它没有显示标题PageTabStrip？

Question

ViewPager工作正常，但我连接pagerTabStrip以显示页眉，我有以下问题。首次启动时，不显示标题。

我转动页面标题后显示。

可能是什么问题？下面取消上市。

<android.support.v4.view.ViewPager
            android:id="@+id/view_pager"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_marginTop="56dp"
            >
            <android.support.v4.view.PagerTabStrip
                android:id="@+id/pagerTabStrip"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_gravity="top">
            </android.support.v4.view.PagerTabStrip>
        </android.support.v4.view.ViewPager>

适配器ViewPager

**

public class ViewPagerAdapter extends FragmentPagerAdapter {
    static final int PAGE_COUNT = 4;
    ArrayList<String> PageTitle = new ArrayList<String >();
    public ViewPagerAdapter(FragmentManager fm) {
        super(fm);
        PageTitle.add("Русские артисты");
        PageTitle.add("Зарубежные артисты");
        PageTitle.add("Поиск");
        PageTitle.add("Добавить артиста");
    }
    @Override
    public Fragment getItem(int position) {
        return PageFragment.newInstance(position);
    }
    @Override
    public int getCount() {
        return PAGE_COUNT;
    }
    public CharSequence getPageTitle(int position) {
        return PageTitle.get(position);
    }
}

片段

public class PageFragment extends Fragment {


    static final String ARGUMENT_PAGE_NUMBER = "arg_page_number";

    int pageNumber;
    int backColor;

    public static PageFragment newInstance(int page) {
        PageFragment pageFragment = new PageFragment();
        Bundle arguments = new Bundle();
        arguments.putInt(ARGUMENT_PAGE_NUMBER, page);
        pageFragment.setArguments(arguments);
        return pageFragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        pageNumber = getArguments().getInt(ARGUMENT_PAGE_NUMBER);

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_page, null);

        TextView tvPage = (TextView) view.findViewById(R.id.tvPage);
        tvPage.setText("Page " + pageNumber);


        return view;
    }


}

MainActivity

pager = (ViewPager) findViewById(R.id.view_pager);
        viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());
        pager.setAdapter(viewPagerAdapter);
        pager.setCurrentItem(2);
        pager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

            }

            @Override
            public void onPageSelected(int position) {

            }

            @Override
            public void onPageScrollStateChanged(int state) {

            }
        });