Using FragmentStatePageAdapter to retain position for fragment with ViewPager

Question

I have an activity that contains a TabLayout and uses a fragment for each tab. In return, that fragment contains another fragment for use with ViewPager. This is my setup:

RoutineActivity.java

public class RoutineActivity extends AppCompatActivity {
private ArrayList<CategoryFragment> categories = new ArrayList<>();
private TabLayout tabs;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_routine);

    // Set the toolbar as the action bar and add the Up button
    Toolbar toolbar = findViewById(R.id.routine_toolbar);
    setSupportActionBar(toolbar);
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

    // Get the intent and extract the string array
    Intent intent = getIntent();
    ArrayList<String> selected =
            intent.getStringArrayListExtra(SelectCategoriesActivity.EXTRA_CATEGORIES);

    // Set the TabLayout
    this.setTabs(selected);
}

// Set the tabs of the TabLayout given a list of tab names
private void setTabs(ArrayList<String> tabs) {
    // Setup the tabs and create a fragment for each category
    this.tabs = findViewById(R.id.category_tabs);
    for (int x = 0; x < tabs.size(); x++) {
        TabLayout.Tab tab = this.tabs.newTab().setText(tabs.get(x));
        // Select the first tab
        if (x == 0) {
            this.tabs.addTab(tab, true);
        } else {
            this.tabs.addTab(tab);
        }

        // Store the created fragment
        CategoryFragment categoryFragment = CategoryFragment.newInstance(tabs.get(x));
        this.categories.add(categoryFragment);
    }

    // Add the first fragment
    getSupportFragmentManager()
            .beginTransaction()
            .add(R.id.fragment_category_container, this.categories.get(0))
            .commit();


    // Add the tab selected listener
    this.tabs.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
        @Override
        public void onTabSelected(TabLayout.Tab tab) {
            RoutineActivity.this.replaceFragment(tab.getPosition());
        }

        @Override
        public void onTabUnselected(TabLayout.Tab tab) {}

        @Override
        public void onTabReselected(TabLayout.Tab tab) {

        }
    });
}

// Replace the fragment corresponding to the category
private void replaceFragment(int position) {
    FragmentManager fm = getSupportFragmentManager();
    FragmentTransaction ft = fm.beginTransaction();
    ft.replace(R.id.fragment_category_container, this.categories.get(position));
    ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
    ft.commit();
} 
}

CategoryFragment.java

public class CategoryFragment extends Fragment {
private ViewPager pager;
private ScreenSlidePagerAdapter pagerAdapter;

public CategoryFragment() {} // Required empty public constructor

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    ArrayList<String> selected = new ArrayList<>();
    selected.add("1 " + getArguments().get("CATEGORY"));
    selected.add("2 " + getArguments().get("CATEGORY"));
    selected.add("3 " + getArguments().get("CATEGORY"));
    selected.add("4 " + getArguments().get("CATEGORY"));

    // Instantiate the PageAdapter
    this.pagerAdapter = new ScreenSlidePagerAdapter(getChildFragmentManager(), selected);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View view = inflater.inflate(R.layout.fragment_category, container, false);

    // Instantiate ViewPager and set its adapter
    this.pager = view.findViewById(R.id.exercise_pager);
    this.pager.setAdapter(this.pagerAdapter);

    return view;
}

// Return a new instance of this fragment
public static CategoryFragment newInstance(String category) {
    Bundle args = new Bundle();
    args.putString("CATEGORY", category);

    CategoryFragment categoryFragment = new CategoryFragment();
    categoryFragment.setArguments(args);

    return categoryFragment;
}

// The page adapter
private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {

    private ArrayList<String> selected;

    public ScreenSlidePagerAdapter(FragmentManager fm, ArrayList<String> data) {
        super(fm);
        this.selected = data;
    }

    @Override
    public Fragment getItem(int position) {
        return ExerciseFragment.newInstance(this.selected.get(position));
    }

    @Override
    public int getCount() {
        return this.selected.size();
    }
}
}

ExerciseFragment.java

public class ExerciseFragment extends Fragment {

public ExerciseFragment() {} // Required empty public constructor

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView = inflater.inflate(
            R.layout.fragment_exercise_page, container, false);

    // Change the header to the category
    TextView tv = rootView.findViewById(R.id.textView_header);
    tv.setText(getArguments().getString("CAT"));

    return rootView;
}

public static ExerciseFragment newInstance(String category) {
    Bundle args = new Bundle();
    args.putString("CAT", category);

    ExerciseFragment exerciseFragment = new ExerciseFragment();
    exerciseFragment.setArguments(args);

    return exerciseFragment;
}

}

LogCat

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.util.SparseArray.get(int)' on a null object reference
at android.support.v4.app.FragmentManagerImpl.getFragment(FragmentManager.java:902)
at android.support.v4.app.FragmentStatePagerAdapter.restoreState(FragmentStatePagerAdapter.java:216)
at android.support.v4.view.ViewPager.onRestoreInstanceState(ViewPager.java:1453)
at android.view.View.dispatchRestoreInstanceState(View.java:16886)
at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:3470)
at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:3476)
at android.view.View.restoreHierarchyState(View.java:16864)
at android.support.v4.app.Fragment.restoreViewState(Fragment.java:500)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1445)
at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1740)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1809)
at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:799)
at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2580)
at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2367)
at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2322)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2229)
at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:700)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6682)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520)

The problem is that when I select a new tab, everything works fine, a new fragment is added, however when I go back to a previous tab, the whole app crashes. I think the problem is how the ViewPager is restoring the fragment but I have no clue how to fix it. I've replaced FragmentStatePagerAdapter to FragmentPagerAdapter and the problem goes away. However, I need to use the former since I will be using many pages (over 20). I just need the CategoryFragment to retain the fragment it left off on. Any ideas?

Can you add the stack trace of the creash? – Juan Aug 13 '17 at 23:40 — Juan, Aug 13 '17 at 23:40
@Juan I've added the LogCat of the error. – Niiks Aug 14 '17 at 00:14 — Niiks, Aug 14 '17 at 00:14

score 7 · Accepted Answer · answered Aug 14 '17 at 02:55

7

I found the problem in the SupportFragmentManager, when you move back the mActive fragment array has the first element null, and that is producing the NullPointerException.

I don't know why but by adding ft.addToBackStack(null); before the commit, the problem dissapears.

The method then remains as:

// Replace the fragment corresponding to the category
    private void replaceFragment(int position) {
        FragmentTransaction ft = fm.beginTransaction();
        ft.replace(R.id.fragment_category_container, this.categories.get(position));
        ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
        ft.addToBackStack(null);
        ft.commit();
    }

answered Aug 14 '17 at 02:55

Juan

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Thanks that did the trick but I switched to setting up the TabLayout with a ViewPager. – Niiks Aug 16 '17 at 14:17
This fixes the issue, but what happens in reality is that the backstack gets piled up. And if you have server call or any code in onResume (or any lifecycle method) of the fragmnet, it will call it multiple times! – Rohit TP May 29 '18 at 16:25

Using FragmentStatePageAdapter to retain position for fragment with ViewPager

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