Flutter “如何修复”；吸气剂'；长度'；被调用时为null“；颤振误差
我试图显示我的资料列表，但它一直失败，snapshot.data似乎为空。请问我该如何解决这个问题。我已经添加了用于模拟的检查表类。我试过把清单.长度改成清单？.长度？？0，但仍然不起作用，我检查了路径“data/checklist.json”是否正确Flutter “如何修复”；吸气剂'；长度'；被调用时为null“；颤振误差,flutter,dart,Flutter,Dart,我试图显示我的资料列表，但它一直失败，snapshot.data似乎为空。请问我该如何解决这个问题。我已经添加了用于模拟的检查表类。我试过把清单.长度改成清单？.长度？？0，但仍然不起作用，我检查了路径“data/checklist.json”是否正确 class CheckList { final String checkListName; final ItemList list; final int total; CheckList({this.checkListName
class CheckList {
final String checkListName;
final ItemList list;
final int total;
CheckList({this.checkListName, this.list, this.total});
factory CheckList.fromJson(Map<String, dynamic> json) {
return new CheckList(
checkListName: json['checkListName'],
list: ItemList.fromJson(json['list']),
total: json['total'],
);
}
static Future<List<CheckList>> browse() async {
String content = await rootBundle.loadString('data/checklist.json');
await Future.delayed(Duration(seconds: 1));
List collection = json.decode(content);
List<CheckList> _checklist =
collection.map((json) => CheckList.fromJson(json)).toList();
return _checklist;
}
}
Future<List<CheckList>> checkList;
@override
void initState() {
super.initState();
checkList = CheckList.browse();
}
body: FutureBuilder(
future: checkList, // a previously-obtained Future<String> or null
builder: (BuildContext context, AsyncSnapshot snapshot) {
switch (snapshot.connectionState) {
case ConnectionState.none:
case ConnectionState.active:
case ConnectionState.waiting:
return Center(child: CircularProgressIndicator());
case ConnectionState.done:
// if (snapshot.hasError) return Text('Error is: ${snapshot.error}');
var checkList = snapshot.data;
return ListView.separated(
itemCount: checkList.length,
itemBuilder: (BuildContext context, int index) {
CheckList checklist = checkList[index];
return ListTile(
title: Padding(
padding: const EdgeInsets.fromLTRB(0, 15, 0, 0),
child: Text(
checklist.checkListName,
style: TextStyle(fontWeight: FontWeight.bold),
),
),
课程清单{
最终字符串checkListName；
最终项目清单；
最终整数合计；
检查表（{this.checkListName，this.list，this.total}）；
工厂清单.fromJson（映射json）{
返回新的检查表(
checkListName:json['checkListName']，
list:ItemList.fromJson（json['list']），
总计：json['total']，
);
}
静态未来浏览（）异步{
String content=await rootBundle.loadString（'data/checklist.json'）；
等待未来。延迟（持续时间（秒：1））；
列表集合=json.decode（内容）；
清单=
collection.map（（json）=>CheckList.fromJson（json））.toList（）；
返回检查表；
}
}
未来清单；
@凌驾
void initState（）{
super.initState（）；
checkList=checkList.browse（）；
}
正文：未来建设者(
future：检查表，//以前获得的future或null
生成器：（BuildContext上下文，异步快照）{
交换机（快照.连接状态）{
案例连接状态。无：
案例连接状态.active：
案例连接状态。正在等待：
返回中心（子项：CircularProgressIndicator（））；
案例连接状态。完成：
//if（snapshot.hasError）返回文本（'Error为：${snapshot.Error}'）；
var检查表=快照数据；
返回ListView.separated(
itemCount：检查表。长度，
itemBuilder:（构建上下文，int索引）{
检查表=检查表[索引]；
返回列表块(
标题：填充(
padding:const EdgeInsets.fromLTRB（0,15,0,0），
子：文本(
checklist.checkListName，
样式：TextStyle（fontWeight:fontWeight.bold），
),
),
假设您有两个清单学校列表和abc列表
我将json文件放在路径assets/data/checklist.json中
对于演示，我没有使用您所有的函数名
您可以在更改json路径后直接运行此代码。
完整工作代码
import 'package:flutter/material.dart';
import 'dart:convert';
void main() => runApp(MyApp());
// To parse this JSON data, do
//
// final checkList = checkListFromJson(jsonString);
List<CheckList> checkListFromJson(String str) => List<CheckList>.from(json.decode(str).map((x) => CheckList.fromJson(x)));
String checkListToJson(List<CheckList> data) => json.encode(List<dynamic>.from(data.map((x) => x.toJson())));
class CheckList {
String checkListName;
List<ListElement> list;
int total;
CheckList({
this.checkListName,
this.list,
this.total,
});
factory CheckList.fromJson(Map<String, dynamic> json) => CheckList(
checkListName: json["checkListName"],
list: List<ListElement>.from(json["list"].map((x) => ListElement.fromJson(x))),
total: json["total"],
);
Map<String, dynamic> toJson() => {
"checkListName": checkListName,
"list": List<dynamic>.from(list.map((x) => x.toJson())),
"total": total,
};
}
class ListElement {
String itemName;
int noOfItems;
int unitPrice;
ListElement({
this.itemName,
this.noOfItems,
this.unitPrice,
});
factory ListElement.fromJson(Map<String, dynamic> json) => ListElement(
itemName: json["itemName"],
noOfItems: json["noOfItems"],
unitPrice: json["unitPrice"],
);
Map<String, dynamic> toJson() => {
"itemName": itemName,
"noOfItems": noOfItems,
"unitPrice": unitPrice,
};
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
// This is the theme of your application.
