UserVisibleHint is false in the selected ViewPager fragment driven by FragmentStatePagerAdapter

Question

I ran into a very difficult problem to diagnose in an Android app. getUserVisibleHint()returns falseto the currently selected fragment in ViewPagerwhen it should return true (because it is visible and selected).

I described cases where I see this behavior as follows:

• The fragment is selected and is currently displayed in ViewPager
• ViewPager is managed FragmentStatePagerAdapter
• A fragment was previously selected, its state was saved and later restored using PagerAdapter
  • minimum 3 tabs in viewpager
  • the user goes to tab 3, then to tab 1, then back to tab 3.
• Application uses support library version 24.0.0 or higher

Answer 1

Debugging showed that FragmentStatePagerAdapterit actually correctly sets the state of the selected tab to setPrimaryItem(ViewGroup container, int position, Object object), but later set it to false inFragmentManager#moveToState(Fragment f, int newState, int transit, int transitionStyle, boolean keepActive)

//from FragmentManager#moveToState(Fragment f, int newState, int transit, int transitionStyle, boolean keepActive)
f.mUserVisibleHint = f.mSavedFragmentState.getBoolean(
        FragmentManagerImpl.USER_VISIBLE_HINT_TAG, true);

f.mSavedFragmentStateabove saved the visible state as false, because it was saved when the fragment was no longer on the screen.

So, the problem here is state loss; the visible state is set to FragmentStatePagerAdapter#setPrimaryItem, but it is lost some time before the method is called onResume.

Correction

​​ , setPrimaryItem PagerAdapter .

public static class SectionsPagerAdapter extends FragmentStatePagerAdapter {
    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public void setPrimaryItem(ViewGroup container, int position, Object object) {
        //Force any pending transactions to save before we set an item as primary
        finishUpdate(null);
        super.setPrimaryItem(container, position, object);
    }

    @Override
    public Fragment getItem(int position) {
        Fragment fragment = new DummySectionFragment();
        Bundle args = new Bundle();
        args.putInt(DummySectionFragment.ARG_SECTION_NUMBER, position + 1);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public int getCount() {
        return 4;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        return "Page " + (position + 1);
    }
}

, FragmentStatePagerAdapter , .

FragmentStatePagerAdapter

, FragmentStatePagerAdapter

@Override
public Object instantiateItem(ViewGroup container, int position) {
    // If we already have this item instantiated, there is nothing
    // to do.  This can happen when we are restoring the entire pager
    // from its saved state, where the fragment manager has already
    // taken care of restoring the fragments we previously had instantiated.
    if (mFragments.size() > position) {
        Fragment f = mFragments.get(position);
        if (f != null) {
            return f;
        }
    }

    if (mCurTransaction == null) {
        mCurTransaction = mFragmentManager.beginTransaction();
    }

    Fragment fragment = getItem(position);
    if (DEBUG) Log.v(TAG, "Adding item #" + position + ": f=" + fragment);
    if (mSavedState.size() > position) {
        Fragment.SavedState fss = mSavedState.get(position);
        if (fss != null) {
            fragment.setInitialSavedState(fss);
        }
    }
    while (mFragments.size() <= position) {
        mFragments.add(null);
    }
    fragment.setMenuVisibility(false);
    fragment.setUserVisibleHint(false);
    mFragments.set(position, fragment);
    mCurTransaction.add(container.getId(), fragment);

    return fragment;
}

@Override
public void setPrimaryItem(ViewGroup container, int position, Object object) {
    Fragment fragment = (Fragment)object;
    if (fragment != mCurrentPrimaryItem) {
        if (mCurrentPrimaryItem != null) {
            mCurrentPrimaryItem.setMenuVisibility(false);
            mCurrentPrimaryItem.setUserVisibleHint(false);
        }
        if (fragment != null) {
            fragment.setMenuVisibility(true);
            fragment.setUserVisibleHint(true);
        }
        mCurrentPrimaryItem = fragment;
    }
}

@Override
public void finishUpdate(ViewGroup container) {
    if (mCurTransaction != null) {
        mCurTransaction.commitNowAllowingStateLoss();
        mCurTransaction = null;
    }
}

Answer 2

, 27.1.1 ( ) . 2018 , 27.1.1 2018 .