错误:对空值使用空检查运算符 [英] error: Null check operator used on a null value
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问题描述
此时，当按钮的uniq_id和数据表的uniq_id匹配时，才会显示该uniq_id用户的数据.但是这个错误在数据显示之前就出现了.
In this, when the uniq_id of the button and the uniq_id of the data table match, only then the data of the user of that uniq_id will show. But this error is coming before the data shows.
确实显示了一次数据，但在出现此错误后，对于两个页面
Did show the data for one time but after this error came, for both the page
这是我的 p_team.dart这是来自 Api 的按钮部分.
This is my p_team.dart This is the part of the button that is coming from the Api.
import 'package:flutter/material.dart';
import 'package:practice/listPost/p_team_list.dart';
import 'package:practice/post/post.dart';
import 'package:practice/post/services.dart';
class PTeam extends StatefulWidget {
@override
_PTeamState createState() => _PTeamState();
}
class _PTeamState extends State<PTeam> with SingleTickerProviderStateMixin {
List<Post>? posts;
@override
void initState() {
Services().getPosts().then((list) {
(() {
posts = list;
print(posts);
});
});
}
@override
Widget build(BuildContext context) {
return Scaffold(
body: Column(
children: [
Container(
margin: const EdgeInsets.all(20.0),
padding: const EdgeInsets.all(10.0),
decoration: BoxDecoration(
border: Border.all(width: 2, color: Colors.greenAccent)),
child: Row(
children: [
Text(
"Total Income:",
style: TextStyle(
fontSize: 30.0,
fontWeight: FontWeight.bold,
fontStyle: FontStyle.italic),
),
SizedBox(
width: 10.0,
),
Text(
"Rs.2000",
style: TextStyle(
fontSize: 20.0,
fontWeight: FontWeight.bold,
fontStyle: FontStyle.italic),
),
],
),
),
SizedBox(
height: 20.0,
),
Flexible(
child: Container(
child: GridView.count(
childAspectRatio: 1.0,
padding: EdgeInsets.only(left: 16, right: 16),
crossAxisCount: 2,
crossAxisSpacing: 18,
mainAxisSpacing: 18,
children: List.generate(
posts!.length,
(index) => GestureDetector(
onTap: () {
print(posts![index].teamUniqId);
Navigator.push(
context,
MaterialPageRoute(
builder: (context) => PTeamList(
teamUniqId: posts![index].teamUniqId,
teamType: posts![index].teamType,
)),
);
},
child: Container(
decoration: BoxDecoration(
color: Color(0xff00ffaa),
borderRadius: BorderRadius.circular(10)),
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Center(
child: Text(posts![index].teamType,
style: TextStyle(
color: Colors.white,
fontSize: 16,
fontWeight: FontWeight.w600)),
),
],
),
),
),
),
),
),
),
],
),
);
}
}
这是 p_team.dart 的按钮 api 数据.
This is my button api's data for p_team.dart.
[{"teamType":"direct team","team_name":"platinum","team_number":"234","team_uniq_id":"1","team_last_update":"10-may-2021"},{"teamType":"left team","team_name":"gold","team_number":"356","team_uniq_id":"2","team_last_update":"10-may-2021"},{"teamType":"right team","team_name":"silver","team_number":"876","team_uniq_id":"3","team_last_update":"10-may-2021"}]
这是我的 p_team_list.dart.
this is my p_team_list.dart.
此时，当按钮的uniq_id与数据表的uniq_id匹配时，才会显示该uniq_id用户的数据.
In this, when the uniq_id of the button and the uniq_id of the data table match, only then the data of the user of that uniq_id will show.
import 'package:flutter/material.dart';
import 'package:practice/listPost/post_list.dart';
import 'package:practice/listPost/services.dart';
class PTeamList extends StatefulWidget {
final teamUniqId;
final teamType;
const PTeamList({Key? key, this.teamUniqId, this.teamType}) : super(key: key);
@override
_PTeamListState createState() => _PTeamListState();
}
class _PTeamListState extends State<PTeamList> {
List<Post>? posts;
@override
void initState() {
Services().getPosts().then((list) {
setState(() {
posts = list;
print("za$posts");
});
});
}
@override
Widget build(BuildContext context) {
// if(widget.teamUniqId==posts.)
return Scaffold(
body:
// Text(widget.teamType),
Flexible(
child: Container(
child: Stack(
children: List.generate(posts!.length, (index) {
if (posts!.length == null) {
print(posts![index].user);
return Center(child: CircularProgressIndicator());
}
else if (widget.teamUniqId == posts![index].teamUniqId) {
return SingleChildScrollView(
scrollDirection: Axis.horizontal,
// Data table widget in not scrollable so we have to wrap it in a scroll view when we have a large data set..
