Flutter 返回地图时FutureBuilder中的空快照
我不明白为什么我在future builder中总是得到空返回,而我确信应该有一个值Flutter 返回地图时FutureBuilder中的空快照,flutter,dart,future,Flutter,Dart,Future,我不明白为什么我在future builder中总是得到空返回,而我确信应该有一个值 //global variables Map<String,bool> students; List<String> studentsList ; List<bool> studentsCheck ; FutureBuilder( future: GetInfoFromBackEnd(), builder: (BuildContext context, AsyncS
//global variables
Map<String,bool> students;
List<String> studentsList ;
List<bool> studentsCheck ;
FutureBuilder(
future: GetInfoFromBackEnd(),
builder: (BuildContext context, AsyncSnapshot snapshot) {
return snapshot.hasData?
CustomScrollView(....some not important widgets here....):CircularProgressIndicator();
},)
Future GetInfoFromBackEnd() async {
students = {
'this info should be taken from backend0':false,
'this info should be taken from backend0':false,
'this info should be taken from backend0':false,
'this info should be taken from backend0':false,
'this info should be taken from backend0':false,
};
studentsList = students.keys;
studentsCheck = students.values;
return students;
}
//全局变量
学生地图;
列出学生名单;
学生名单;
未来建设者(
future:GetInfoFromBackEnd(),
生成器:(BuildContext上下文,异步快照){
返回snapshot.hasData?
CustomScrollView(……此处的一些不重要的小部件……):CircularProgressIndicator();
},)
将来的GetInfoFromBackEnd()异步{
学生={
“此信息应取自后端0”:false,
“此信息应取自后端0”:false,
“此信息应取自后端0”:false,
“此信息应取自后端0”:false,
“此信息应取自后端0”:false,
};
studentsList=students.keys;
studentsCheck=students.values;
留学生;
}
我对这段代码有一个变通方法,但我需要了解为什么会出现这种情况,以及发生了什么情况,提供了包含代码的基本HelloWorld颤振示例。我为future添加了一个延迟,以便ProgressIndicator显示两秒钟。作品如我所愿:
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
Map<String, bool> students;
void _incrementCounter() {
setState(() {
_counter++;
});
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: FutureBuilder(
future: GetInfoFromBackEnd(),
builder: (BuildContext context, AsyncSnapshot<Map> snapshot) {
return snapshot.hasData
? new Text(snapshot.data.toString())
: new CircularProgressIndicator();
}),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
Future<Map> GetInfoFromBackEnd() async {
return Future.delayed(Duration(seconds: 2), () {
students = {
'this info should be taken from backend0': false,
'this info should be taken from backend1': false,
'this info should be taken from backend2': false,
'this info should be taken from backend3': false,
'this info should be taken from backend4': false,
};
return students;
});
}
}
导入“包装:颤振/材料.省道”;
void main()=>runApp(MyApp());
类MyApp扩展了无状态小部件{
//此小部件是应用程序的根。
@凌驾
小部件构建(构建上下文){
返回材料PP(
标题:“颤振演示”,
主页:MyHomePage(标题:“颤振演示主页”),
);
}
}
类MyHomePage扩展StatefulWidget{
MyHomePage({Key,this.title}):超级(Key:Key);
最后的字符串标题;
@凌驾
_MyHomePageState createState()=>\u MyHomePageState();
}
类_MyHomePageState扩展状态{
int _计数器=0;
学生地图;
void _incrementCounter(){
设置状态(){
_计数器++;
});
}
@凌驾
小部件构建(构建上下文){
返回脚手架(
