我正在学习MVVM和使用Room和Viewmodel的教程。我手动插入了一些项目到数据库中，它应该用这些项目更新RecyclerView。但是，当我运行它时，连接到适配器的arrayList显示为0。我不确定我哪里出错了，因为我遵循了教程，但是在检查了几次步骤后，我仍然卡住了

主要活动：

public class MainActivity extends AppCompatActivity {

    private NoteViewModel noteViewModel;

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

        RecyclerView recyclerView = findViewById(R.id.recycler_view);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        recyclerView.setHasFixedSize(true);
        NoteAdapter noteAdapter = new NoteAdapter();
        recyclerView.setAdapter(noteAdapter);

        noteViewModel = new ViewModelProvider(this).get(NoteViewModel.class);

        //noteViewModel = new ViewModelProvider(this).get(NoteViewModel.class);
        noteViewModel.getAllNotes().observe(this, new Observer<List<Note>>() {
            @Override
            public void onChanged(List<Note> notes) {
                noteAdapter.setNotes(notes);
                Toast.makeText(MainActivity.this, "onChanged", Toast.LENGTH_SHORT).show();

            }
        });

    }

视图模型：

public class NoteViewModel extends AndroidViewModel {

    private NoteRepository repository;
    private LiveData<List<Note>> allNotes;


    public NoteViewModel(@NonNull Application application) {
        super(application);
        repository = new NoteRepository(application);
        allNotes = repository.getAllNotes();
    }

    public void insert(Note note){
        repository.insert(note);
    }

    public void update(Note note){
        repository.update(note);
    }

    public void delete(Note note){
        repository.delete(note);
    }

    public void deleteAllNotes(){
        repository.deleteAllNotes();
    }

    public LiveData<List<Note>> getAllNotes() {
        return allNotes;
    }
}

存储库：

public class NoteRepository {
    private NoteDao noteDao;
    private LiveData<List<Note>> allNotes;

    public NoteRepository(Application application){
        NoteDatabase database = NoteDatabase.getInstance(application);
        noteDao = database.noteDao();
        allNotes = noteDao.getAllNotes();
    }

    public void insert(Note note){
        new InsertNoteAsyncTask(noteDao).execute(note);
    }

    public void update(Note note){
        new UpdateNoteAsyncTask(noteDao).execute(note);
    }

    public void delete(Note note){
        new DeleteNoteAsyncTask(noteDao).execute(note);
    }

    public void deleteAllNotes(){
        new DeleteAllNotesAsyncTask(noteDao).execute();
    }

    public LiveData<List<Note>> getAllNotes(){
        return allNotes;
    }

    private static class InsertNoteAsyncTask extends AsyncTask<Note, Void, Void>{
        private NoteDao noteDao;

        private InsertNoteAsyncTask(NoteDao noteDao){
            this.noteDao = noteDao;
        }

        @Override
        protected Void doInBackground(Note... notes) {
            noteDao.insert(notes[0]);
            return null;
        }
    }
    private static class UpdateNoteAsyncTask extends AsyncTask<Note, Void, Void>{
        private NoteDao noteDao;

        private UpdateNoteAsyncTask(NoteDao noteDao){
            this.noteDao = noteDao;
        }

        @Override
        protected Void doInBackground(Note... notes) {
            noteDao.update(notes[0]);
            return null;
        }
    }
    private static class DeleteNoteAsyncTask extends AsyncTask<Note, Void, Void>{
        private NoteDao noteDao;

        private DeleteNoteAsyncTask(NoteDao noteDao){
            this.noteDao = noteDao;
        }

        @Override
        protected Void doInBackground(Note... notes) {
            noteDao.delete(notes[0]);
            return null;
        }
    }
    private static class DeleteAllNotesAsyncTask extends AsyncTask<Void, Void, Void>{
        private NoteDao noteDao;

        private DeleteAllNotesAsyncTask(NoteDao noteDao){
            this.noteDao = noteDao;
        }

        @Override
        protected Void doInBackground(Void... voids) {
            noteDao.deleteAllNotes();
            return null;
        }
    }
}

适配器：

public class NoteAdapter extends RecyclerView.Adapter<NoteAdapter.MyViewHolder> {
    private List<Note> notes = new ArrayList<>();


    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View itemView = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.note_item, parent, false);
        return new MyViewHolder(itemView);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        Note currentNote = notes.get(position);
        holder.tvTitle.setText(currentNote.getTitle());
        holder.tvDescription.setText(currentNote.getDescription());
        holder.tvPriority.setText(String.valueOf(currentNote.getPriority()));
    }

    @Override
    public int getItemCount() {
        return notes.size();
    }

    public void setNotes(List<Note> notes){
        this.notes = notes;
        notifyDataSetChanged();
    }
    public class MyViewHolder extends RecyclerView.ViewHolder{

        private TextView tvTitle;
        private TextView tvDescription;
        private TextView tvPriority;

        public MyViewHolder(@NonNull View itemView) {
            super(itemView);
            tvTitle = itemView.findViewById(R.id.tvTitle);
            tvDescription = itemView.findViewById(R.id.tvDescription);
            tvPriority = itemView.findViewById(R.id.tvPriority);
        }
    }

数据库类：

@Database(entities = {Note.class}, version = 1)
public abstract class NoteDatabase extends RoomDatabase {

    private static NoteDatabase instance;

    public abstract NoteDao noteDao();

    public static synchronized NoteDatabase getInstance(Context context){
        if(instance == null){
            instance = Room.databaseBuilder(context.getApplicationContext(),
                    NoteDatabase.class, "note_database")
                    .fallbackToDestructiveMigration()
                    .addCallback(roomCallback)
                    .build();
        }
        return instance;
    }
    private static RoomDatabase.Callback roomCallback = new RoomDatabase.Callback(){
        @Override
        public void onCreate(@NonNull SupportSQLiteDatabase db) {
            super.onCreate(db);
            new PopulateDbAsyncTask(instance).execute();
        }
    };

    private static class PopulateDbAsyncTask extends AsyncTask<Void, Void, Void>{
        private NoteDao noteDao;

        public PopulateDbAsyncTask(NoteDatabase db){
            noteDao = db.noteDao();
        }

        @Override
        protected Void doInBackground(Void... voids) {
            noteDao.insert(new Note("Title 1", "Description 1", 1));
            noteDao.insert(new Note("Title 2", "Description 2", 2));
            noteDao.insert(new Note("Title 3", "Description 3", 3));

            return null;
        }
    }

道：

@Dao
public interface NoteDao {

    @Insert
    void insert(Note note);

    @Update
    void update(Note note);

    @Delete
    void delete(Note note);

    @Query("DELETE FROM note_table")
    void deleteAllNotes();

    @Query("SELECT * FROM note_table ORDER BY priority DESC") // '*' means all columns
    LiveData<List<Note>> getAllNotes();

}

感谢您的任何意见