在我的Flutter课程中浏览课程，但无法运行以下代码

Question

我试图从firestore获取项目，以在应用程序上的列表视图中显示，但获得“构建函数返回null。有问题的小部件是：streambuilder，构建函数必须切勿返回null”。我只想从ListView中显示的Firstore列表“帖子”。

构建函数返回的null。令人讨厌的小部件是StreamBuilder＆lt; querysnapshot＆gt;构建功能绝不能返回null。

请帮助我解决应用程序

import 'package:flutter/material.dart';
import 'package:flash_chat/constants.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:cloud_firestore/cloud_firestore.dart';

class ChatScreen extends StatefulWidget {
  static const String id = 'chat_screen';

  @override
  _ChatScreenState createState() => _ChatScreenState();
}

class _ChatScreenState extends State<ChatScreen> {
  final _fireStore = FirebaseFirestore.instance;
  final _auth = FirebaseAuth.instance;
  User loggedInUser;
  String messageText;

  @override
  void initState() {

    super.initState();
    getCurrentUser();
  }

  void getCurrentUser() async {
    try {
      final user = await _auth.currentUser;
      if (user != null) {
        loggedInUser = user;
        print(loggedInUser.email);
      }
    } catch (e) {
      print(e);
    }
  }



  void messageStream() async {
    await for (var snapShot in _fireStore.collection('messages').snapshots()) {
      for (var message in snapShot.docs) {
        print(message.data());
      }
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        leading: null,
        actions: <Widget>[
          IconButton(
              icon: Icon(Icons.close),
              onPressed: () {
                messageStream();
                // _auth.signOut();
                // Navigator.pop(context);
              }),
        ],
        title: Text('⚡️Chat'),
        backgroundColor: Colors.lightBlueAccent,
      ),
      body: SafeArea(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceBetween,
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            StreamBuilder<QuerySnapshot>(
                stream: _fireStore.collection('messages').snapshots(),
                builder: (context, snapshot){
                  if(snapshot.hasData){
                    final messages = snapshot.data.docs;
                    List<Text> messageWidgets = [];
                    for (var message in messages){
                      final messageText = (message.data as Map<String, dynamic>)['text'];
                      final messageSender = (message.data as Map<String, dynamic>)['sender'];
                      final messageWidget = Text('$messageText from $messageSender');
                      messageWidgets.add(messageWidget);
                    } return Column(
                      children: messageWidgets,
                    );
                  }
                },),
            Container(
              decoration: kMessageContainerDecoration,
              child: Row(
                crossAxisAlignment: CrossAxisAlignment.center,
                children: <Widget>[
                  Expanded(
                    child: TextField(
                      onChanged: (value) {
                        messageText = value;
                      },
                      decoration: kMessageTextFieldDecoration,
                    ),
                  ),
                  FlatButton(
                    onPressed: () {
                      _fireStore.collection('messages').add({
                        'text': messageText,
                        'sender': loggedInUser.email
                      });
                    },
                    child: Text(
                      'Send',
                      style: kSendButtonTextStyle,
                    ),
                  ),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }
}

Answer 1

因此，您描述的问题是streamBuilder返回null，即使widget。然后，让我们专注于代码的那一部分。
我清理了格式，以使其更容易理解。

StreamBuilder<QuerySnapshot>(
  stream: _fireStore.collection('messages').
  builder: (context, snapshot){
    if(snapshot.hasData) {
      final messages = snapshot.data.docs;
      List<Text> messageWidgets = [];
      for (var message in messages){
        final messageText = (message.data as Map<String, dynamic>)['text'];
        final messageSender = (message.data as Map<String, dynamic>)['sender'];
        final messageWidget = Text('$messageText from $messageSender');
        messageWidgets.add(messageWidget);
      }
      return Column(
        children: messageWidgets,
      );
    }
  },
),

Builder函数将返回小部件。但是，如果语句，您的代码立即以开始，并且以下代码仅在snapshot.hasdata是正确时执行。因此，列仅在这种情况下返回。

当快照尚未完成时，IE具有 ，或者投掷 error 没有else/shartback部分，代码将简单返回而无需给出widget返回呼叫者，返回null。
在这些情况下，可以通过提供回退回报来解决：

StreamBuilder<QuerySnapshot>(
  ...
  builder: (context, snashot) {
    if (snapshot.hasData) {
      ...
      return Column(...);
    }
    if (snapshot.hasError) {
      return Container(
        child: Text("An error occured..."),
       );
    }
    return Container(
      child: Text("No data received yet...Loading..."),
    );
  }
);