如何按顺序从firebase获取数据？

Question

我目前正在开发聊天应用程序，问题是我无法从 firebase 获取数据，在聊天屏幕上没有显示任何聊天内容，在 firebase 上对我的数据进行排序以将它们像真正的聊天应用程序一样带到我的聊天屏幕，我使用了 orderBy('createAt',descending: true).snapshots() 属性，它根据时间对 firebase 上的数据进行排序，但在使用此参数之前，我可以看到消息，但现在我可以看到，感谢您的回答，有美好的一天..

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

final _firestore=FirebaseFirestore.instance;
User loggedInUser;

class ChatScreen extends StatefulWidget {
  static String id="chat_screen";
  @override
  _ChatScreenState createState() => _ChatScreenState();
}

class _ChatScreenState extends State<ChatScreen> {

  final messageTextController = TextEditingController();
  final _auth = FirebaseAuth.instance;

  String messageText;

  void initState(){
    super.initState();
    getCurrentUser();
  }

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


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        leading: null,
        actions: <Widget>[
          IconButton(
              icon: Icon(Icons.close),
              onPressed: () {
                _auth.signOut();
                Navigator.pop(context);
              }),
        ],
        title: Text('Chat'),
        backgroundColor: Colors.lightBlueAccent,
      ),
      body: SafeArea(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceBetween,
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            MessagesStream(),
            Container(
              decoration: kMessageContainerDecoration,
              child: Row(
                crossAxisAlignment: CrossAxisAlignment.center,
                children: <Widget>[
                  Expanded(
                    child: TextField(
                      controller: messageTextController,
                      onChanged: (value) {
                        messageText = value;
                      },
                      decoration: kMessageTextFieldDecoration,
                    ),
                  ),
                  FlatButton(
                    onPressed: () {
                      messageTextController.clear();
                      _firestore.collection('messages').add({
                        'text': messageText,
                        'sender': loggedInUser.email,
                        'createdAt': FieldValue.serverTimestamp(),
                      });
                    },
                    child: Text(
                      'Send',
                      style: kSendButtonTextStyle,
                    ),
                  ),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class MessagesStream extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return StreamBuilder<QuerySnapshot>(
      stream: _firestore.collection('messages').
      orderBy('createAt', descending: true).snapshots(),
      builder: (context, snapshot) {
        if (!snapshot.hasData) {
          return Center(
            child: CircularProgressIndicator(
              backgroundColor: Colors.lightBlueAccent,
            ),
          );
        }
        final messages = snapshot.data.docs.reversed;
        List<MessageBubble> messageBubbles = [];
        for (var message in messages) {
          final messageText = message.data()['text'];
          final messageSender = message.data()['sender'];

          final currentUser = loggedInUser.email;

          final messageBubble = MessageBubble(
            sender: messageSender,
            text: messageText,
            isMe: currentUser == messageSender,
          );

          messageBubbles.add(messageBubble);
        }
        return Expanded(
          child: ListView(
            reverse: true,
            padding: EdgeInsets.symmetric(horizontal: 10.0, vertical: 20.0),
            children: messageBubbles,
          ),
        );
      },
    );
  }
}

class MessageBubble extends StatelessWidget {
  MessageBubble({this.sender, this.text, this.isMe});

  final String sender;
  final String text;
  final bool isMe;

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: EdgeInsets.all(10.0),
      child: Column(
        crossAxisAlignment:
        isMe ? CrossAxisAlignment.end : CrossAxisAlignment.start,
        children: <Widget>[
          Text(
            sender,
            style: TextStyle(
              fontSize: 12.0,
              color: Colors.black54,
            ),
          ),
          Material(
            borderRadius: isMe
                ? BorderRadius.only(
                topLeft: Radius.circular(30.0),
                bottomLeft: Radius.circular(30.0),
                bottomRight: Radius.circular(30.0))
                : BorderRadius.only(
              bottomLeft: Radius.circular(30.0),
              bottomRight: Radius.circular(30.0),
              topRight: Radius.circular(30.0),
            ),
            elevation: 5.0,
            color: isMe ? Colors.lightBlueAccent : Colors.white,
            child: Padding(
              padding: EdgeInsets.symmetric(vertical: 10.0, horizontal: 20.0),
              child: Text(
                text,
                style: TextStyle(
                  color: isMe ? Colors.white : Colors.black54,
                  fontSize: 15.0,
                ),
              ),
            ),
          ),
        ],
      ),
    );
  }
}
[![Here is my database screenshot][1]][1]