onDestroyView() {
      super.onDestroyView()
      binding.pager.adapter = null
   }
   

Answer 4

Using viewpager2 with FragmentStateAdapter has a similar behaviour than FragmentStatePagerAdapter

Actually there are two ways of solving this problem

First one is seting setSaveEnabled() to false into our viewpager2

viewpager2.setSaveEnabled(false)

The other one is overriding restoreState at our adapter and return null

Check : https://developer.android.com/reference/androidx/viewpager2/adapter/FragmentStateAdapter

Important

Please note that this should not be the recommended way of disabling the state, this should work for some use cases but for production apps consider saving the state of the fragment or re-emiting the state to the UI from a state holder like viewmodel

Answer 5

All you have to do is to set your viewpager2 in the layout file with the following property

android:saveEnabled="true"

Answer 6

My problem solved with implementing new Navigation Component. It itself handles fragment transition. Anyone who across with this problem can change their navigation method.