appBar:appBar(
标题:文本(widget.title),
),
正文:未来建设者(
future:GetInfoFromBackEnd(),
生成器:(BuildContext上下文,异步快照){
返回snapshot.hasData
?新文本(snapshot.data.toString())
:新的循环压缩机指示器();
}),
浮动操作按钮:浮动操作按钮(
按下时:\ u递增计数器,
工具提示:“增量”,
子:图标(Icons.add),
),
);
}
将来的GetInfoFromBackEnd()异步{
返回未来。延迟(持续时间(秒:2),(){
学生={
“此信息应取自后端0”:false,
“此信息应取自后端1”:false,
“此信息应取自后端2”:false,
“此信息应取自后端3”:false,
“此信息应取自后端4”:false,
};
留学生;
});
}
}
提供了包含代码的HelloWorld颤振基本示例。我为future添加了一个延迟,以便ProgressIndicator显示两秒钟。作品如我所愿:
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
Map<String, bool> students;
void _incrementCounter() {
setState(() {
_counter++;
});
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: FutureBuilder(
future: GetInfoFromBackEnd(),
builder: (BuildContext context, AsyncSnapshot<Map> snapshot) {
return snapshot.hasData
? new Text(snapshot.data.toString())
: new CircularProgressIndicator();
}),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
Future<Map> GetInfoFromBackEnd() async {
return Future.delayed(Duration(seconds: 2), () {
students = {
'this info should be taken from backend0': false,
'this info should be taken from backend1': false,
'this info should be taken from backend2': false,
'this info should be taken from backend3': false,
'this info should be taken from backend4': false,
};
return students;
});
}
}
导入“包装:颤振/材料.省道”;
void main()=>runApp(MyApp());
类MyApp扩展了无状态小部件{
//此小部件是应用程序的根。
@凌驾
小部件构建(构建上下文){
返回材料PP(
标题:“颤振演示”,
主页:MyHomePage(标题:“颤振演示主页”),
);
}
}
类MyHomePage扩展StatefulWidget{
MyHomePage({Key,this.title}):超级(Key:Key);
最后的字符串标题;
@凌驾
_MyHomePageState createState()=>\u MyHomePageState();
}
类_MyHomePageState扩展状态{
int _计数器=0;
学生地图;
void _incrementCounter(){
设置状态(){
_计数器++;
});
}
@凌驾
小部件构建(构建上下文){
返回脚手架(
appBar:appBar(
标题:文本(widget.title),
),
正文:未来建设者(
future:GetInfoFromBackEnd(),
生成器:(BuildContext上下文,异步快照){
返回snapshot.hasData
?新文本(snapshot.data.toString())
:新的循环压缩机指示器();
}),
浮动操作按钮:浮动操作按钮(
按下时:\ u递增计数器,
工具提示:“增量”,
子:图标(Icons.add),
),
);
}
将来的GetInfoFromBackEnd()异步{
返回未来。延迟(持续时间(秒:2),(){
学生={
“此信息应取自后端0”:false,
“此信息应取自后端1”:false,
“此信息应取自后端2”:false,
“此信息应取自后端3”:false,
“此信息应取自后端4”:false,
};
留学生;
});
}
}
问题不在FutureBuilder或地图中,事实上,当我试图获取学生地图值和列表中的每个键时,它破坏了未来并阻止它返回正确值,因此我在snapshot.data中得到空值;下面是我如何修复它的:
studentsList = students.keys.toList();
studentsCheck = students.values.toList();
return students;
注意:错误是
类型“\u CompactIterable”不是类型“List”的子类型
问题不在FutureBuilder或地图中,事实上,当我试图获取学生地图值和列表中的每个键时,它破坏了未来并阻止它返回正确值,因此我在snapshot.data中得到空值;下面是我如何修复它的:
studentsList = students.keys.toList();
studentsCheck = students.values.toList();
return students;
注意:错误是
类型“\u CompactIterable”不是类型“List”的子类型
什么是nullsnapshot.data
?我使用了您的示例,snapshot和hasData都不为null。顺便说一句,如果您想在映射中使用模拟数据进行测试,您至少需要有不同的键。否则,您将在映射中只得到一个元素。通过向AsyncSnapshot和Future添加类型,我将使代码更加类型安全。使编码更容易,您将在编辑器中拥有intellisense。@pskink是的,这是快照数据Marc确定,请稍候