android - Firebase DB 无法显示查询的对象列表

Question

出于某种原因，我的 Adapter 没有获得更新的对象列表，经过一些调试后我发现了这一点:当我在UpdateUI()，由于某种原因查询不会启动并返回一个空列表。也许我按错误的顺序做某事？ (只是一些随机文本，因为我无法发布问题)

public class ProductListFragment extends Fragment {
    private RecyclerView mRecyclerView;
    private ListFragmentAdapter mAdapter;
    private List<Product> mProducts;

    public ProductListFragment() {
        // Required empty public constructor
    }
    public static ProductListFragment newInstance() {
       return new ProductListFragment();
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setHasOptionsMenu(true);


    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_product_list, container, false);
        mRecyclerView = (RecyclerView)view.findViewById(R.id.listRecyclerView);
        mRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
        updateUI();
        return view;
    }

    @Override
    public void onStart() {
        super.onStart();
        updateUI();
    }

    private void updateUI() {
        mProducts = DbOps.get(getActivity()).getProducts();

        if (mAdapter == null) {
            mAdapter = new ListFragmentAdapter(mProducts);
            mRecyclerView.setAdapter(mAdapter);
        } else {
            mAdapter.notifyDataSetChanged();
        }
    }


public class DbOps {
    private static DbOps sDbOps;
    private FirebaseDatabase mDb;
    private DatabaseReference dbRef;
    private List<Product> mProducts;
    private Product pr;

    public static DbOps get(Context ctx) {
        if (sDbOps == null) {
            sDbOps = new DbOps(ctx);
        }
        return sDbOps;
    }

    private DbOps(Context ctx) {
        mDb = FirebaseDatabase.getInstance();
        dbRef = mDb.getReference().child("products");
        if(mProducts == null) {
            getProducts();
        }
    }

    public Product getProduct(final String mId) {
        Query query = dbRef.child(mId);
        query.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                if(dataSnapshot.exists()) {
                    for (DataSnapshot obj: dataSnapshot.getChildren()) {
                        if(obj.getValue(Product.class).getId().equals(mId)) {
                            pr = obj.getValue(Product.class);
                            break;
                        }
                    }
                }
            }
            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
        return pr;
    }

    public void addProduct(Product product) {
        dbRef.child(product.getId()).setValue(product);
    }


    public List<Product> getProducts() {
        ValueEventListener listener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                for(DataSnapshot mObject : dataSnapshot.getChildren());
                mProducts = new ArrayList<>();
                Product pr = dataSnapshot.getValue(Product.class);
                mProducts.add(pr);
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        };
        dbRef.addValueEventListener(listener);
        return mProducts;
    }

}

Answer 1

您不能以您的方式返回 Product 类的对象，由于 onDataChange() 方法的异步行为，它始终为空您正在尝试从数据库中获取数据。所以要解决这个问题，你有两个选择:

1. 您需要在 onDataChange() 方法中使用和声明 pr 对象，如下所示:

   query.addListenerForSingleValueEvent(new ValueEventListener() {
       @Override
       public void onDataChange(DataSnapshot dataSnapshot) {
           if(dataSnapshot.exists()) {
               for (DataSnapshot obj: dataSnapshot.getChildren()) {
                   if(obj.getValue(Product.class).getId().equals(mId)) {
                       Product pr = obj.getValue(Product.class);
                       //do what you want to do with pr
                   }
               }
           }
       }
       @Override
       public void onCancelled(DatabaseError databaseError) {}
   });
   

2. 如果您想在onDataChange() 方法 之外使用pr 对象的值，那么请参阅我在post 中的回答.