How to fix "LateInitializationError: Field 'data' has not been initialized error"
How to fix "LateInitializationError: Field 'data' has not been initialized error"
我正在使用 flutter 应用程序。
我正在尝试显示来自 json.
的数据
我遇到了以下错误：
LateInitializationError: Field 'data' has not been initialized error
我的main.dart文件如下：
import 'package:flutter/material.dart';
import 'dart:async';
import 'dart:core';
import 'dart:convert';
import 'package:http/http.dart' as http;
void main() => runApp(MaterialApp(
home: HomePage(),
));
class HomePage extends StatefulWidget{
@override
HomePageState createState()=> new HomePageState();
}
class HomePageState extends State<HomePage>{
final String url = "http://127.0.0.1:5000/restaurant";
late List data;
@override
void initState() {
super.initState();
getJsonData();
}
Future<String> getJsonData() async{
var response = await http.get(Uri.parse(url));
print(response.body);
setState((){
var convertDataToJson = json.decode(response.body);
data = convertDataToJson['restaurant'];
});
return "success";
}
@override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar (
title: new Text("Where to Eat")
),
body: new ListView.builder(
itemCount: data == null ? 0 : data.length,
itemBuilder: (BuildContext context, int index) {
return new Container(
child: new Center(
child: new Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: <Widget>[
new Card(
child: new Container(
child: new Text(data[index]['city']['details']),
padding: const EdgeInsets.all(20.0),
),
),
],
),
),
);
},
)
);
}
}
导致我出现问题的代码行是：
late List data;
我已经尝试使用这个 Whosebug post 中的解决方案。
但是，这对我不起作用。它导致了其他错误，也导致代码无法编译。
如何使用我的代码正确修复此错误，以便显示和解析 json 数据？
这是我正在使用的json：
{"city": "Al-Khobar",
"details": [
{
"id": 1,
"Area": "Near Corniche",
"Restaurant": "Naranj Damascus Restaurant",
"Street": "Firas Bin Al Nadr Street",
"location link": "https:\/\/www.tripadvisor.com\/Restaurant_Review-g298545-d22895043-Reviews-Naranj_Damascus_Restaurant-Al_Khobar_Eastern_Province.html#MAPVIEW",
"City": "Al Khobar",
"Zip": 34447,
"Country": "Saudi Arabia",
"phone": 966508446622,
"meals": "Breakfast, Lunch, Dinner, Late Night",
"cuisine": "International, Barbecue, Grill, Diner, Middle Eastern",
"price range": "' to "
},
{
"id": 2,
"Area": "Near Corniche",
"Restaurant": "The Butcher Shop and Grill",
"Street": "Prince Turkey Street",
"location link": "https:\/\/www.tripadvisor.com\/Restaurant_Review-g298545-d10691837-Reviews-The_Butcher_Shop_and_Grill-Al_Khobar_Eastern_Province.html#MAPVIEW",
"City": "Al Khobar",
"Zip": " ",
"Country": "Saudi Arabia",
"phone": 966138085182,
"meals": "Lunch, Dinner",
"cuisine": "Steakhouse",
"price range": "' to "
},
{
"id": 3,
"Area": "Near Corniche",
"Restaurant": "Kosebasi, Al Khobar",
"Street": "Prince Turkey Street",
"location link": "https:\/\/www.tripadvisor.com\/Restaurant_Review-g298545-d9874670-Reviews-Kosebasi_Al_Khobar-Al_Khobar_Eastern_Province.html#MAPVIEW",
"City": "Al Khobar",
"Zip": " ",
"Country": "Saudi Arabia",
"phone": 966138030089,
"meals": "Lunch, Dinner",
"cuisine": "Turkish, Middle Eastern, Barbecue",
"price range": "' to "
},
{
"id": 4,
"Area": "Near Corniche",
"Restaurant": "Bun & Patty",
"Street": "Prince Turkey Street Al Yarmouk",
"location link": "https:\/\/www.tripadvisor.com\/Restaurant_Review-g298545-d8054714-Reviews-Bun_Patty-Al_Khobar_Eastern_Province.html#MAPVIEW",
"City": "Al Khobar",
"Zip": 344233213,
"Country": "Saudi Arabia",
"phone": " ",
"meals": "Lunch, Dinner",
"cuisine": "American, Fast Food",
"price range": " "
}
]
},
{"city": "Dammam",
"details": [
{
"id": 5,
"Area": "Near Corniche",
"Restaurant": "Abu Nawas",
