ListView inside a TabView with Firebase data

I'm trying to input data from the firebase into a listView that is already inside a TabView, but when I try to load the data from the firebase nothing comes. I started the ListView within the onCreateView method, like this:

    Firebase dref;
    ListView listView;
    ArrayList<String> list = new ArrayList<>();

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

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(getActivity().getApplicationContext(),
                android.R.layout.simple_list_item_1 , list);
        Firebase.setAndroidContext(getContext());
        dref = new Firebase("https://intercampi-bd69d.firebaseio.com");
        listView = (ListView) rootView.findViewById(R.id.list_view1);
        listView.setAdapter(adapter);

        dref.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(com.firebase.client.DataSnapshot dataSnapshot) {
                String input = dataSnapshot.getValue(String.class);
                list.add(input);

            }

            @Override
            public void onCancelled(FirebaseError firebaseError) {

            }
        });
        return rootView;
    }

here is the complete activity code:

public class HorarioActivity extends AppCompatActivity implements ListAdapter {


private SectionsPagerAdapter mSectionsPagerAdapter;



private ViewPager mViewPager;


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

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.container);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
    tabLayout.setupWithViewPager(mViewPager);

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });

}


@Override
public boolean onCreateOptionsMenu(Menu menu) {

    getMenuInflater().inflate(R.menu.menu_horario, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {

    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

@Override
public boolean areAllItemsEnabled() {
    return false;
}

@Override
public boolean isEnabled(int position) {
    return false;
}

@Override
public void registerDataSetObserver(DataSetObserver observer) {

}

@Override
public void unregisterDataSetObserver(DataSetObserver observer) {

}

@Override
public int getCount() {
    return 0;
}

@Override
public Object getItem(int position) {
    return null;
}

@Override
public long getItemId(int position) {
    return 0;
}

@Override
public boolean hasStableIds() {
    return false;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    return null;
}

@Override
public int getItemViewType(int position) {
    return 0;
}

@Override
public int getViewTypeCount() {
    return 0;
}

@Override
public boolean isEmpty() {
    return false;
}


public static class PlaceholderFragment extends Fragment {

    private static final String ARG_SECTION_NUMBER = "section_number";
    Firebase dref;
    ListView listView;
    ArrayList<String> list = new ArrayList<>();
    public PlaceholderFragment() {
    }


    public static PlaceholderFragment newInstance(int sectionNumber) {
        PlaceholderFragment fragment = new PlaceholderFragment();
        Bundle args = new Bundle();
        args.putInt(ARG_SECTION_NUMBER, sectionNumber);
        fragment.setArguments(args);
        return fragment;
    }

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

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(getActivity().getApplicationContext(),
                android.R.layout.simple_list_item_1 , list);
        Firebase.setAndroidContext(getContext());
        dref = new Firebase("https://intercampi-bd69d.firebaseio.com");
        listView = (ListView) rootView.findViewById(R.id.list_view1);
        listView.setAdapter(adapter);

        dref.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(com.firebase.client.DataSnapshot dataSnapshot) {
                String input = dataSnapshot.getValue(String.class);
                list.add(input);

            }

            @Override
            public void onCancelled(FirebaseError firebaseError) {

            }
        });
        return rootView;
    }
}


public class SectionsPagerAdapter extends FragmentPagerAdapter {

    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        // getItem is called to instantiate the fragment for the given page.
        // Return a PlaceholderFragment (defined as a static inner class below).
        return PlaceholderFragment.newInstance(position + 1);
    }

    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        switch (position) {
            case 0:
                return "SECTION 1";
            case 1:
                return "SECTION 2";
            case 2:
                return "SECTION 3";
        }
        return null;
    }
  }
}

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Thanks in advance for any feedback