Fragment里面的Android ViewPager只加载一次v2

Question

我的问题是similtar：

Android ViewPager Inside Fragment Loaded Only Once

不同之处在于它的答案对我不起作用。

我有4个片段。在其中一个片段中，有一个片段X ，ViewPager可以切换到另外两个片段。我第一次加载FragmentX时ViewPager正常工作。问题是如果我切换到其他4个片段中的一个，当我回到FragmentX时，ViewPager内部没有任何内容。

片段X

...
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment

    View view = inflater.inflate(R.layout.fragment_campus, container, false);

    TabLayout tabLayout = (TabLayout)view.findViewById(R.id.tab_layout);
    tabLayout.addTab(tabLayout.newTab().setText("Mapa"));
    tabLayout.addTab(tabLayout.newTab().setText("Eventos"));
    tabLayout.setTabGravity(TabLayout.GRAVITY_FILL);

    vpPager = (ViewPager)view.findViewById(R.id.vpPager);
    adapterViewPager = new PagerAdapter(getChildFragmentManager());
    vpPager.setAdapter(adapterViewPager);
    vpPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));
    tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
        @Override
        public void onTabSelected(TabLayout.Tab tab) {
            vpPager.setCurrentItem(tab.getPosition());
        }

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

        }

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

        }
    });

    return view;
}

PageAdapter

public class PagerAdapter extends FragmentPagerAdapter {

    private int NUM_ITEMS = 2;

    public PagerAdapter(FragmentManager fragmentManager) {
        super(fragmentManager);
    }

    // Returns total number of pages
    @Override
    public int getCount() {
        return NUM_ITEMS;
    }

    // Returns the fragment to display for that page
    @Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0:
                return MapFragment.newInstance(0);
            case 1:
                return EventosFragment.newInstance(1);
            default:
                return null;
        }
    }

    // Returns the page title for the top indicator
    @Override
    public CharSequence getPageTitle(int position) {
        return "Page " + position;
    }


}

MainActivity

public boolean onNavigationItemSelected(MenuItem item) {
    int id = item.getItemId();
    ...
    if (...) {
        if (campusFragment == null) // the problem is not solved by commenting this condition
            campusFragment = new CampusFragment(); // Fragment X
        fragmentManager.beginTransaction().replace(R.id.content_frame, campusFragment).commit();
    }
}