从 Firebase 数据库读取数据列表时为空模型对象

Question

我正在使用 Firebase 实时数据库来填充回收站视图，使用材料设计卡片视图来显示不同类别的业务。

在我的导航抽屉中，我有一个类别列表，我将在这些类别中显示我所在地区的不同类型的企业。到目前为止，我只有一个片段可以放大视图，以确保在我到达其他片段之前一切正常nav categories，这是 Firebase 数据库的屏幕截图database

当我点击“会计/财务”片段时，我从 Android Studios 中得到一个显示 java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference

这会将我带到我的 Recyclerview 适配器类中的 getItemCount()。它返回 null 因为 Model 类为 null。如果我硬编码一个像 10 这样的数字以在getItemCount() 中返回，那么它将在onBindViewHolder() 中出错，我从企业列表中获取位置。这是我认为与该问题有关的代码。部分代码将被删除以使其尽可能简洁。

这是我从 Firebase 数据库获取数据并尝试将数据分配给 Businesses.java 模型类以从 recyclerview 读取的主要活动。

public class HomeScreenActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener
{
    private List<Businesses> mBusinesses = new ArrayList<>();
    private RecyclerViewAdapter mRecyclerViewAdapter;

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

        DatabaseReference database = FirebaseDatabase.getInstance().getReference().child("accounting");
        database.addChildEventListener(new ChildEventListener()
        {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s)
            {
                Businesses model = dataSnapshot.getValue(Businesses.class);
                mBusinesses.add(model);
            }
        });
    }


    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item)
    {
        int id = item.getItemId();
        displaySelectedScreen(id);
        item.setCheckable(true);
        setTitle(item.getTitle());

        return true;
    }

    private void displaySelectedScreen(int itemId)
    {
        Fragment fragment = null;

        switch (itemId)
        {
            case R.id.nav_accounting_finance:
                fragment = new AccountingFinanceFrag();
                break;
        }

        if (fragment != null)
        {
            FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
            ft.replace(R.id.fragment_container, fragment);
            ft.commit();
        }
    }

}

这是我的模型类，当从“会计/财务”片段调用 recyclerview 时返回 null

@IgnoreExtraProperties
public class Businesses
{
    public String Name;
    public String About;
    public String Picture;
    public long Phone;

    public Businesses()
    {

    }

    public Businesses(String name, String about, String picture)
    {
        Name = name;
        About = about;
        Picture = picture;
    }
}

这是我的 Recyclerview 适配器类，我在其中获取了应该包含 Firebase 数据库数据的空模型对象。

public class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerViewAdapter.RecViewHolder>
{
    private Context mContext;
    private List<Businesses> mBusinesses;

    public RecyclerViewAdapter(Context context, List<Businesses> businesses)
    {
        mContext = context;
        mBusinesses = businesses;
    }

    @Override
    public RecViewHolder onCreateViewHolder(ViewGroup parent, int viewType)
    {
        LayoutInflater inflater = LayoutInflater.from(mContext);
        View view = inflater.inflate(R.layout.card, parent, false);
        return new RecViewHolder(view);
    }

    @Override
    public void onBindViewHolder(RecViewHolder holder, int position)
    {
        Businesses businesses = mBusinesses.get(position);
        holder.mName.setText(businesses.Name);
        holder.mAbout.setText(businesses.About);
        holder.bindGalleryItem(businesses);
    }

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

    public class RecViewHolder extends RecyclerView.ViewHolder
    {
        private CardView mCardView;
        private Businesses mBusinesses;
        private ImageView mImageView;
        private TextView mName, mAbout;

        RecViewHolder(View itemView)
        {
            super(itemView);
            mCardView = (CardView) itemView.findViewById(R.id.card_view);
            mImageView = (ImageView) itemView.findViewById(R.id.buisness_image);
            mName = (TextView) itemView.findViewById(R.id.business_name);
            mAbout = (TextView) itemView.findViewById(R.id.buisness_about);
        }

        public void bindGalleryItem(Businesses businesses)
        {
            mBusinesses = businesses;
            Glide.with(mContext)
                    .load(businesses.Picture)
                    .placeholder(R.mipmap.ic_launcher)
                    .diskCacheStrategy(DiskCacheStrategy.RESULT)
                    .override(200, 200)
                    .into(mImageView);
        }
    }
}

这是 recyclerview 被夸大的片段。

public class AccountingFinanceFrag extends Fragment
{
    private List<Businesses> mBusinesses;
    private RecyclerView mRecyclerView;
    private RecyclerView.Adapter mAdapter;
    private TextView mNameText, mAboutText;
    private ImageView mImageView;

        @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
    {
        View v = inflater.inflate(R.layout.fragment_category, container, false);

        mNameText = (TextView) v.findViewById(R.id.business_name);
        mAboutText = (TextView) v.findViewById(R.id.buisness_about);
        mImageView = (ImageView) v.findViewById(R.id.buisness_image);

        mRecyclerView = (RecyclerView) v.findViewById(R.id.fragment_recyclerview);
        mAdapter = new RecyclerViewAdapter(getContext(), mBusinesses );
        mRecyclerView.setAdapter(mAdapter);
        mRecyclerView.setLayoutManager(new LinearLayoutManager(getContext()));

        return v;
    }
}

数据库的 JSON 文件

 {
  "accounting" : {
    "buisness_0" : {
      "About" : "dummy text",
      "Address" : "123 test ave test, test 00000",
      "Email" : "test@example.com",
      "Name" : "Test test",
      "Owner" : "Test owner",
      "Phone" : 4120000000,
      "Picture" : "add link"
    },
    "business_1" : {
      "About" : "dummy text",
      "Address" : "123 test ave test, test 00000",
      "Email" : "test@example.com",
      "Name" : "Test test",
      "Owner" : "Test owner",
      "Phone" : 4120000000,
      "Picture" : "add link"
    }
  }
}

Answer 1

您的活动中的mBusinesses 和您的片段中的mBusinesses 是不同的东西，这就是发生混淆的地方。

在您的活动中，您初始化并向mBusinesses 添加条目，但是在创建 RecyclerView 时，您将片段的mBusinesses 对象传递给它，该对象尚未初始化或添加任何数据，因此NullPointerException！

为此，Fragment 需要将数据提供给 RecyclerView，可能是通过在 Fragment 中加载数据，或者 Activity 将数据传递给 Fragment 或其他方式。