因此，我正在创建一个移动聊天应用程序（仅供学习）。我试图使其使用time_formatter包推送消息的时间戳（来自Firebase）以将其显示在屏幕上。它实际上正在工作，但是每次我发送一条消息时，仿真器屏幕都会变红约1秒钟，并抛出此错误：
════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building MessageBubble(dirty):
The getter 'millisecondsSinceEpoch' was called on null.
Receiver: null
Tried calling: millisecondsSinceEpoch
The relevant error-causing widget was:
MessageBubble file:///C:/Users/cauer/Desktop/Apps-Flutter/flash_chat_flutter/lib/screens/chat_screen.dart:141:33
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1 MessageBubble.build (package:flash_chat/screens/chat_screen.dart:208:32)
#2 StatelessElement.build (package:flutter/src/widgets/framework.dart:4576:28)
#3 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4502:15)
#4 Element.rebuild (package:flutter/src/widgets/framework.dart:4218:5)
此后，它在屏幕上准确显示了我想要的内容，但是我确实想知道发生了什么并对其进行修复。对不起，我在这里知道。如果需要，我会发布更多代码或任何其他信息，但我可以肯定这是由此处的某些原因引起的。
import 'package:flutter/cupertino.dart';
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';
import 'package:time_formatter/time_formatter.dart';
final _firestore = Firestore.instance;
FirebaseUser 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;
@override
void initState() {
super.initState();
getCurrentUser();
}
void getCurrentUser() async {
try {
final user = await _auth.currentUser();
if (user != null) {
loggedInUser = user;
}
} catch (e) {
print(e);
}
}
void messageStream() async {
await for (var snapshot in _firestore.collection('messages').snapshots()) {
for (var message in snapshot.documents) {
print(message.data);
}
}
}
@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,
'timestamp': FieldValue.serverTimestamp(),
}).then((value) => print('${value.documentID} added'));
messageTextController.clear();
},
child: Text(
'Send',
style: kSendButtonTextStyle,
),
),
],
),
),
],
),
),
);
}
}
class MessagesStream extends StatelessWidget {
@override
Widget build(BuildContext context) {
return StreamBuilder<QuerySnapshot>(
stream: _firestore
.collection('messages')
.orderBy('timestamp', descending: false)
.snapshots(),
builder: (context, snapshot) {
if (!snapshot.hasData) {
return Center(
child: CircularProgressIndicator(
backgroundColor: Colors.lightBlueAccent,
),
);
}
final messages = snapshot.data.documents.reversed;
List<MessageBubble> messageBubbles = [];
for (var message in messages) {
final messageText = message.data['text'];
final messageSender = message.data['sender'];
final messageTime = message.data['timestamp']; //add this
final currentUser = loggedInUser.email;
final messageBubble = MessageBubble(
sender: messageSender,
text: messageText,
isMe: currentUser == messageSender,
time: messageTime,
);
messageBubbles.add(messageBubble);
}
return Expanded(
child: ListView(
reverse: true,
padding: EdgeInsets.symmetric(horizontal: 10.0, vertical: 20.0),
children: messageBubbles,
),
);
},
);
}
}
class MessageBubble extends StatelessWidget {
MessageBubble({this.sender, this.text, this.isMe, this.time});
final String sender;
final String text;
final bool isMe;
final Timestamp time;
@override
Widget build(BuildContext context) {
return Padding(
padding: EdgeInsets.all(10.0),
child: Column(
crossAxisAlignment:
isMe ? CrossAxisAlignment.end : CrossAxisAlignment.start,
children: <Widget>[
Text(
'$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(
topRight: Radius.circular(30.0),
bottomLeft: Radius.circular(30.0),
bottomRight: Radius.circular(30.0)),
elevation: 5.0,
color: isMe ? Colors.lightBlueAccent : Colors.white,
child: Padding(
padding: EdgeInsets.symmetric(vertical: 12.0, horizontal: 20.0),
child: Text(
'$text',
style: TextStyle(
color: isMe ? Colors.white : Colors.black54,
fontSize: 15,
),
),
),
),
Text(
'${formatTime(time.millisecondsSinceEpoch)}',
style: TextStyle(
fontSize: 11.0,
color: Colors.black54,
),
),
],
),
);
}
}
将不胜感激，已经在寻找答案但找不到答案。
This is what the screen displays in the moment that the message is sent
答案 0 :(得分：2)
我认为这应该可行。
单独创建一个函数，以便您可以在时间为空的情况下进行处理。
Widget handleerr() async{
await Future.delayed(const Duration(milliseconds: 2000), () {
setState(() {
Text(
'${formatTime(time.millisecondsSinceEpoch)}',
style: TextStyle(
fontSize: 11.0,
color: Colors.black54,
)
});
});
}
答案 1 :(得分：2)
如此修改MessageBubble构造函数
MessageBubble({this.sender, this.text, this.isMe, Timestamp time}):time = time ?? Timestamp.now();
问题是Firestore在本地写入数据时{{1}}为空