how to read local json file in flutter - json

Question

I am trying to read a local json file named "catalog.json" I wrote all the nessessary codes but it's showing this error "lateinitializationError: Field 'catalogdata' has not been initialized."
then i tried by initializing the 'catalogdata' variable but then it shows that 'catalogdata' variable is empty . I dont know how to solve it . Please help me.
my code
import 'package:flutter/material.dart';
import 'dart:convert';
import 'package:flutter/services.dart';
class Homepage extends StatefulWidget {
const Homepage({Key? key}) : super(key: key);
#override
_HomepageState createState() => _HomepageState();
}
class _HomepageState extends State<Homepage> {
late List catalogdata;
Future<String> loadData() async {
var data = await rootBundle.loadString("assets/images/files/catalog.json");
setState(() {
catalogdata = json.decode(data);
});
return "success";
}
#override
void initState() {
// TODO: implement initState
this.loadData();
super.initState();
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Homepage"),
),
body: Center(
child: Text(
catalogdata[0],
style: TextStyle(fontSize: 20),
),
),
);
}
}

Answer 1

Here’s sample.json:
{
"items": [
{
"id": "p1",
"name": "Item 1",
"description": "Description 1"
},
{
"id": "p2",
"name": "Item 2",
"description": "Description 2"
},
{
"id": "p3",
"name": "Item 3",
"description": "Description 3"
}
]
}
The code which is used to fetch data from the JSON file (see the full code below):
Future<void> readJson() async {
final String response = await rootBundle.loadString('assets/sample.json');
final data = await json.decode(response);
// ...
}
Declare the json file in the assets section in your pubspec.yaml file:
flutter:
assets:
- assets/sample.json
main.dart
import 'package:flutter/material.dart';
import 'dart:convert';
import 'package:flutter/services.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return const MaterialApp(
// Hide the debug banner
debugShowCheckedModeBanner: false,
title: 'Kindacode.com',
home: HomePage(),
);
}
}
class HomePage extends StatefulWidget {
const HomePage({Key? key}) : super(key: key);
#override
_HomePageState createState() => _HomePageState();
}
class _HomePageState extends State<HomePage> {
List _items = [];
// Fetch content from the json file
Future<void> readJson() async {
final String response = await rootBundle.loadString('assets/sample.json');
final data = await json.decode(response);
setState(() {
_items = data["items"];
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
centerTitle: true,
title: const Text(
'Kindacode.com',
),
),
body: Padding(
padding: const EdgeInsets.all(25),
child: Column(
children: [
ElevatedButton(
child: const Text('Load Data'),
onPressed: readJson,
),
// Display the data loaded from sample.json
_items.isNotEmpty
? Expanded(
child: ListView.builder(
itemCount: _items.length,
itemBuilder: (context, index) {
return Card(
margin: const EdgeInsets.all(10),
child: ListTile(
leading: Text(_items[index]["id"]),
title: Text(_items[index]["name"]),
subtitle: Text(_items[index]["description"]),
),
);
},
),
)
: Container()
],
),
),
);
}
}

Answer 2

it's showing this error "lateinitializationError: Field 'catalogdata'
has not been initialized." then I tried by initializing the
'catalogdata'
While using late before variables make sure that, the variable must be initialized later. Otherwise, you can encounter a runtime error when the variable is used.
If you didn't add the correct location catalog.json in pubsec.yaml your variable catalog didn't gets the correct value so the late variable is not initialized.
So you must add asset path in pubsec.yaml
assets:
- assets/
- assets/images/files/catalog.json
Another case here is JSON.decode() return map<string,dynamic> value here you set list.maybe that also cause the problem and not initialised.
instead of this late List catalogdata; use this late var catalogdata; or late Map<string,dynamic> catalogdata;
Sample Code
Catalog.json
{
"name": "lava",
"Catagory": "man"
}
Main.dart
import 'dart:convert';
import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
void main() {
runApp(MyApp());
}
class AppRoutes {
static String detail = "/Detail";
static String Page2 = "/FilterBeacon";
static String Page1 = "/FilterPoint";
static String home = "/";
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
static const String _title = 'Flutter Code Sample';
#override
Widget build(BuildContext context) {
return MaterialApp(
debugShowCheckedModeBanner: false,
// key: constItem.navigatorKey,
initialRoute: "/",
routes: {
AppRoutes.home: (context) => Home(),
},
title: _title,
// home: ,
);
}
}
class Home extends StatefulWidget {
const Home({Key? key}) : super(key: key);
#override
State<Home> createState() => _HomeState();
}
var _index = 0;
class _HomeState extends State<Home> {
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(),
body: Homepage(),
);
}
}
class Homepage extends StatefulWidget {
const Homepage({Key? key}) : super(key: key);
#override
_HomepageState createState() => _HomepageState();
}
class _HomepageState extends State<Homepage> {
late var catalogdata;
Future<String> loadDatas() async {
var data = await rootBundle.loadString("assets/images/files/catalog.json");
// setState(() {
catalogdata = json.decode(data);
// });
return "success";
}
Future<String> loadData() async {
var data = await rootBundle.loadString("assets/images/files/catalog.json");
setState(() {
catalogdata = json.decode(data);
});
return "success";
}
#override
void initState() {
loadData();
// loadData().then((value) => catalogdata=value);
} // String jsons = "";
// #override
// Future<void> initState() async {
// super.initState();
// await loadData();
// }
#override
Widget build(BuildContext context) {
var futureBuilder = FutureBuilder(
future: loadData(),
builder: (context, AsyncSnapshot snapshot) {
if (snapshot.connectionState == ConnectionState.waiting)
return Center(child: CircularProgressIndicator());
else if (snapshot.connectionState == ConnectionState.done)
return Center(
child: Text(
catalogdata.toString(),
style: TextStyle(),
),
);
else
return Container();
});
return Scaffold(
appBar: AppBar(
title: Text("Homepage"),
),
body: Center(
child: Text(
catalogdata.toString(),
style: TextStyle(),
),
),
);
}
}