如何在StreamBuilder中修复空值检查操作符

Question

我正面临一个Null检查操作符，用于空值尝试使用StreamBuilder从FireStore数据库获取数据时的问题。我在整个代码中找不到任何额外的空检查操作符。请检查代码

下面是输出跟踪

======== Exception caught by widgets library =======================================================
The following _CastError was thrown building StreamBuilder<QuerySnapshot<Object?>>(dirty, dependencies: [MediaQuery], state: _StreamBuilderBaseState<QuerySnapshot<Object?>, AsyncSnapshot<QuerySnapshot<Object?>>>#9d300):
Null check operator used on a null value
The relevant error-causing widget was: 
StreamBuilder<QuerySnapshot<Object?>> StreamBuilder:file:///Users/milansmac/Desktop/chaudhary_chicken_users_app/lib/widgets/category_widget.dart:20:14
When the exception was thrown, this was the stack: 
#0      _CategoryWidgetState.build.<anonymous closure> (package:chaudhary_chicken_users_app/widgets/category_widget.dart:53:49)
#1      StreamBuilder.build (package:flutter/src/widgets/async.dart:444:81)
#2      _StreamBuilderBaseState.build (package:flutter/src/widgets/async.dart:124:48)
#3      StatefulElement.build (package:flutter/src/widgets/framework.dart:4992:27)
#4      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4878:15)
#5      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5050:11)
#6      Element.rebuild (package:flutter/src/widgets/framework.dart:4604:5)
#7      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4859:5)
#8      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:5041:11)
#9      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4853:5)
...     Normal element mounting (12 frames)
#21     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3863:16)
#22     MultiChildRenderObjectElement.inflateWidget (package:flutter/src/widgets/framework.dart:6435:36)
====================================================================================================

下面是我的代码

class CategoryWidget extends StatefulWidget {
const CategoryWidget({Key? key}) : super(key: key);
@override
State<CategoryWidget> createState() => _CategoryWidgetState();
}
class _CategoryWidgetState extends State<CategoryWidget> {
@override
Widget build(BuildContext context) {
return Padding(
padding: const EdgeInsets.all(8.0),
child: StreamBuilder<QuerySnapshot>(
stream: FirebaseFirestore.instance
.collection("sellers")
.doc("5wzmKHapUiMGAtQHozRAsqYCZDk2")
.collection("menus")
.snapshots(),
builder: (context, AsyncSnapshot<QuerySnapshot> snapshot) {
if (snapshot.hasError) {
return Center(child: circularProgress(),);
} else {
return SizedBox(
height: 170,
width: MediaQuery.of(context).size.width,
child: Column(
children: [
Row(
children: const [
Text('Categories', style: TextStyle(
color: Colors.black,
fontWeight: FontWeight.bold,
fontSize: 24
),
),
],
),
Expanded(
child: InkWell(
onTap: (){
},
splashColor: const Color(0xfffb9e5a),
child: ListView.builder(
scrollDirection: Axis.horizontal,
itemCount: snapshot.data!.docs.length,
itemBuilder: (BuildContext context, int index) {
Menus model = Menus.fromJson(snapshot.data!.docs[index].data()! as Map<String, dynamic>,);
return CategoriesDesign(
model: model,
context: context,
);
},
),
),
),
],
),
);
}
},
),
);
}
}

此小部件用作参考，将在另一个屏幕中用于显示类别。

Answer 1

您必须检查3个条件

if (snapshot.hasError) {
return Container(child:Text("error"),);
}  
else if(snapshot.hasData){

return 
.....
your widget listview
.....

}  else{
return Center(child: circularProgress(),);
}