Fragment no longer exists for key FragmentStateAdapter with Viewpager2

问题

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.

回答1:

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

回答2:

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

回答3:

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!!)

来源：https://stackoverflow.com/questions/59486504/fragment-no-longer-exists-for-key-fragmentstateadapter-with-viewpager2