如果没有匹配的数据，Firebase实时数据库将触发"null"错误 [英] Firebase Real Time Database triggers `null` error if there is no matching data

问题描述

在我扑动的代码中，我试图从Firebase实时数据库中获取数据.下面是我的代码.

In my flutter code, I am trying to get data from the Firebase Real-Time Database. Below is my code.

final DatabaseReference reference = FirebaseDatabase.instance.reference().child('chat_room');

    return Scaffold(
      body: StreamBuilder(
          stream:
              reference.orderByChild("email").equalTo("[e-mail]").onValue,
          builder: (context, snapshot) {
            if (snapshot == null || !snapshot.hasData) {
              return Container(child: Center(child: Text("No data")));
            } else {
              Map<dynamic, dynamic> map = snapshot.data.snapshot.value;
              return ListView.builder(
                  itemCount: map.values.toList().length,
                  itemBuilder: (context, index) {
                    String imageURL = map.values.toList()[index]["imageUrl"];
                    return Container(
                      margin: EdgeInsets.only(top: 10),
                      child: ListTile(
                        leading: CircleAvatar(
                          radius: 30.0,
                          backgroundImage: NetworkImage(imageURL),
                          backgroundColor: Colors.transparent,
                        ),
                        title: Text(
                          map.values.toList()[index]["email"],
                        ),
                      ),
                    );
                  });
            }
          }),
    );

注意，我正在加载 email 等于 [e-mail] 的数据.如果有 [e-mail] 的记录，该代码将非常有用.但是，如果数据库为空或没有 [e-mail] 的记录，我将收到以下错误

Notice, I am loading data where the email is equal to [e-mail]. The code works great if there are record for [e-mail]. But if the database is empty or no records for [e-mail], I AM getting the below error

The following NoSuchMethodError was thrown building StreamBuilder<Event>(dirty, state: _StreamBuilderBaseState<Event, AsyncSnapshot<Event>>#ad47f):
The getter 'values' was called on null.
Receiver: null
Tried calling: values

The relevant error-causing widget was
StreamBuilder<Event>
package:xxx/…/chat/chat_list_supplier.dart:19
When the exception was thrown, this was the stack
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1      _ChatListSupplierState.build.<anonymous closure>
package:xxx/…/chat/chat_list_supplier.dart:28

我该如何解决?

推荐答案

问题是存在快照，但是快照中没有数据.最容易抓住这一点:

The problem is that there is a snapshot, but the snapshot contains no data. It's easiest to catch this in:

  builder: (context, snapshot) {
    if (snapshot == null || !snapshot.hasData || snapshot.data.snapshot.value == null) {
      return Container(child: Center(child: Text("No data")));
    } else {
      ...

这篇关于如果没有匹配的数据，Firebase实时数据库将触发"null"错误的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！