I use the code below to setup my TabLayout inside a Fragment. The problem is that the tabs are not swiping even though I used registerOnPageChangeCallback(). When I add fragment containing tablayout to a bottom navigation view then when swiping the tablayout goes to a white screen with no fragments. What am I doing wrong? If i use tablayout in activity it can be swiped as usual.

public class BookcaseFragment extends Fragment {

    private TabLayout tabLayout;
    private ViewPager2 mViewPager;

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

    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        tabLayout = view.findViewById(R.id.tab_layout_bookcase);
        mViewPager = view.findViewById(R.id.bookcase_viewpager);

        BookcaseViewPagerAdapter adapter = new BookcaseViewPagerAdapter(this);
        mViewPager.setAdapter(adapter);

        new TabLayoutMediator(tabLayout, mViewPager, (tab, position) -> {
            switch (position){
                case 0:
                    tab.setText("following");
                    break;
                case 1:
                    tab.setText("downloaded");
                    break;
                case 2:
                    tab.setText("watched");
                    break;
            }
        }).attach();

        mViewPager.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
            @Override
            public void onPageSelected(int position) {
                super.onPageSelected(position);
                tabLayout.getTabAt(position).select();
                System.out.println(position);
            }
        });

    }
}
public class BookcaseViewPagerAdapter extends FragmentStateAdapter {


    public BookcaseViewPagerAdapter(@NonNull Fragment fragment) {
        super(fragment);
    }

    @NonNull
    @Override
    public Fragment createFragment(int position) {
        switch (position){
            case 0:
                return new FollowingFragment();
            case 1:
                return new DownloadedFragment();
            case 2:
                return new JustWatchedFragment();
            default:
                return new FollowingFragment();
        }
    }

    @Override
    public int getItemCount() {
        return 3;
    }
}

CodePudding user response：

This could be caused by the viewpager being null. Did you make the viewpager in the fragment itself?

I was able to get it to swipe as long as I put the viewpager inside the fragments xml.

written in kotlin, sorry, but should still be the same idea.

class BookcaseFragment : Fragment() {

    private var tabLayout: TabLayout? = null
    private var mViewPager: ViewPager2? = null
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_bookcase, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        tabLayout = view.findViewById(R.id.tab_layout_bookcase)
        mViewPager = view.findViewById(R.id.bookcase_pager)

        val adapter = BookcaseViewPagerAdapter(this)
        adapter.addFragment(BlankFragment())
        adapter.addFragment(BlankFragment())
        adapter.addFragment(BlankFragment())
        mViewPager?.adapter = adapter

        tabLayout?.let { tabs->
            mViewPager?.let { pager->

                TabLayoutMediator(tabs, pager,
                    TabConfigurationStrategy { tab: TabLayout.Tab, position: Int ->
                        when (position) {
                            0 -> tab.text = "following"
                            1 -> tab.text = "downloaded"
                            2 -> tab.text = "watched"
                        }
                    }).attach()

                pager.registerOnPageChangeCallback(object : OnPageChangeCallback() {
                    override fun onPageSelected(position: Int) {
                        super.onPageSelected(position)
                        tabs.getTabAt(position)!!.select()
                        println(position)
                    }
                })

            }
        }
    }

}
class BookcaseViewPagerAdapter(fragment: Fragment) :
    FragmentStateAdapter(fragment) {
    private val arrayList: ArrayList<Fragment> = ArrayList()

    fun getItem(position: Int): Fragment {
        return arrayList[position]
    }

    fun addFragment(fragment: Fragment) {
        arrayList.add(fragment)
    }

    override fun getItemCount(): Int {
        return arrayList.size
    }

    override fun createFragment(position: Int): Fragment {
        return arrayList[position]
    }
}

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout 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=".BookcaseFragment">

    <androidx.viewpager2.widget.ViewPager2
        android:id="@ id/bookcase_pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

    </androidx.viewpager2.widget.ViewPager2>

    <com.google.android.material.tabs.TabLayout
        android:id="@ id/tab_layout_bookcase"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
        <com.google.android.material.tabs.TabItem
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"/>
        <com.google.android.material.tabs.TabItem
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"/>
        <com.google.android.material.tabs.TabItem
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"/>

    </com.google.android.material.tabs.TabLayout>

</FrameLayout>

you can find more information in this answer. Proper implementation of ViewPager2 in Android