"Street": "Al Adama – Prince Mansour Street",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d805085-Reviews-Abu_Nawas-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Dammam",
"Zip": "31461",
"Country": "Saudi Arabia",
"phone": 966138266363,
"meals": " ",
"cuisine": "Lebanese, Mediterranean, Middle Eastern",
"price range": "' to "
},
{
"id": 6,
"Area": "Near Corniche",
"Restaurant": "Heritage Village",
"Street": "Prince Turkey Street",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d805123-Reviews-Heritage_Village-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Dammam",
"Zip": " ",
"Country": "Saudi Arabia",
"phone": 96638090000,
"meals": "Lunch, Dinner",
"cuisine": "Middle Eastern, Vegetarian Friendly, Halal",
"price range": "' to "
},
{
"id": 7,
"Area": "Near Corniche",
"Restaurant": "Manoosha Alreef",
"Street": "Prince Faisal Bin Fahad Road Khobar North",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d10221865-Reviews-Manoosha_Alreef-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Dammam",
"Zip": "34426",
"Country": "Saudi Arabia",
"phone": 966539222673,
"meals": "Breakfast, Lunch, Dinner",
"cuisine": "Bakeries, Lebanese, Fast Food",
"price range": "' to "
},
{
"id": 8,
"Area": "Near Corniche",
"Restaurant": "American, Steakhouse",
"Street": "Prince Mohammad Bin Fahad St.",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d2659493-Reviews-Steak_House-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Dammam",
"Zip": 11372,
"Country": "Saudi Arabia",
"phone": "96638335468",
"meals": "Lunch, Dinner",
"cuisine": "American, Fast Food",
"price range": " to "
}
]
},
{"city": "Jeddah",
"details": [
{
"id": 9,
"Area": "Blvd Mall",
"Restaurant": "Nando's Mall Of Arabia",
"Street": "Blvd Mall of Arabia Mall of Arabia, Gate No-5",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g295419-d20090707-Reviews-Nando_s_Mall_Of_Arabia-Jeddah_Makkah_Province.html#MAPVIEW",
"City": "Jeddah",
"Zip": "23532",
"Country": "Saudi Arabia",
"phone": 966503446623,
"meals": "Breakfast, Lunch, Dinner",
"cuisine": "Fast Food",
"price range": "' to "
},
{
"id": 10,
"Area": "Near Corniche",
"Restaurant": "Heritage Village",
"Street": "Prince Turkey Street",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d805123-Reviews-Heritage_Village-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Jeddah",
"Zip": " ",
"Country": "Saudi Arabia",
"phone": 96638090000,
"meals": "Lunch, Dinner",
"cuisine": "Middle Eastern, Vegetarian Friendly, Halal",
"price range": "' to "
},
{
"id": 11,
"Area": "Near Corniche",
"Restaurant": "Manoosha Alreef",
"Street": "Prince Faisal Bin Fahad Road Khobar North",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d10221865-Reviews-Manoosha_Alreef-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Jeddah",
"Zip": "34426",
"Country": "Saudi Arabia",
"phone": 966539222673,
"meals": "Breakfast, Lunch, Dinner",
"cuisine": "Bakeries, Lebanese, Fast Food",
"price range": "' to "
},
{
"id": 12,
"Area": "Near Corniche",
"Restaurant": "American, Steakhouse",
"Street": "Prince Mohammad Bin Fahad St.",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d2659493-Reviews-Steak_House-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Jeddah",
"Zip": 11372,
"Country": "Saudi Arabia",
"phone": "96638335468",
"meals": "Lunch, Dinner",
"cuisine": "American, Fast Food",
"price range": " to "
}
]
}
]
谢谢
late List data; 应更改为 List? data;。如果某物是 non-nullable 但 late，则需要开发人员确保此变量在使用前已初始化。
由于在build方法中使用，在data == null ? 0 : data.length;中会报错
将其更改为可为 null List? 可以解决此问题。
这是如何使用 FutureBuilder 进行操作的基本示例。它不调用 http 并使用有限的数据工作，仅模拟延迟，但您可以根据需要进行调整。您可以在 dartpad.dev.
