Question

尽管接收到快照并在ArrayList onDataChange()函数中添加了数据，但此函数仍然没有返回任何数据，但最终，它返回的ArrayList大小为0。

List<ProductEntity> feed_data() {
        final List<ProductEntity> feededProducts = new ArrayList<>();

    progressDialog = new ProgressDialog(this);
    progressDialog.setMessage("Please wait...");
    progressDialog.show();
    mDatabase = FirebaseDatabase.getInstance().getReference("/products");

    //adding an event listener to fetch values
    mDatabase.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot snapshot) {
            //dismissing the progress dialog
            progressDialog.dismiss();

            //iterating through all the values in database
            for (DataSnapshot postSnapshot : snapshot.getChildren()) {
                ProductEntity upload = postSnapshot.getValue(ProductEntity.class);
                Log.d("error", "onDataChange: " + upload.about);
                feededProducts.add(upload);
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Log.d("Error", "onCancelled: " + databaseError.getMessage());

            NestedScrollView root_layout = findViewById(R.id.category_root_layout) ;
            Snackbar.make(root_layout, "Internal Error!!", Snackbar.LENGTH_SHORT).show();
        }
    });
    return feededProducts ;
}

Answer 1

onDataChange（DataSnapshot快照）是异步调用的。当我们从Firebase获取数据时，就会调用它。

因此，在onDataChange（DataSnapshot snapshot）方法之前调用返回feededProducts，因此每次都会返回空列表。

你必须在onDataChange（DataSnapshot snapshot）内的适配器上调用notifydatasetchanged

Answer 2

您需要创建一个接收数据的接口。

interface DataReceivedListener{
    void onDataReceived(List<ProductEntity> data);
}

void feed_data(DataReceivedListener listener) {
    final List<ProductEntity> feededProducts = new ArrayList<>();

    progressDialog = new ProgressDialog(this);
    progressDialog.setMessage("Please wait...");
    progressDialog.show();
    mDatabase = FirebaseDatabase.getInstance().getReference("/products");

    //adding an event listener to fetch values
    mDatabase.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot snapshot) {
            //dismissing the progress dialog
            progressDialog.dismiss();

            //iterating through all the values in database
            for (DataSnapshot postSnapshot : snapshot.getChildren()) {
                ProductEntity upload = 
                postSnapshot.getValue(ProductEntity.class);
                Log.d("error", "onDataChange: " + upload.about);
                feededProducts.add(upload);
            }

            listener.onDataReceived(feededProducts);

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Log.d("Error", "onCancelled: " + databaseError.getMessage());

            NestedScrollView root_layout = 
            findViewById(R.id.category_root_layout) ;
            Snackbar.make(root_layout, "Internal Error!!", 
            Snackbar.LENGTH_SHORT).show();
        }
    });
}

在Activty中实现DataReceivedListener接口并覆盖'onDataReceived（）'函数并将'this'传递给'feed_data（DataReceivedListener listener）'或创建DataReceivedListener作为变量并将该变量传递给'feed_data（DataReceivedListener listener））'

当在'onDataChange（DataSnapshot snapshot）'中接收到数据时，它会调用接口的函数，然后调用被覆盖的函数或者初始化'DataReceivedListener'变量的函数。取决于你是如何做到的。

将代码放在“onDataReceived（列表数据）”函数中需要数据的位置。

Answer 3

我需要在我的Arralist中添加所有数据之后初始化并设置适配器，如此指出above这样就解决了我的问题：

mDatabase.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot snapshot) {

                for (DataSnapshot postSnapshot : snapshot.getChildren()) {
                    ProductEntity upload = postSnapshot.getValue(ProductEntity.class);
                    productList.add(upload);
                }

                productsRecyclerAdapter = new ProductsRecyclerAdapter(productList, CategoryActivity.this);
                products.setAdapter(productsRecyclerAdapter);
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                NestedScrollView root_layout = findViewById(R.id.category_root_layout);
                Snackbar.make(root_layout, "Internal Error!!", Snackbar.LENGTH_SHORT).show();
            }
        });

FireBase onDataChange（）不起作用

3 个答案: