FragmentStatePagerAdapter没有调用getItem（）

Question

i have seen the relative post to my issue but, it's not solve my probleme. I'm trying to get a swipe view with 3 fragment. In the main fragment i have this:

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

    ConversionsPagerAdapter cPageAdapter = new ConversionsPagerAdapter(getActivity().getSupportFragmentManager());
    ViewPager vp = vue.findViewById(R.id.conversionsPager);
    cPageAdapter.getItem(0);
    vp.setAdapter(cPageAdapter);


   Button convKmhKts = (Button) vue.findViewById(R.id.convKmhKts);
   convKmhKts.setText("...");
...

Here is the class of my FragmentStatePagerAdapter:

public class ConversionsPagerAdapter extends FragmentStatePagerAdapter{
    public ConversionsPagerAdapter(FragmentManager fm) {
        super(fm);
        Log.i("ARKA", "FragmentStatePagerAdapter CONSTRUCTOR");
    }

    @Override
    public Fragment getItem(int i) {
        return ConversionFragment.newInstance(i);
    }

    @Override
    public int getCount() {
        return 1;
    }
}

And finnaly the Fragment which display the Tab:

public class ConversionFragment extends Fragment {
    public static final String ID_CONVERSION = "id_conversion";

    public static ConversionFragment newInstance(int pos) {
        ConversionFragment stf = new ConversionFragment();
        Bundle args = new Bundle();
        args.putInt("pos", pos);
        stf.setArguments(args);
        return stf;
    }


    public ConversionFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {
        // The last two arguments ensure LayoutParams are inflated
        // properly.

        Bundle args = getArguments();
        int idConversion = args.getInt(ID_CONVERSION);
        Log.i("idConversion", String.valueOf(idConversion));
        int idVue;

        switch (idConversion){
            case 0: idVue = R.layout.fragment_conversions_vitesse;
                break;
            case 1: idVue = R.layout.fragment_conversions_vitesse;
            default: idVue = R.layout.fragment_conversions_vitesse;
                break;
        }

        View rootView = inflater.inflate(idVue, null);
        return rootView;
    }
}

The fact is, the getItem() is never called. I try to change getActivity().getSupportFragmentManager() by getChildFragmentManager(), but it seem it's not the good way... When i'm trying to acces my button i get this:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

At the line where i'm using the button.

Answer 1

If you use ViewPager inside fragment you have to use chield fragment manager so try replace getActivity().getSupportFragmentManager() with getChildFragmentManager() . If you use one fragment manager for all stuff, it will bring some bugs to you.

For more information

Answer 2

You are doing wrong here

cPageAdapter.getItem(0);
vp.setAdapter(cPageAdapter);

Set adapter to view pager first and then try to call get item method.

vp.setAdapter(cPageAdapter);
cPageAdapter.getItem(0);

Answer 3

Ok, the probleme was that i try to acces a component which is not inflated:

convKmhKts.setText("...");

I acces to it in the child fragment and now it's ok....