上 运行
import 'package:flutter/material.dart';
void main() => runApp(MaterialApp(
home: HomePage(),
));
class HomePage extends StatefulWidget {
@override
HomePageState createState() => HomePageState();
}
class HomePageState extends State<HomePage> {
late final Future<List<Map<String, dynamic>>> _getJsonDataFuture;
@override
void initState() {
super.initState();
_getJsonDataFuture = getJsonData();
}
Future<List<Map<String, dynamic>>> getJsonData() async {
// simulate delay
await Future.delayed(const Duration(seconds: 2));
var response = [
{"id": 1, "name": "first"},
{"id": 2, "name": "second"},
{"id": 3, "name": "third"}
];
return Future.value(response);
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: const Text("Where to Eat")),
body: FutureBuilder<List<Map<String, dynamic>>>(
future: _getJsonDataFuture,
builder: (context, snapshot) {
// while loading
if (snapshot.connectionState == ConnectionState.waiting) {
return const Center(child: CircularProgressIndicator());
}
// on error
if (snapshot.hasError) {
return Center(child: Text('Snapshot error: ${snapshot.error}'));
}
// on data received from future
if (snapshot.hasData && snapshot.data != null) {
return ListView.builder(
itemCount: snapshot.data!.length,
itemBuilder: (BuildContext context, int index) => Card(
child: Container(
child: Text(snapshot.data![index]['name']),
padding: const EdgeInsets.all(20.0),
),
));
}
// on missing data
return const Center(child: Text('Missing data'));
}));
}
}
我正在使用 flutter 应用程序。 我正在尝试显示来自 json.
的数据我遇到了以下错误：
LateInitializationError: Field 'data' has not been initialized error
我的main.dart文件如下：
import 'package:flutter/material.dart';
import 'dart:async';
import 'dart:core';
import 'dart:convert';
import 'package:http/http.dart' as http;
void main() => runApp(MaterialApp(
home: HomePage(),
));
class HomePage extends StatefulWidget{
@override
HomePageState createState()=> new HomePageState();
}
class HomePageState extends State<HomePage>{
final String url = "http://127.0.0.1:5000/restaurant";
late List data;
@override
void initState() {
super.initState();
getJsonData();
}
Future<String> getJsonData() async{
var response = await http.get(Uri.parse(url));
print(response.body);
setState((){
var convertDataToJson = json.decode(response.body);
data = convertDataToJson['restaurant'];
});
return "success";
}
@override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar (
title: new Text("Where to Eat")
),
body: new ListView.builder(
itemCount: data == null ? 0 : data.length,
itemBuilder: (BuildContext context, int index) {
return new Container(
child: new Center(
child: new Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: <Widget>[
new Card(
child: new Container(
child: new Text(data[index]['city']['details']),
padding: const EdgeInsets.all(20.0),
),
),
],
),
),
);
},
)
);
}
}
导致我出现问题的代码行是：
late List data;
我已经尝试使用这个 Whosebug post
如何使用我的代码正确修复此错误，以便显示和解析 json 数据？
这是我正在使用的json：
{"city": "Al-Khobar",
"details": [
{
"id": 1,
"Area": "Near Corniche",
"Restaurant": "Naranj Damascus Restaurant",
"Street": "Firas Bin Al Nadr Street",
"location link": "https:\/\/www.tripadvisor.com\/Restaurant_Review-g298545-d22895043-Reviews-Naranj_Damascus_Restaurant-Al_Khobar_Eastern_Province.html#MAPVIEW",
"City": "Al Khobar",
"Zip": 34447,
"Country": "Saudi Arabia",
"phone": 966508446622,
"meals": "Breakfast, Lunch, Dinner, Late Night",
"cuisine": "International, Barbecue, Grill, Diner, Middle Eastern",
"price range": "' to "
},
{
"id": 2,
"Area": "Near Corniche",
"Restaurant": "The Butcher Shop and Grill",
"Street": "Prince Turkey Street",
"location link": "https:\/\/www.tripadvisor.com\/Restaurant_Review-g298545-d10691837-Reviews-The_Butcher_Shop_and_Grill-Al_Khobar_Eastern_Province.html#MAPVIEW",
"City": "Al Khobar",
"Zip": " ",
"Country": "Saudi Arabia",
"phone": 966138085182,
"meals": "Lunch, Dinner",
"cuisine": "Steakhouse",
"price range": "' to "
},
{
"id": 3,
"Area": "Near Corniche",
"Restaurant": "Kosebasi, Al Khobar",
"Street": "Prince Turkey Street",
"location link": "https:\/\/www.tripadvisor.com\/Restaurant_Review-g298545-d9874670-Reviews-Kosebasi_Al_Khobar-Al_Khobar_Eastern_Province.html#MAPVIEW",
"City": "Al Khobar",
"Zip": " ",
"Country": "Saudi Arabia",
"phone": 966138030089,
"meals": "Lunch, Dinner",
"cuisine": "Turkish, Middle Eastern, Barbecue",
"price range": "' to "
},
{
"id": 4,
"Area": "Near Corniche",
"Restaurant": "Bun & Patty",
"Street": "Prince Turkey Street Al Yarmouk",
"location link": "https:\/\/www.tripadvisor.com\/Restaurant_Review-g298545-d8054714-Reviews-Bun_Patty-Al_Khobar_Eastern_Province.html#MAPVIEW",
"City": "Al Khobar",
"Zip": 344233213,
"Country": "Saudi Arabia",
"phone": " ",
"meals": "Lunch, Dinner",
"cuisine": "American, Fast Food",
"price range": " "
}
]
},
{"city": "Dammam",
"details": [
{
"id": 5,
"Area": "Near Corniche",
"Restaurant": "Abu Nawas",
"Street": "Al Adama – Prince Mansour Street",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d805085-Reviews-Abu_Nawas-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Dammam",
"Zip": "31461",
"Country": "Saudi Arabia",
"phone": 966138266363,
"meals": " ",
"cuisine": "Lebanese, Mediterranean, Middle Eastern",
"price range": "' to "
},
{
"id": 6,
"Area": "Near Corniche",
"Restaurant": "Heritage Village",
"Street": "Prince Turkey Street",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d805123-Reviews-Heritage_Village-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Dammam",
"Zip": " ",
"Country": "Saudi Arabia",
"phone": 96638090000,
"meals": "Lunch, Dinner",
"cuisine": "Middle Eastern, Vegetarian Friendly, Halal",
"price range": "' to "
},
{
"id": 7,
"Area": "Near Corniche",
"Restaurant": "Manoosha Alreef",
"Street": "Prince Faisal Bin Fahad Road Khobar North",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d10221865-Reviews-Manoosha_Alreef-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Dammam",
"Zip": "34426",
"Country": "Saudi Arabia",
"phone": 966539222673,
"meals": "Breakfast, Lunch, Dinner",
"cuisine": "Bakeries, Lebanese, Fast Food",
"price range": "' to "
},
{
"id": 8,
"Area": "Near Corniche",
"Restaurant": "American, Steakhouse",
"Street": "Prince Mohammad Bin Fahad St.",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d2659493-Reviews-Steak_House-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Dammam",
"Zip": 11372,
"Country": "Saudi Arabia",
"phone": "96638335468",
"meals": "Lunch, Dinner",
"cuisine": "American, Fast Food",
"price range": " to "
}
]
},
{"city": "Jeddah",
"details": [
{
"id": 9,
"Area": "Blvd Mall",
"Restaurant": "Nando's Mall Of Arabia",
