Android Firebase数据库如何从子节点检索所有数据 [英] android firebase database how to retrieve all data from child node

问题描述

我有一个应用程序，它可以从Firebase检索大量数据，但是我似乎无法使其正常工作.

I've got an app where it retrieves a bunch of data from firebase but I cant seem to make it work.

database = FirebaseDatabase.getInstance();
    myRef = database.getReference().child("Sold").child("Item");

myListView = (ListView)findViewById(R.id.listView);
    final ArrayAdapter<String> myArrayAdapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_1, myArrayList);


    myListView.setAdapter(myArrayAdapter);

    myRef.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
            String value = dataSnapshot.getValue(String.class);
            myArrayList.add(value);
            myArrayAdapter.notifyDataSetChanged();
        }

它不会给我任何错误，但是会关闭我的应用程序. 我试图访问URL/Sold/Item并在listview中传递数据

It doesnt give me any error but it just closes my app. Im trying to access URL/Sold/Item and pass the data in my listview

推荐答案

您正在使用以下内容创建引用:

You're creating a reference with this:

database = FirebaseDatabase.getInstance();
myRef = database.getReference().child("Sold").child("Item");

然后添加ChildEventListener.这意味着，将首先使用DUMMY 1节点的快照调用onChildAdded中的DataSnapshot，然后使用DUMMY 2节点的快照进行调用.

And then you add a ChildEventListener. That means that the DataSnapshot in your onChildAdded will be called with a snapshot of the DUMMY 1 node first, and then with a snapshot of the DUMMY 2 node.

您正在onChildAdded中呼叫dataSnapshot.getValue(String.class).但是，由于DUMMY 1和DUMMY 2具有多个属性，因此它们没有单个字符串值.因此该调用会返回null，这可能是导致您出现问题的原因.

You're calling dataSnapshot.getValue(String.class) in onChildAdded. But since DUMMY 1 and DUMMY 2 have multiple properties, they don't have a single string value. So that call returns null, which is probably what's causing you problems.

如果要将DUMMY 1，DUMMY 2键本身添加到适配器，则可以调用dataSnapshot.getKey().否则，您可以使用dataSnapshot.child("Description").getValue(String.class)获取特定子属性的值.

If you want to add the DUMMY 1, DUMMY 2 key itself to the adapter, you can call dataSnapshot.getKey(). Otherwise you can get the value of the specific child properties with dataSnapshot.child("Description").getValue(String.class).

所以:

myRef.addChildEventListener(new ChildEventListener() {
    @Override
    public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
        String key = dataSnapshot.getKey();
        String description = dataSnapshot.child("Description").getValue(String.class);
        Long quantity = dataSnapshot.child("Quantity").getValue(Long.class);
        myArrayList.add(key+": "+description+" ("+quantity+")");
        myArrayAdapter.notifyDataSetChanged();
    }

---

如果您想将整个值读入表示该项目的类(所谓的POJO)，则最简单的类是:

---

If you'd like to read the entire value into a class representing the item (a so-called POJO), the simplest class is:

public class Item {
  public String Description;
  public Long Quantity;
}

请注意，字段名称(包括大小写)必须与数据库中属性的名称完全匹配.您可以像这样使用上面的类:

Note that the name (including the case) of the fields must exactly match the names of the properties in your database. You can use the above class like this:

myRef.addChildEventListener(new ChildEventListener() {
    @Override
    public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
        String key = dataSnapshot.getKey();
        Item item = dataSnapshot.getValue(Item.class);
        myArrayList.add(key+": "+item.description+" ("+item.quantity+")");
        myArrayAdapter.notifyDataSetChanged();
    }

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