所以，这是我的学生列表的代码。.我正在使用sqlflite数据库存储学生集合。.但是当我运行它时..这显示了我的futurebuilder方法中的错误。我在snapshot.data.lenght行中遇到错误，错误显示该长度不应为null，这就是为什么我尝试了一些条件但仍然无法正常工作的原因
class StudentListPage extends StatefulWidget {
@override
_StudentListPageState createState() => _StudentListPageState();
}
class _StudentListPageState extends State<StudentListPage> {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Student List'),
actions: <Widget>[
IconButton(
icon: Icon(Icons.add,color: Colors.white,),
onPressed: (){
Navigator.of(context).push(MaterialPageRoute(
builder: (context)=>AddStudentPage()
));
},
)
],
),
body: FutureBuilder(
future: DBHelpers.getAllStudent(),
builder: (context,AsyncSnapshot<List<StudentModels>> snapshot)=>
snapshot.connectionState==ConnectionState.waiting?
Center(
child: CircularProgressIndicator(),
):
Padding(
child:snapshot.data.length !=0? ListView.builder(
itemCount: snapshot.data.length,
itemBuilder: (context,index)=>StudentItems(snapshot.data[index])):
Center(
child: Column(
mainAxisSize: MainAxisSize.min,
children: <Widget>[
RaisedButton(
elevation: 5,
color: Theme.of(context).primaryColor,
child: Text("Click me To add Employee", style: TextStyle(color: Colors.white),),
shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(12)),
onPressed: (){
},
)
],
),
),
),
),
);
}
}
那是我的数据库助手代码：
class DBHelpers{
static final String CREATE_TABLE=''' create table $TABLE_STUDENT(
$COL_STUDENT_ID integer primary key autoincrement,
$COL_STUDENT_NAME text not null,
$COL_STUDENT_PHONE integer not null,
$COL_STUDENT_EMAIL text not null,
$COL_STUDENT_FAVOURITE text )''';
static Future<Database> open() async{
final dbpath=await getDatabasesPath();
final path=Path.join(dbpath,'student.db');
return openDatabase(path,version: 1,onCreate: (db,version) async{
db.execute(CREATE_TABLE);
});
}
static Future<int> insertStudent(String table,Map<String,dynamic> map) async{
final db=await open();
return db.insert(table, map,conflictAlgorithm: ConflictAlgorithm.replace);
}
static Future<List<StudentModels>> getAllStudent() async{
final db=await open();
final List<Map<String,dynamic>> map=await db.query(TABLE_STUDENT,orderBy: COL_STUDENT_NAME);
return List.generate(map.length, (index){
return StudentModels.fromMap(map[index]);
});
}
}
这是我的学生模型：
final String TABLE_STUDENT='tbl_student';
final String COL_STUDENT_ID='col_id';
final String COL_STUDENT_NAME='col_name';
final String COL_STUDENT_EMAIL='col_email';
final String COL_STUDENT_PHONE='col_phone';
final String COL_STUDENT_FAVOURITE='col_fav';
class StudentModels{
int id;
String name;
String email;
int phoneNumber;
bool isFavourite;
StudentModels({
this.id, this.name, this.email, this.phoneNumber, this.isFavourite=false
});
@override
String toString() {
return 'StudentModels{id: $id, name: $name, email: $email, phoneNumber: $phoneNumber, isFavourite: $isFavourite}';
}
Map<String,dynamic> toMap(){
var map=<String,dynamic>{
COL_STUDENT_NAME:name,
COL_STUDENT_EMAIL:email,
COL_STUDENT_PHONE:phoneNumber,
COL_STUDENT_FAVOURITE:isFavourite,
};
if(id!=null){
map[COL_STUDENT_ID]=id;
}
return map;
}
StudentModels.fromMap(Map<String,dynamic> map){
name=map[COL_STUDENT_NAME];
id=map[COL_STUDENT_ID];
email=map[COL_STUDENT_EMAIL];
phoneNumber=map[COL_STUDENT_PHONE];
isFavourite=map[COL_STUDENT_FAVOURITE];
}
}
这是我每次运行它时都会遇到的错误：
E/SQLiteLog(11830): (1) no such table: tbl_student
I/flutter (11830): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (11830): The following NoSuchMethodError was thrown building FutureBuilder<List<StudentModels>>(dirty, state:
I/flutter (11830): _FutureBuilderState<List<StudentModels>>#1afc9):
I/flutter (11830): The getter 'length' was called on null.
I/flutter (11830): Receiver: null
I/flutter (11830): Tried calling: length
I/flutter (11830):
I/flutter (11830): The relevant error-causing widget was:
I/flutter (11830): FutureBuilder<List<StudentModels>>
I/flutter (11830): file:///D:/flutterapps/first_flutter_app/lib/pages/student_list.dart:29:13
I/flutter (11830):
I/flutter (11830): When the exception was thrown, this was the stack:
I/flutter (11830): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
I/flutter (11830): #1 _StudentListPageState.build.<anonymous closure> (package:first_flutter_app/pages/student_list.dart:37:31)
I/flutter (11830): #2 _FutureBuilderState.build (package:flutter/src/widgets/async.dart:732:55)
I/flutter (11830): #3 StatefulElement.build (package:flutter/src/widgets/framework.dart:4619:28)
I/flutter (11830): #4 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4502:15)
I/flutter (11830): #5 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4675:11)
I/flutter (11830): #6 Element.rebuild (package:flutter/src/widgets/framework.dart:4218:5)
I/flutter (11830): #7 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2627:33)
I/flutter (11830): #8 WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:883:20)
I/flutter (11830): #9 RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:284:5)
I/flutter (11830): #10 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1113:15)
I/flutter (11830): #11 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1052:9)
I/flutter (11830): #12 SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:968:5)
I/flutter (11830): #16 _invoke (dart:ui/hooks.dart:261:10)
I/flutter (11830): #17 _drawFrame (dart:ui/hooks.dart:219:3)
I/flutter (11830): (elided 3 frames from dart:async)
I/flutter (11830):
I/flutter (11830): ════════════════════════════════════════════════════════════════════════════════════════════════════
E/SQLiteLog(11830): (1) no such table: tbl_student
════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building FutureBuilder<List<StudentModels>>(dirty, state: _FutureBuilderState<List<StudentModels>>#1afc9):
The getter 'length' was called on null.
Receiver: null
Tried calling: length
The relevant error-causing widget was:
FutureBuilder<List<StudentModels>> file:///D:/flutterapps/first_flutter_app/lib/pages/student_list.dart:29:13
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1 _StudentListPageState.build.<anonymous closure> (package:first_flutter_app/pages/student_list.dart:37:31)
#2 _FutureBuilderState.build (package:flutter/src/widgets/async.dart:732:55)
#3 StatefulElement.build (package:flutter/src/widgets/framework.dart:4619:28)
#4 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4502:15)
答案 0 :(得分：2)
在快照接收器上尝试
builder: (context,AsyncSnapshot<List<StudentModels>> snapshot) {
//progress UI
if (snapshot.connectionState != ConnectionState.done)
return Center(child: Circular Progress Indicator());
//load null data UI
if (!snapshot.hasData || snapshot.data == null)
return Container();
//load empty data UI
if (snapshot.data.isEmpty)
return Container();
//enter your code to show data
//show the list of student model
...