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java - 为什么我在 if 语句之外的返回值是 null？

问题描述

我想有人已经遇到过这个问题，但没有一个解决方案对我有用。这是背景。我正在查询 firestore 以查看文档中是否存在某个索引。如果是这样（因此下面的 if 语句），则返回同一文档中的一个字段。编辑：底部的工作代码

我尝试将返回值声明为全局、本地、最终值，但没有任何效果。

我正在记录结果并在哪里检查“项目图像 1：”我得到了我正在寻找的 url 字符串，但是当我再次检查“项目图像 2：”时，该值为空。当价值观相同时，怎么可能呢？

private String getImage(String classRestaurantId, final String menuItemName) {
        CollectionReference restaurantQuery = firebaseFirestore.collection("Restaurant_Data").document(classRestaurantId).collection("Menu_Card");
        restaurantQuery.get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
            @Override
            public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                for (QueryDocumentSnapshot menuDocument : queryDocumentSnapshots) {
                    if (menuDocument.getString("menuItemName").equals(menuItemName)) {
                        orderItemImage = menuDocument.getString("menuItemImage");
                        Log.d(TAG, "item image 1:"+ orderItemImage);
                    }
                }
            }
        });
        Log.d(TAG, "item image 2:"+ orderItemImage);
        return orderItemImage;
    }

这是我的 OnBindViewHolder：

public void onBindViewHolder(@NonNull final MealPlanDetailsAdapter.MealPlanDetailsViewHolder mpDetailsViewHolder, final int position) {



        // Menu items
        mpDetailsViewHolder.mealplanMenuRecycler.setHasFixedSize(true);
        mpDetailsViewHolder.mealplanMenuRecycler.setLayoutManager(new LinearLayoutManager(adapterContext, LinearLayoutManager.HORIZONTAL, false));

        String orderId = mealplanDetailsArrayList.get(position).getOrderDetails();

        FirebaseUser firebaseUser = FirebaseAuth.getInstance().getCurrentUser();
        if (firebaseUser != null) {
            for (UserInfo profile : firebaseUser.getProviderData()) {
                userId = profile.getUid();
            }
        }

        // Get order details
        final String restaurantId = mealplanDetailsArrayList.get(position).getRestaurantId();
        CollectionReference orderReference = firebaseFirestore.collection("Customer_Data").document(userId).collection("Meal_Plans").document(orderId).collection("Placed_Order");
        orderReference.get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
            @Override
            public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                mealplanMenuArrayList = new ArrayList<>();
                for (QueryDocumentSnapshot orderDocument : queryDocumentSnapshots) {
                    // Get image
                    final String menuItemName = orderDocument.getString("menuItemName");


                    //String menuItemImage = getImage(restaurantId, menuItemName);

                    // Start of image collection
                    CollectionReference restaurantQuery = firebaseFirestore.collection("Restaurant_Data").document(restaurantId).collection("Menu_Card");
                    restaurantQuery.get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                        @Override
                        public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                            for (QueryDocumentSnapshot menuDocument : queryDocumentSnapshots) {
                                if (menuDocument.getString("menuItemName").equals(menuItemName)) {
                                    orderItemImage = menuDocument.getString("menuItemImage");
                                    Log.d(TAG, "item image 1:"+ orderItemImage);
                                }
                            }
                        }
                    });
                    // End of image collection


                    MealplanMenu mealplanMenu = new MealplanMenu(menuItemName, orderDocument.getString("menuItemQuantity"),orderItemImage);
                    mealplanMenuArrayList.add(mealplanMenu);
                }
                mealplanMenuAdapter = new MealplanMenuAdapter(adapterContext, mealplanMenuArrayList);
                mpDetailsViewHolder.mealplanMenuRecycler.setAdapter(mealplanMenuAdapter);
            }
        });
    }