//
// Try running your application with "flutter run". You'll see the
// application has a blue toolbar. Then, without quitting the app, try
// changing the primarySwatch below to Colors.green and then invoke
// "hot reload" (press "r" in the console where you ran "flutter run",
// or simply save your changes to "hot reload" in a Flutter IDE).
// Notice that the counter didn't reset back to zero; the application
// is not restarted.
primarySwatch: Colors.blue,
),
home: ListContent(),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
// This widget is the home page of your application. It is stateful, meaning
// that it has a State object (defined below) that contains fields that affect
// how it looks.
// This class is the configuration for the state. It holds the values (in this
// case the title) provided by the parent (in this case the App widget) and
// used by the build method of the State. Fields in a Widget subclass are
// always marked "final".
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
// This call to setState tells the Flutter framework that something has
// changed in this State, which causes it to rerun the build method below
// so that the display can reflect the updated values. If we changed
// _counter without calling setState(), then the build method would not be
// called again, and so nothing would appear to happen.
_counter++;
});
}
@override
Widget build(BuildContext context) {
// This method is rerun every time setState is called, for instance as done
// by the _incrementCounter method above.
//
// The Flutter framework has been optimized to make rerunning build methods
// fast, so that you can just rebuild anything that needs updating rather
// than having to individually change instances of widgets.
return Scaffold(
appBar: AppBar(
// Here we take the value from the MyHomePage object that was created by
// the App.build method, and use it to set our appbar title.
title: Text(widget.title),
),
body: Center(
// Center is a layout widget. It takes a single child and positions it
// in the middle of the parent.
child: Column(
// Column is also a layout widget. It takes a list of children and
// arranges them vertically. By default, it sizes itself to fit its
// children horizontally, and tries to be as tall as its parent.
//
// Invoke "debug painting" (press "p" in the console, choose the
// "Toggle Debug Paint" action from the Flutter Inspector in Android
// Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
// to see the wireframe for each widget.
//
// Column has various properties to control how it sizes itself and
// how it positions its children. Here we use mainAxisAlignment to
// center the children vertically; the main axis here is the vertical
// axis because Columns are vertical (the cross axis would be
// horizontal).
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.display1,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}
class ListContent extends StatefulWidget {
@override
_ListContentState createState() => _ListContentState();
}
class _ListContentState extends State<ListContent> {
List<CheckList> _words = List<CheckList>();
Future<List<CheckList>> _getWords() async {
var dictionaryData = await DefaultAssetBundle.of(context)
.loadString('assets/data/checklist.json');
var words = List<CheckList>();
if (dictionaryData != null) {
words = checkListFromJson(dictionaryData);
print("words lenght ${words.length}");
} else {
print('fail');
}
return words;
}
@override
Widget build(BuildContext context) {
return FutureBuilder(
builder: (context, AsyncSnapshot<List<CheckList>> snapshot) {
switch (snapshot.connectionState) {
case ConnectionState.none:
return new Text('none');
case ConnectionState.waiting:
return new Center(child: new CircularProgressIndicator());
case ConnectionState.active:
return new Text('');
case ConnectionState.done:
if (snapshot.hasError) {
return new Text(
'${snapshot.error}',
style: TextStyle(color: Colors.red),
);
} else {
return ListView.builder(
itemCount: snapshot.data.length,
itemBuilder: (context, index) {
CheckList checkList = snapshot.data[index];
return Card(
elevation: 6.0,
child: Padding(
padding: const EdgeInsets.only(
top: 6.0, bottom: 6.0, left: 8.0, right: 8.0),
child: Row(
crossAxisAlignment: CrossAxisAlignment.start,
children: <Widget>[
Text(
checkList.checkListName,
),
Spacer(),
Text(
checkList.total.toString(),
),
],
),
));
});
}
}
},
future: _getWords(),
);
}
}
Future<List<CheckList>> checkList;
未来清单；
List<CheckList> checkList;
清单清单；
@覆盖
void initState（）{
super.initState（）；
检查表=[]；//这很重要
这个.getCheckList（）；
}
Future getCheckList（）异步{
检查表=等待检查表。浏览（）；
设置状态（{}）；
}
List<CheckList> checkList;
@override
void initState() {
super.initState();
checkList = []; //It is important
this.getCheckList();
}
Future<void> getCheckList() async{
checkList = await CheckList.browse();
setState({});
}