ListActivity.java

import android.app.ProgressDialog;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.ImageButton;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.Task;
import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.firebase.FirebaseApp;
import com.google.firebase.firestore.DocumentSnapshot;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.QuerySnapshot;

import org.jetbrains.annotations.NotNull;
import org.w3c.dom.Document;

import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.List;


public class ListActivity extends AppCompatActivity  {

    List<Collection> collectionList = new ArrayList<>();
    private RecyclerView mRecyclerView;
    RecyclerView.LayoutManager layoutManager;

    ImageButton mAddBtn;

    FirebaseFirestore db;

    private CustomAdapter adapter;

    ProgressDialog pd;

    @Override
    protected void onCreate (Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_list);

        db = FirebaseFirestore.getInstance();

        mRecyclerView = findViewById(R.id.recycler_view);
        mAddBtn = findViewById(R.id.addBtn);


        //       свойства просмотра recycler view
        mRecyclerView.setHasFixedSize(true);
        layoutManager = new LinearLayoutManager(this);
        mRecyclerView.setLayoutManager(layoutManager);
        adapter = new CustomAdapter(ListActivity.this, new ArrayList<>());
        mRecyclerView.setAdapter(adapter);

        pd = new ProgressDialog(this);

        showData();

        mAddBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(new Intent(ListActivity.this, Define.class));
                finish();
            }
        });


    }

    //        показ recyclerView
    private void showData() {

        db.collection("Documents").get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
            @Override
            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                if (pd != null && pd.isShowing()) {
                    pd.dismiss(); }
//                в удачном случае здесь появяятся файлы. Но нельзя просто так взять и вывести данные 👌
                for (DocumentSnapshot doc: task.getResult()) {
                    Collection collection = new Collection(doc.getString("id"),
                            doc.getString("title"));
//                    doc.getString() #todo description
                    collectionList.add(collection);
                }
//               TODO переписать участок кода (чтобы создание не было привязано к готовому листу)
//               CustomAdapter adapter = nre CustomAdapter()  recyclerView.setAdapter(adapter) см. Intellij

                adapter.putData(collectionList);
                adapter.notifyDataSetChanged();


            }
        }).addOnFailureListener(new OnFailureListener() {
            @Override
            public void onFailure(@NonNull @NotNull Exception e) {
                pd.dismiss();

                Toast.makeText(ListActivity.this, e.getMessage(), Toast.LENGTH_SHORT).show();

            }
        });
    }