如何遍历购物车中的物品，2)将每个物品添加到新位置，3)从旧位置删除物品

本文介绍了如何遍历购物车中的物品，2)将每个物品添加到新位置，3)从旧位置删除物品的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

正在使用Recycler firestore Adapter显示项目.我希望将recyclerview视图中的项目添加到另一个数据库中，并通过单击按钮清除初始数据库.我尝试了此操作，但出现了一个错误，即com.google.firebase.firestore.QuerySnapshot无法转换为java.util.List

The items are being displayed using Recycler firestore Adapter. I want to have the items in the recyclerview view to be added in another database and the initial database cleared by a click of a button. I tried this but I got the error that com.google.firebase.firestore.QuerySnapshot cannot be cast to java.util.List

public void placeOrder() { FirebaseFirestore rootRef = FirebaseFirestore.getInstance(); Query query = rootRef .collection(String.valueOf(new StringBuffer("/").append(user.getUid()).append("cart"))); Task task = query.get(); task.addOnSuccessListener(new OnSuccessListener<List<Object>>() { @Override public void onSuccess(List<Object> list) { CollectionReference notebookRef; mAuth = FirebaseAuth.getInstance(); user = mAuth.getCurrentUser(); if (user != null) { notebookRef = db.collection(String.valueOf(new StringBuffer("/").append(user.getUid()).append("order"))); String name = String.valueOf(list.get(1)); String delivery = String.valueOf(list.get(2)); String amount = String.valueOf(list.get(3)); OrderItem orderItem = new OrderItem(name, amount, delivery); notebookRef.add(orderItem); Toast.makeText(context, "Items Added", Toast.LENGTH_SHORT).show(); } } }); }[![This is my Database][1]][1]

推荐答案

Query.get()方法返回Task<QuerySnapshot>.由于您在代码中声明了Task<List>，因此会导致出现错误:com.google.firebase.firestore.QuerySnapshot cannot be cast to java.util.List.

The Query.get() method returns a Task<QuerySnapshot>. Since you declared a Task<List> in your code, this results in the error that you get: com.google.firebase.firestore.QuerySnapshot cannot be cast to java.util.List.

请阅读从集合中读取多个文档的文档，因为它包含您需要的精确代码段.

Please study the documentation on reading multiple documents from a collection, as it contains the precise code snippet you need.

尽管您要从一个集合中获取整个集合还是单个文档，但我还是不能完全确定.您可以这样初始化query:

One thing I'm not completely sure of though it whether you are getting an entire collection or a single document from a collection. You initialize query like this:

Query query = rootRef.collection( String.valueOf(new StringBuffer("/").append(user.getUid()).append("cart")));

据我所知，这导致了"/uidOfUser/cart"的路径，该路径将指向以当前用户命名的集合中的cart 文档.如果确实是您的数据库结构，则需要遵循获取单个文档，该文档将返回Task<DocumentSnapshot>.

As far as I can see this leads to a path of "/uidOfUser/cart", which would point to the cart document in a collection named after the current user. If that is indeed your database structure, you'll need to follow the example from the documentation on getting a single document, which returns a Task<DocumentSnapshot>.