"Street": "Blvd Mall of Arabia Mall of Arabia, Gate No-5",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g295419-d20090707-Reviews-Nando_s_Mall_Of_Arabia-Jeddah_Makkah_Province.html#MAPVIEW",
"City": "Jeddah",
"Zip": "23532",
"Country": "Saudi Arabia",
"phone": 966503446623,
"meals": "Breakfast, Lunch, Dinner",
"cuisine": "Fast Food",
"price range": "' to "
},
{
"id": 10,
"Area": "Near Corniche",
"Restaurant": "Heritage Village",
"Street": "Prince Turkey Street",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d805123-Reviews-Heritage_Village-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Jeddah",
"Zip": " ",
"Country": "Saudi Arabia",
"phone": 96638090000,
"meals": "Lunch, Dinner",
"cuisine": "Middle Eastern, Vegetarian Friendly, Halal",
"price range": "' to "
},
{
"id": 11,
"Area": "Near Corniche",
"Restaurant": "Manoosha Alreef",
"Street": "Prince Faisal Bin Fahad Road Khobar North",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d10221865-Reviews-Manoosha_Alreef-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Jeddah",
"Zip": "34426",
"Country": "Saudi Arabia",
"phone": 966539222673,
"meals": "Breakfast, Lunch, Dinner",
"cuisine": "Bakeries, Lebanese, Fast Food",
"price range": "' to "
},
{
"id": 12,
"Area": "Near Corniche",
"Restaurant": "American, Steakhouse",
"Street": "Prince Mohammad Bin Fahad St.",
"location link": "https://www.tripadvisor.com/Restaurant_Review-g298547-d2659493-Reviews-Steak_House-Dammam_Eastern_Province.html#MAPVIEW",
"City": "Jeddah",
"Zip": 11372,
"Country": "Saudi Arabia",
"phone": "96638335468",
"meals": "Lunch, Dinner",
"cuisine": "American, Fast Food",
"price range": " to "
}
]
}
]
谢谢
late List data; 应更改为 List? data;。如果某物是 non-nullable 但 late，则需要开发人员确保此变量在使用前已初始化。
由于在build方法中使用，在data == null ? 0 : data.length;中会报错
将其更改为可为 null List? 可以解决此问题。
这是如何使用 FutureBuilder 进行操作的基本示例。它不调用 http 并使用有限的数据工作，仅模拟延迟，但您可以根据需要进行调整。您可以在 dartpad.dev.
import 'package:flutter/material.dart';
void main() => runApp(MaterialApp(
home: HomePage(),
));
class HomePage extends StatefulWidget {
@override
HomePageState createState() => HomePageState();
}
class HomePageState extends State<HomePage> {
late final Future<List<Map<String, dynamic>>> _getJsonDataFuture;
@override
void initState() {
super.initState();
_getJsonDataFuture = getJsonData();
}
Future<List<Map<String, dynamic>>> getJsonData() async {
// simulate delay
await Future.delayed(const Duration(seconds: 2));
var response = [
{"id": 1, "name": "first"},
{"id": 2, "name": "second"},
{"id": 3, "name": "third"}
];
return Future.value(response);
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: const Text("Where to Eat")),
body: FutureBuilder<List<Map<String, dynamic>>>(
future: _getJsonDataFuture,
builder: (context, snapshot) {
// while loading
if (snapshot.connectionState == ConnectionState.waiting) {
return const Center(child: CircularProgressIndicator());
}
// on error
if (snapshot.hasError) {
return Center(child: Text('Snapshot error: ${snapshot.error}'));
}
// on data received from future
if (snapshot.hasData && snapshot.data != null) {
return ListView.builder(
itemCount: snapshot.data!.length,
itemBuilder: (BuildContext context, int index) => Card(
child: Container(
child: Text(snapshot.data![index]['name']),
padding: const EdgeInsets.all(20.0),
),
));
}
// on missing data
return const Center(child: Text('Missing data'));
}));
}
}