ViewPager not showing fragments

Question

I can't get this to work. The basic app functions as it's supposed to but the ViewPager on one of the layout files doesn't show any fragment at all. I checked the code multiple times but I can't find a bug. When I tried debugging it I noticed that it doesn't even initialize the fragments classes which it's supposed to when you select a tab. My MainActivity:

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.setDrawerListener(toggle);
    toggle.syncState();

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);

    new GetData(this).execute();

    TabLayout tab_layout = (TabLayout) findViewById(R.id.tab_layout);
    tab_layout.addTab(tab_layout.newTab().setText("CASUAL"));
    tab_layout.addTab(tab_layout.newTab().setText("RANKED"));
    tab_layout.setTabGravity(TabLayout.GRAVITY_FILL);

    final ViewPager viewPager = (ViewPager) findViewById(R.id.view_pager);
    PagerAdapter adapter = new PagerAdapter(getSupportFragmentManager(), tab_layout.getTabCount());
    viewPager.setAdapter(adapter);
    tab_layout.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
        @Override
        public void onTabSelected(TabLayout.Tab tab) {
            viewPager.setCurrentItem(tab.getPosition());
        }

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

        }

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

        }
    });

}

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

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

    return super.onOptionsItemSelected(item);
}

@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    int id = item.getItemId();

    if (id == R.id.nav_stats) {
        // Handle the camera action
    } else if (id == R.id.nav_about) {

    }

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}
}

My PagerAdapter:

public class PagerAdapter extends FragmentStatePagerAdapter {
int mNumOfTabs;

public PagerAdapter(FragmentManager fm, int NumOfTabs) {
    super(fm);
    this.mNumOfTabs = NumOfTabs;
    }

@Override
public Fragment getItem(int position) {

    switch (position) {
        case 0:
            CasualStats tab1 = new CasualStats();
            return tab1;
        case 1:
            RankedStats tab2 = new RankedStats();
            return tab2;
        default:
            return null;
        }
     }

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

Layout with the ViewPager:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent"><![CDATA[

        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="fill_parent"
        android:layout_below="@id/tab_layout"/>
]]>

<android.support.design.widget.TabLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_alignParentStart="true"
    android:id="@+id/tab_layout">

</android.support.design.widget.TabLayout>

<view
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    class="android.support.v4.view.ViewPager"
    android:layout_alignParentStart="true"
    android:id="@+id/view_pager"
    android:layout_below="@+id/tab_layout" />

</RelativeLayout>

The classes for the tab fragments which are supposed to be displayed in the ViewPager are just normal auto generated Android Studio fragment classes.

One of the fragments:

public class CasualStats extends Fragment{
// TODO: Rename parameter arguments, choose names that match
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
private static final String ARG_PARAM1 = "param1";
private static final String ARG_PARAM2 = "param2";

// TODO: Rename and change types of parameters
private String mParam1;
private String mParam2;

private OnFragmentInteractionListener mListener;

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

/**
 * Use this factory method to create a new instance of
 * this fragment using the provided parameters.
 *
 * @param param1 Parameter 1.
 * @param param2 Parameter 2.
 * @return A new instance of fragment CasualStats.
 */
// TODO: Rename and change types and number of parameters
public static CasualStats newInstance(String param1, String param2) {
    CasualStats fragment = new CasualStats();
    Bundle args = new Bundle();
    args.putString(ARG_PARAM1, param1);
    args.putString(ARG_PARAM2, param2);
    fragment.setArguments(args);
    return fragment;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (getArguments() != null) {
        mParam1 = getArguments().getString(ARG_PARAM1);
        mParam2 = getArguments().getString(ARG_PARAM2);
    }
}

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

// TODO: Rename method, update argument and hook method into UI event
public void onButtonPressed(Uri uri) {
    if (mListener != null) {
        mListener.onFragmentInteraction(uri);
    }
}

@Override
public void onDetach() {
    super.onDetach();
    mListener = null;
}

/**
 * This interface must be implemented by activities that contain this
 * fragment to allow an interaction in this fragment to be communicated
 * to the activity and potentially other fragments contained in that
 * activity.
 * <p>
 * See the Android Training lesson <a href=
 * "http://developer.android.com/training/basics/fragments/communicating.html"
 * >Communicating with Other Fragments</a> for more information.
 */
public interface OnFragmentInteractionListener {
    // TODO: Update argument type and name
    void onFragmentInteraction(Uri uri);
}
}

The xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.app.anzel.r6s.CasualStats">

<!-- TODO: Update blank fragment layout -->

<TextView
    android:text="1st fragment"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_alignParentStart="true"
    android:layout_marginStart="28dp"
    android:layout_marginTop="22dp"
    android:id="@+id/textView"
    android:visibility="visible" />
</RelativeLayout>

Answer 1

The problem is that you have nested fragments and use getFragmentManager(); You should use getChildFragmentManager() instead.

java:

viewPagerAdapter = new PagerAdapter(getChildFragmentManager())

kotlin:

viewPagerAdapter = PagerAdapter(childFragmentManager)