Question

I have the next sequence:

• create activity;
• place a fragment into it;
• go to next fragment;
• return to previous fragment with back button.

Okay, let’s do it.

This is how I go to next fragment in navigation:

public static void addFragment(Fragment currentFragment, Fragment fragment, int frameLayout) {
    FragmentTransaction fragmentTransaction = currentFragment.getFragmentManager().beginTransaction();
    fragmentTransaction.replace(frameLayout, fragment);
    fragmentTransaction.addToBackStack(null);
    fragmentTransaction.commit();
}

public static void replaceFragment(Fragment currentFragment, Fragment fragment, int frameLayout) {
    FragmentManager fragmentManager = currentFragment.getFragmentManager();
    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
    Fragment topFragment = fragmentManager.findFragmentById(frameLayout);
    int transactionsCount = fragmentManager.getBackStackEntryCount();
    if (transactionsCount > 0 && topFragment == currentFragment) {
        fragmentManager.popBackStack();
        fragmentTransaction.replace(frameLayout, fragment);
        fragmentTransaction.addToBackStack(null);
    }   else {
        fragmentTransaction.replace(frameLayout, fragment);
    }
    fragmentTransaction.commit();
}

In onCreateView of first fragment I load some data and hide activity indicator when it’s done

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    LinearLayout homeLayout = new LinearLayout(activity);
    inflater.inflate(R.layout.screen_home, homeLayout);

    setupCategoryButtons();
    setupHomeGenderRadioButton(mainLayout);
    setupMagazinesPreviews(mainLayout);

    return homeLayout;
}

private void setupCategoryButtons() {
    if(categoriesButtons.size() > 0) {
        View categoriesWaitIndicator = activity.findViewById(R.id.categoriesWaitIndicator);
        categoriesWaitIndicator.setVisibility(View.INVISIBLE);

        LinearLayout categoriesButtonsLayout = (LinearLayout)activity.findViewById(R.id.categoryButtonsLayout);

        for(CategoryButton categoryButton : categoriesButtons) {
            categoriesButtonsLayout.addView(categoryButton);
        }

        refreshCategoriesButtons();
    }
}

Nothing special. And it works perfectly. Until I return to this screen.

When I return to this screen I getting NullPointerException on those lines:

categoriesWaitIndicator.setVisibility(View.INVISIBLE);
categoriesButtonsLayout.addView(categoryButton);

It seems that the app cannot find those views! I tried to do it in onResume but it has the same effect. What am I doing wrong?

Answer 1

I don’t know exactly what this fragment does, but it’s manipulating views in the Activity. Since it’s possible for the fragment to exist before the Activity view is built or destroyed, it could be trying to get views that have been destroyed and/or modified by elements beyond its control. It would be much better if the fragment was isolated so it handled it’s own view.