findFragmentById returns null after implementing ViewPager

Question

I followed this tutorial to implement ViewPager (without creating a tablayout) : https://www.javacodegeeks.com/2013/04/android-tutorial-using-the-viewpager.html

It works, but I have a trouble :

When I try to do findFragmentById(R.id.my_fragment), it returns me null. And I think it's because that in my activity_main.xml there is nothing else than ViewPager

I've tried to add a <fragment> in my activity_main.xml, now I can access my fragment by using findFragmentById(R.id.my_fragment) but my ViewPager doesn't work right now

There is a way to get an access to my Fragment ? I need to access his views.

Here is my activity_main.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"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <androidx.viewpager.widget.ViewPager
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</RelativeLayout>`enter code here`

Here is my MainActivity's onCreate

public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView((int) R.layout.activity_main);
        scan_fragment = (ScanFragment) getSupportFragmentManager().findFragmentById(R.id.scan_fragment);
        nfcButton = (ImageButton) findViewById(R.id.nfc_button);
        integrator = new IntentIntegrator(this);
        pendingIntent = PendingIntent.getActivity(this, 0, new Intent(this, getClass()).addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP), 0);
        //
        anim = ObjectAnimator.ofFloat(this.nfcButton, "scaleX", new float[]{0.9f});
        anim2 = ObjectAnimator.ofFloat(this.nfcButton, "scaleY", new float[]{0.9f});
        List<Fragment> fragments = getFragments();
        pageAdapter = new MyPageAdapter(getSupportFragmentManager(), fragments);
        ViewPager pager = (ViewPager) findViewById(R.id.viewpager);
        pager.setAdapter(pageAdapter);
    }

    private List<Fragment> getFragments() {
        List<Fragment> fList = new ArrayList<Fragment>();
        fList.add(ScanFragment.newInstance("Fragment 1"));
        fList.add(WebFragment.newInstance("Fragment 2"));
        return fList;
    }

MyPageAdapter

class MyPageAdapter extends FragmentPagerAdapter {

    private List<Fragment> fragments;

    public MyPageAdapter(FragmentManager fm, List<Fragment> fragments) {
        super(fm);
        this.fragments = fragments;
    }

    @Override
    public Fragment getItem(int position) {
        return this.fragments.get(position);
    }


    @Override
    public int getCount() {
        return this.fragments.size();
    }
}

The important part of my Fragment :

    public interface OnButtonClickedListener {
        public void onButtonClicked(View view);
    }

    public static final ScanFragment newInstance(String message)
    {
        ScanFragment f = new ScanFragment();
        Bundle bdl = new Bundle(1);
        bdl.putString(EXTRA_MESSAGE, message);
        f.setArguments(bdl);
        return f;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_scan, container, false);
        mainActivity = (MainActivity) getActivity();
        MainActivity.display = 0;
        qrButton = (ImageButton) view.findViewById(R.id.main_button);
        nfcButton = (ImageButton) view.findViewById(R.id.nfc_button);
        anim = ObjectAnimator.ofFloat(this.nfcButton, "scaleX", new float[]{0.9f});
        anim2 = ObjectAnimator.ofFloat(this.nfcButton, "scaleY", new float[]{0.9f});
        nfcButton.setOnClickListener(this);
        qrButton.setOnClickListener(this);
        return view;
    }

And my fragment's XML just contains few buttons and images

Answer 1

I'm sorry to disappoint you, but that Tutorial you are following is BROKEN and you should not follow it.

• See #3 in https://proandroiddev.com/the-seven-actually-10-cardinal-sins-of-android-development-491d2f64c8e0,

• follow this gist https://gist.github.com/Zhuinden/c643f03a023a9cbe83fff6c75c948d3b#file-myfragmentpageradapter-kt

• and refer to https://stackoverflow.com/a/54280113/2413303

.

class MyFragmentPagerAdapter(
    private val context: Context,
    fragmentManager: FragmentManager
) : FragmentPagerAdapter(fragmentManager) {
    override fun getCount() = 2

    override fun getItem(position: Int) = when(position) {
        0 -> FirstFragment()
        1 -> SecondFragment()
        else -> throw IllegalStateException("Unexpected position $position")
    }

    override fun getPageTitle(position: Int): CharSequence = when(position) {
        0 -> context.getString(R.string.first)
        1 -> context.getString(R.string.second)
        else -> throw IllegalStateException("Unexpected position $position")
    }
}

class ParentFragment: Fragment() {
    override fun onCreateView(...) = ...

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        viewPager.adapter = MyFragmentPagerAdapter(requireContext(), childFragmentManager)
        tabLayout.setupWithViewPager(viewPager)
    }
}

And

Fragment fragment = childFragmentManager.findFragmentByTag("android:switcher:" + viewPager.getId() + ":" + fragmentPosition);

Answer 2

Perhaps the error occurs because ViewPager does not keep all the fragments active, only the current one and the other 2 at its side in onPause (). The others do not have the views created for the Activity, but we can access them through the ViewPager since the instances of the Fragments have been saved.

I hope the following code can help you:

/******************************************/
    //Your ADAPTER.
    class MyPageAdapter extends FragmentPagerAdapter {

        private List<Fragment> fragments;

        //Your code...


        /**
         * Use this method for get the fragments list.
         */
        public List<Fragment> getFragments(){
            return fragments;
        }

    }


    /******************************************/
    //In your ACTIVITY.
    private List<Fragment> getFragments() {
        pageAdapter.getFragments();
    }

    /**
    * Use this to find a view and cast for your needs.
    */
    private View findById(int fragmentIndex, int viewId){
        Fragment fragment = this.getFragments().get(fragmentIndex);
        View view = ((YourFragment) fragment).getView(); //Casting to YourFragment and catching root view.
        return view.findViewById(viewId);
    }



    /******************************************/
    //In your FRAGMENTS.
    //Must save fragment rootView to access to findById() method. For example:
    public class YourFragment extends Fragment{

        private View view;

        @Override
            public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                     Bundle savedInstanceState) {
                view = inflater.inflate(R.layout.fragment_scan, container, false);
                //Your code....
                return view;
        }

        public View getView(){
            return view;
        }

    }