child: SingleChildScrollView(
child: DataTable(
columns: [
// DataColumn(
// label: Text('Verified'),
// tooltip: 'represents if user is verified.'),
DataColumn(
label: Text('S No'),
tooltip: 'represents first S no of the user'),
DataColumn(
label: Text('Sponsor ID'),
tooltip: 'represents Sponsor ID of the user'),
DataColumn(
label: Text('User ID'),
tooltip: 'represents User ID of the user'),
DataColumn(
label: Text('Name'),
tooltip: 'represents Name of the user'),
DataColumn(
label: Text('Mobile'),
tooltip: 'represents Mobile of the user'),
DataColumn(
label: Text('Date'),
tooltip: 'represents Date of the user'),
DataColumn(
label: Text('a'),
tooltip: 'represents Date of the user'),
DataColumn(
label: Text('b'),
tooltip: 'represents Date of the user'),
DataColumn(
label: Text('c'),
tooltip: 'represents Date of the user'),
DataColumn(
label: Text('d'),
tooltip: 'represents Date of the user'),
],
rows: posts![index]
.user
.map((data) =>
DataRow(
cells: [
// I want to display a green color icon when user is verified and red when unverified
DataCell(Text(data.userName)),
DataCell(Text(data.userMotherName)),
DataCell(Text(data.userAddress)),
DataCell(Text(data.userSponsorId)),
DataCell(Text(data.sponsorId)),
DataCell(Text(data.email)),
DataCell(Text(data.city)),
DataCell(Text(data.state)),
DataCell(Text(data.userMobile)),
DataCell(Text(data.dob)),
]))
.toList(),
),
),
);
}
return Text("");
}),
),
),
),
);
}
}
这是我的数据表 api 的 p_team.dart 数据.
This is my data table api's data for p_team.dart.
[{"teamType":"direct Team","team_uniq_id":"1","user":[{"user_name":"deepak","user_mother_name":"Accomodation","user_address":"varanasi","user_mobile":"5678989","user_sponsor_id":"123456","sponsor_id":"3456","email":"abc@gmai.com","city":"varanasi","state":"India","dob":"12-5-1996"},{"user_name":"deepak","user_mother_name":"Accomodation","user_address":"varanasi","user_mobile":"5678989","user_sponsor_id":"123456","sponsor_id":"3456","email":"abc@gmai.com","city":"varanasi","state":"India","dob":"12-5-1996"},{"user_name":"deepak","user_mother_name":"Accomodation","user_address":"varanasi","user_mobile":"5678989","user_sponsor_id":"123456","sponsor_id":"3456","email":"abc@gmai.com","city":"varanasi","state":"India","dob":"12-5-1996"}]},{"teamType":"left Team","team_uniq_id":"2","user":[{"user_name":"Ashu","user_mother_name":"manju","user_address":"Mirzapur","user_mobile":"222222","user_sponsor_id":"123456","sponsor_id":"3456","email":"abc@gmai.com","city":"varanasi","state":"India","dob":"12-5-1996"},{"user_name":"Ashutodh","user_mother_name":"manju1","user_address":"Mirzapur1","user_mobile":"2222221","user_sponsor_id":"1234561","sponsor_id":"34561","email":"abc@gmai.com1","city":"varanasi1","state":"India1","dob":"12-5-19961"}]},{"teamType":"Right Team","team_uniq_id":"3","user":[{"user_name":"tosh","user_mother_name":"snju","user_address":"Allahabad","user_mobile":"44444444","user_sponsor_id":"333456","sponsor_id":"6666666","email":"jkl@gmai.com","city":"lucknow","state":"India","dob":"15-3-1956"}]},{"teamType":"Total Team","team_uniq_id":"4","user":[{"user_name":"tosh","user_mother_name":"snju","user_address":"Allahabad","user_mobile":"44444444","user_sponsor_id":"333456","sponsor_id":"6666666","email":"jkl@gmai.com","city":"lucknow","state":"India","dob":"15-3-1956"},{"user_name":"deepak","user_mother_name":"Accomodation","user_address":"varanasi","user_mobile":"5678989","user_sponsor_id":"123456","sponsor_id":"3456","email":"abc@gmai.com","city":"varanasi","state":"India","dob":"12-5-1996"},{"user_name":"deepak","user_mother_name":"Accomodation","user_address":"varanasi","user_mobile":"5678989","user_sponsor_id":"123456","sponsor_id":"3456","email":"abc@gmai.com","city":"varanasi","state":"India","dob":"12-5-1996"},{"user_name":"tosh","user_mother_name":"snju","user_address":"Allahabad","user_mobile":"44444444","user_sponsor_id":"333456","sponsor_id":"6666666","email":"jkl@gmai.com","city":"lucknow","state":"India","dob":"15-3-1956"}]}]
推荐答案
这个问题的原因是您在空值上使用了 bang ! 运算符.这表示 Object 不能为 null，如果是则抛出错误.
The cause of this issue is that you're using a bang ! operator on a null value. This indicates that the Object can't be null, and throws an error if so.
您可以在开始时添加一个空检查，或者如果值可以为空，最好使用可空的 ? 运算符.
You can either add a null check at the start or if the value can be null, it's best to use a nullable ? operator instead.
这个错误可以很容易地用
The error can be easily demonstrated with
int? foo; // nullable var
debugPrint('${foo!}'); // with the ! operator, Flutter expects the value to never be null
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