class MyClass extends StatelessWidget {
@override
Widget build(BuildContext context) {
var _userSnapshot;
List<dynamic> myContactsList;
return StreamProvider<DocumentSnapshot>.value(
value: DatabaseService("").myUser,
builder: (context, child) => Expanded(
child: Container(
child: (_userSnapshot = Provider.of<DocumentSnapshot>(context)) ==
null ||
(myContactsList = _userSnapshot.data["contacts"]) == null
? Padding(
// if no contacts were found
padding: EdgeInsets.all(10.0),
child: Column(
children: <Widget>[
Text(
"No recent chats yet",
),
],
),
)
: ListView.builder(
itemCount: _userSnapshot.data["contacts"].length,
itemBuilder: (context, index) => FutureBuilder(
future: DatabaseService("").getUserById(
myContactsList.elementAt(index),
),
builder: (context, snapshot) => GestureDetector(
onTap: () => Navigator.push(
context,
MaterialPageRoute(
builder: (_) => ChatScreen(
myContactsList.elementAt(index),
snapshot.data["displayName"],
),
),
),
child: Container(
margin: EdgeInsets.only(top: 5, bottom: 5, right: 20),
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceBetween,
children: <Widget>[
Row(
children: <Widget>[
FutureBuilder(
future: DatabaseService("").getUserById(
myContactsList.elementAt(index),
),
builder: (context, snapshot) =>
CircleAvatar(
radius: 33,
backgroundImage: snapshot
.data["imageUrl"] ==
null
? snapshot.data["isMale"] == true
? AssetImage(
"assets/images/male.png")
: AssetImage(
"assets/images/female.png")
: AssetImage(
"assets/images/male.png"),
),
),
SizedBox(width: 10),
Column(
crossAxisAlignment:
CrossAxisAlignment.start,
children: [
FutureBuilder<DocumentSnapshot>(
future: DatabaseService("").getUserById(
myContactsList.elementAt(index),
),
builder: (context, snapshot) => Text(
snapshot.data["displayName"],
style: TextStyle(
color: Colors.grey,
fontSize: 18,
fontWeight: FontWeight.bold,
),
),
),
SizedBox(height: 5),
Container(
width:
MediaQuery.of(context).size.width *
0.45,
child: Text(
"Something N. $index ",
style: TextStyle(
color: Colors.blueGrey,
fontSize: 18,
fontWeight: FontWeight.w600,
),
overflow: TextOverflow.ellipsis,
),
),
],
),
],
),
Column(
children: <Widget>[
Text(
"TIME"
),
Container(
height: 20,
width: 40,
decoration: BoxDecoration(
color: Theme.of(context).primaryColor,
borderRadius: BorderRadius.circular(30),
),
alignment: Alignment.center,
child: Text(
"NEW",
style: TextStyle(
color: Colors.white,
fontSize: 11,
fontWeight: FontWeight.bold,
),
),
),
],
),
],
),
),
),
),
),
),
),
),
);
}
}
每次与我的应用进行交互并在云Firestore DB中触发更改时，我都会使用StreamProvider()获取快照。当发生这种情况时，我在下面列出了一些异常，但是半秒钟后屏幕已修复，这可能是我猜出新快照的原因。
就像下面列出的一样，问题来自我使用的FutureBuilder()。
这是错误：
The following NoSuchMethodError was thrown building FutureBuilder<DocumentSnapshot>(dirty, state: _FutureBuilderState<DocumentSnapshot>#98dd9):
The method '[]' was called on null.
Receiver: null
Tried calling: []("imageUrl")
The relevant error-causing widget was:
FutureBuilder<DocumentSnapshot> file:///home/pss/Desktop/android/flutter_firebase_chat_app/lib/widgets/recent_chats.dart:90:35
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1 RecentChats.build.<anonymous closure>.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:flutter_firebase_chat_app/widgets/recent_chats.dart:98:56)
#2 _FutureBuilderState.build (package:flutter/src/widgets/async.dart:740:55)
#3 StatefulElement.build (package:flutter/src/widgets/framework.dart:4663:28)
#4 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4546:15)
...
════════════════════════════════════════════════════════════════════════════════════════════════════
Another exception was thrown: NoSuchMethodError: The method '[]' was called on null.
════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The method '[]' was called on null.
Receiver: null
Tried calling: []("displayName")
The relevant error-causing widget was:
FutureBuilder<DocumentSnapshot> file:///home/pss/Desktop/android/flutter_firebase_chat_app/lib/widgets/recent_chats.dart:114:39
错误指向以下两行：
backgroundImage: snapshot.data["imageUrl"] == null
和
builder: (context, snapshot) => Text(snapshot.data["displayName"]
因此，我不确定自己是否诚实。我认为这是FutureBuilder()的原因，没有它们，我也不会例外。
我是Flutter的新手，因此任何建议将不胜感激！
答案 0 :(得分：1)
您要呼叫_userSnapshot.data[]，但data是null，因此必须首先确保_userSnapshot不为null，也要确保_userSnapshot.data。
同时，考虑放置一个加载器，您可以使用CircularProgressIndicator作为简单的开始，也可以使用pub.dev中的Shimmer库进行一些很酷的占位符加载动画（如Facebook）。
答案 1 :(得分：0)
您可以将initialData参数设置为StreamProvider，在流还没有发出任何东西的情况下使用此数据。 您还应该在访问_userSnapshot.data！= null之前检查其键之一。