即使我调用 notifyDataSetChanged()，RecyclerView 也不显示项目

Question

I'm using a RecyclerView to show datas but even if I call notifyDataSetChanged() after I got the list from Room db I can't see the items on the first creation of the fragment, but only on second one.

MainActivity.class

package it.bastoner.taboom;

import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.util.Log;
import android.view.MenuItem;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.room.Room;

import com.google.android.material.bottomnavigation.BottomNavigationView;
import com.google.android.material.navigation.NavigationBarView;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import it.bastoner.taboom.database.CardDAO;
import it.bastoner.taboom.database.CardEntity;
import it.bastoner.taboom.database.DatabaseTaboom;
import it.bastoner.taboom.fragments.AddFragment;
import it.bastoner.taboom.fragments.BaseCardFragment;
import it.bastoner.taboom.fragments.PlayFragment;
import it.bastoner.taboom.fragments.UpdateFragment;

public class MainActivity extends AppCompatActivity {

private static final String TAG = "MainActivity";
private List<CardEntity> cardList;
private boolean cardListIsUpdated;

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

    Log.d(TAG, ">>MainActivity created");

    cardList = new ArrayList<CardEntity>();

    // Setting bottomNavigation
    BottomNavigationView bottomNav = findViewById(R.id.bottom_nav);
    bottomNav.setOnItemSelectedListener(new NavigationBarView.OnItemSelectedListener() {

        //TODO ANIMATION

        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {

            // By using switch we can easily get the selected fragment by using there id.
            Fragment selectedFragment = null;
            switch (item.getItemId()) {
                case R.id.add_nav:
                    selectedFragment = new AddFragment(cardList);
                    break;
                case R.id.play_nav:
                    selectedFragment = new PlayFragment(cardList);
                    break;
                case R.id.update_nav:
                    selectedFragment = new UpdateFragment(cardList);
                    break;
            }
            // It will help to replace the
            // one fragment to other.
            getSupportFragmentManager()
                    .beginTransaction()
                    .replace(R.id.fragment_container, selectedFragment)
                    .commit();
            return true;
        }

    });

    // As soon as the application opens the play fragment should be shown to the user
    bottomNav.setSelectedItemId(R.id.play_nav);

    loadCardList();

}

private void loadCardList() {

    CardDAO cardDAO = DatabaseTaboom.getDatabase(getApplicationContext()).cardDao();

    // Setting cardsList
    ExecutorService executor = Executors.newSingleThreadExecutor();

    Runnable runnable = new Runnable() {
        @Override
        public void run() {
            cardList = cardDAO.getAll();
            Log.d(TAG, ">>Cardlist: " + cardList);
            BaseCardFragment actualFragment = (BaseCardFragment) getSelectedFragment();
            if (actualFragment != null)
                actualFragment.updateUI();
            else
                Log.d(TAG, ">>Null fragment");
        }
    };

    executor.execute(runnable);

}

private Fragment getSelectedFragment() {

    FragmentManager fragmentManager = getSupportFragmentManager();
    List<Fragment> fragmentList = fragmentManager.getFragments();
    for (Fragment f: fragmentList) {
        if (f != null && f.isVisible())
            return f;
    }
    return null;
}

}

PlayFragment.class

package it.bastoner.taboom.fragments;

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.media.MediaPlayer;
import android.os.Bundle;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.recyclerview.widget.GridLayoutManager;
import androidx.recyclerview.widget.PagerSnapHelper;
import androidx.recyclerview.widget.RecyclerView;
import androidx.recyclerview.widget.SnapHelper;

import android.os.CountDownTimer;
import android.text.InputFilter;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.util.List;
import java.util.Locale;

import it.bastoner.taboom.R;
import it.bastoner.taboom.adapter.RecyclerViewAdapter;
import it.bastoner.taboom.database.CardEntity;
import it.bastoner.taboom.filters.MinMaxFilter;
import it.bastoner.taboom.listeners.MyDialogListener;


public class PlayFragment extends BaseCardFragment implements MyDialogListener {

    private static final String TAG = "PlayFragment";

    private RecyclerView recyclerView;

    private TextView timerTextView;
    private Button playPauseButton;
    private Button resetButton;
    private CountDownTimer countDownTimer;
    private long startTimeInMillis;
    private boolean timerIsRunning;
    private long timeLeftInMillis = startTimeInMillis;

    private MediaPlayer endTimerSound;

    public PlayFragment(List<CardEntity> cardList) {
        super(cardList);
        //Log.d(TAG, "PlayFragment created");
    }

    @Override
    public void updateUI() {
        recyclerView.getAdapter().notifyDataSetChanged();
        Log.d(TAG, ">>Update, number of items: " + recyclerView.getAdapter().getItemCount());
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        Log.d(TAG, ">>OnCreateView");
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_play, container, false);
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        Log.d(TAG, ">>OnViewCreated");

        setRecyclerView();

        setTimerSound();

        setTimer();

        setDialogModifyTimer();

        updateUI();

    }

   // other methods...

}

As you can see in the console the cardList has received elements before the call of UpdateUI() but in update the first time is called no elements are seen. Can you help me? I don't understand why, thank you.

Answer 1

I think you need to set the Data in the RecyclerAdapter. Then call it inside your fragment with a ViewModel. I'm using Kotlin sorry. This works only if u are using RoomDB with viewModel and repositories

In your RecyclerAdapter, you first need to set an empty list

private var timerList = emptyList<EntityClassName>()
    
    // Now use a method to set the Data
    fun setData(timer : List<EntityClassName>){
        this.timerList = timer
        notifyDataSetChanged()
    }

Then you call this on your Fragment inside OnViewCreated.

sharedViewModel.readAllData.observe(viewLifecycleOwner,Observer {
            data -> myAdapter.setData(data)})