为什么我的返回值在if语句之外为空？
为什么我的返回值在if语句之外为空？
提问于 2019-06-09 17:13:27
我认为有人已经有了这个问题，但是没有一个解决方案对我有效。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(因此使用下面的If语句)，则返回同一文档中的一个字段。编辑:在底层中工作的代码
我尝试将返回值声明为全局、局部、最终，但都不起作用。
我正在记录结果并检查"item image 1:“，我得到了我正在寻找的url字符串，但当我再次检查"item image 2:”时，该值为空。当这些值是相同的时候，这怎么可能呢？
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的最终有效代码。如果人们没有为嵌套回收器视图寻找类似的解决方案，我会感到惊讶，在这种视图中，数据是从firestore查询的。
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);
}
}
});回答 1
Stack Overflow用户
回答已采纳
发布于 2019-06-09 17:29:37
restaurantQuery.get().addOnSuccessListener(...);在这里，您将向您的查询添加一个侦听器。这意味着addOnSuccessListener将立即返回，但稍后在后台执行查询并返回结果时将调用onSuccess。因此，此代码将在调用onSuccess之前执行：
Log.d(TAG, "item image 2:"+ orderItemImage);
return orderItemImage;这意味着orderItemImage仍然是null，因为它不是由onSuccess方法设置的。您可以将依赖于orderItemImage的代码放在onSuccess方法中：
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：
这两行依赖于orderItemImage，
MealplanMenu mealplanMenu = new MealplanMenu(menuItemName, orderDocument.getString("menuItemQuantity"),orderItemImage);
mealplanMenuArrayList.add(mealplanMenu);因此您可以将它们放在onSuccess中用于餐厅查询，还可以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);
}
});页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/56513349
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