这是来自 ElektroWeak 的最终工作代码。如果人们不是在寻找与从 Firestore 查询数据的嵌套回收器视图类似的解决方案，我会感到惊讶。

final String restaurantId = mealplanDetailsArrayList.get(position).getRestaurantId();
        CollectionReference orderReference = firebaseFirestore.collection("Customer_Data").document(userId).collection("Meal_Plans").document(orderId).collection("Placed_Order");
        orderReference.get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
            @Override
            public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                final ArrayList<MealplanMenu> mealplanMenuArrayList = new ArrayList<>();
                for (QueryDocumentSnapshot orderDocument : queryDocumentSnapshots) {
// Get image
                    final String menuItemName = orderDocument.getString("menuItemName");
                    final String menuItemQuantity = orderDocument.getString("menuItemQuantity");

//String menuItemImage = getImage(restaurantId, menuItemName);

                    CollectionReference restaurantQuery = firebaseFirestore.collection("Restaurant_Data").document(restaurantId).collection("Menu_Card");
                    restaurantQuery.get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                        @Override
                        public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                            for (QueryDocumentSnapshot menuDocument : queryDocumentSnapshots) {
                                if (menuDocument.getString("menuItemName").equals(menuItemName)) {
                                    String menuItemImage = menuDocument.getString("menuItemImage");
                                    Log.d(TAG, "item image 1:"+ orderItemImage);

                                    MealplanMenu mealplanMenu = new MealplanMenu(menuItemName, menuItemQuantity, menuItemImage);
                                    mealplanMenuArrayList.add(mealplanMenu);
                                }
                            }
                            mealplanMenuAdapter = new MealplanMenuAdapter(adapterContext, mealplanMenuArrayList);
                            mpDetailsViewHolder.mealplanMenuRecycler.setAdapter(mealplanMenuAdapter);
                        }

                    });

// Log.d(TAG, "item image 2:"+ orderItemImage);
                }
            }
        });

标签： javaandroid

解决方案

restaurantQuery.get().addOnSuccessListener(...);

在这里，您正在向查询添加侦听器。这意味着addOnSuccessListener将立即返回，但onSuccess稍后会在查询在后台执行并返回结果时调用。因此，此代码将在onSuccess调用之前执行：

Log.d(TAG, "item image 2:"+ orderItemImage);
return orderItemImage;

这意味着orderItemImage仍然是null因为它不是通过onSuccess方法设置的。您可以放置​​取决于orderItemImage内部onSuccess方法的代码：

restaurantQuery.get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
    @Override
    public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
        for (QueryDocumentSnapshot menuDocument : queryDocumentSnapshots) {
            if (menuDocument.getString("menuItemName").equals(menuItemName)) {
                orderItemImage = menuDocument.getString("menuItemImage");
                Log.d(TAG, "item image 1:"+ orderItemImage);

                // Place your code here, or call methods that depend on orderItemImage
            }
        }
    }
});

编辑：

这两行取决于orderItemImage,

MealplanMenu mealplanMenu = new MealplanMenu(menuItemName, orderDocument.getString("menuItemQuantity"),orderItemImage);
mealplanMenuArrayList.add(mealplanMenu);

因此您可以将它们放在里面onSuccess以进行餐厅查询，并将适配器创建代码移到那里

CollectionReference restaurantQuery = firebaseFirestore.collection("Restaurant_Data").document(restaurantId).collection("Menu_Card");
restaurantQuery.get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
    @Override
    public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
        for (QueryDocumentSnapshot menuDocument : queryDocumentSnapshots) {
            if (menuDocument.getString("menuItemName").equals(menuItemName)) {
                orderItemImage = menuDocument.getString("menuItemImage");
                Log.d(TAG, "item image 1:"+ orderItemImage);

                MealplanMenu mealplanMenu = new MealplanMenu(menuItemName, orderDocument.getString("menuItemQuantity"),orderItemImage);
                mealplanMenuArrayList.add(mealplanMenu);
            }
        }
        mealplanMenuAdapter = new MealplanMenuAdapter(adapterContext, mealplanMenuArrayList);    
        mpDetailsViewHolder.mealplanMenuRecycler.setAdapter(mealplanMenuAdapter);
    }
});

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