The Activity does not update the content of a ViewPager2

Question

I implemented an Android application that must display the content of a list in an Activity, using ViewPager2. See an extract of the code below:

public class MainActivity extends AppCompatActivity {
    ViewPager2 viewPager2;
    DataPageViewAdapter dataPageViewAdapter;
    private Button btnFind;
    private Control ctrl;
    private List<Movie> movies;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        viewPager2 = findViewById(R.id.view_pager_2);
        ctrl = new Control();
        movies = ctrl.loadAllMovies();        
        btnFind = findViewById(R.id.btn_find);
        btnFind.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(!movies.isEmpty()) {
                    dataPageViewAdapter = new DataPageViewAdapter(getApplicationContext(), movies);
                    dataPageViewAdapter.notifyDataSetChanged();
                    viewPager2.setAdapter(dataPageViewAdapter);
                }else{
                    Toast.makeText(this, "No movies loaded yet!", Toast.LENGTH_LONG).show();
                }
            }
        });       
        
    }
}

There is a Movie class with the data stored and a Control class that returns a list of Movies. The application fulfills its purpose, but I have to press the menu action twice so that the information in the Activity is observed. How can I make the content appear at once when pressing the button? Thanks in advance!

Answer 1

You should set the adapter first and then call notifyDataSetChanged()

@Override
public void onClick(View v) {
    if(!movies.isEmpty()) {
        dataPageViewAdapter = new DataPageViewAdapter(getApplicationContext(), movies);
        viewPager2.setAdapter(dataPageViewAdapter);
        dataPageViewAdapter.notifyDataSetChanged();
    } else {
        Toast.makeText(this, "No movies loaded yet!", Toast.LENGTH_LONG).show();
    }
}

Answer 2

This is my Adapter and Holder for the ViewPager2

package com.example.test.view;

import android.annotation.SuppressLint;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;
import com.bumptech.glide.Glide;
import java.util.List;
import com.example.test.R;
import com.example.test.models.Configuration.APIVars;
import com.example.test.models.Movies.Movie;

public class DataPageViewAdapter extends RecyclerView.Adapter<ViewHolderA> {
    Context context;
    List<Movie> myMovies;
    LayoutInflater li;

    public DataPageViewAdapter(Context context, List<Movie> movies){
        this.context = context;
        this.myMovies = movies;
        li = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

    @NonNull
    @Override
    public ViewHolderA onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View mv = li.inflate(R.layout.movie_view_detail,parent,false);
        return new ViewHolderA(mv);
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolderA holder, int position) {
        holder.bindData(myMovies.get(position));
    }

    @Override
    public int getItemCount() {
        return myMovies.size();
    }
}
class ViewHolderA extends RecyclerView.ViewHolder{
    private final ImageView movie_poster;
    private final TextView tv_title;
    private final TextView tv_id;
    private final TextView tv_overview;
    private final TextView tv_genre;
    private final TextView tv_release_date;
    private final TextView tv_vote_count;
    private final TextView tv_vote_avg;
    private final View mv;

    /*My ViewHolderA Constructor*/
    public ViewHolderA(@NonNull View itemView) {
        super(itemView);
        mv = itemView;
        movie_poster = mv.findViewById(R.id.movie_poster);
        tv_id = mv.findViewById(R.id.tv_id);
        tv_title = mv.findViewById(R.id.tv_title);
        tv_overview = mv.findViewById(R.id.tv_overview);
        tv_genre = mv.findViewById(R.id.tv_genre);
        tv_release_date = mv.findViewById(R.id.tv_release_date);
        tv_vote_avg = mv.findViewById(R.id.tv_vote_avg);
        tv_vote_count = mv.findViewById(R.id.tv_vote_count);
    }

    /*Set all the movie data to show*/
    @SuppressLint("SetTextI18n")
    public void bindData(Movie mMovie){
        if(mMovie != null) {            
            Glide.with(mv).load(APIVars.BASE_IMG_URL + mMovie.getBackdrop_path()).into(movie_poster);
            tv_title.setText("Title: " + mMovie.getTitle());
            tv_id.setText("ID: " + mMovie.getId());
            tv_overview.setText("Overview: " + mMovie.getOverview());
            tv_genre.setText("Genre: " + mMovie.getGenres());
            tv_release_date.setText("Release Date: " + mMovie.getRelease_date());
            tv_vote_count.setText("Vote Count: " + mMovie.getVote_count());
            tv_vote_avg.setText("Vote AVG: " + mMovie.getVote_average());
        }
    }
}

I don't see anything wrong that could affect performance, but no matter where I put the notifyDataSetChanged function, the result is still that I have to press the button twice for it to display the information.