How to read a value from an object - Unhandled Exception - flutter

Question

After days of useless attempts, I'm new with Flutter, I can only ask for help from those who know more than me
I need to read the value of 'G'
that I get from an await that executes a query to a database and returns an eventObject object composed of:
id:
object:
(see better in the figure)
having a futures that returns exactly
return new EventObject (
       id: ...,
       object: gruppiJsonParsing.gruppo);
and gruppiJsonParsing that returns an instance of the Gruppi class with:
final String utente;
final String G;
with:
var groupName = eventObject.object;
I only get:
Instance of 'Gruppi'
then, since as in figure 0 = ['Gruppi']
I would think I could write
Gruppi groupName = eventObject.object;
String groupUser = groupName.G;
But so I get instead
Unhandled Exception: type 'List <Groups'' is not a subtype of type 'Gruppi'
Thanks for any precious suggestions
Aa required here the code of GruppiJsonParsing.dart
import 'package:flutter_client_php_backend/models/Gruppi.dart';
class GruppiJsonParsing {
final String result;
final String message;
final List<Gruppi> gruppo;
GruppiJsonParsing({this.result, this.message, this.gruppo});
factory GruppiJsonParsing.fromJson(Map<String, dynamic> json) {
return GruppiJsonParsing(
result: json['result'],
message: json['message'],
gruppo: parseGruppi(json['gruppo']));
}
static List<Gruppi> parseGruppi(json) {
List<Gruppi> gruppiList = new List<Gruppi>();
for (var log in json) {
gruppiList.add(new Gruppi(G: log['G']));
}
return gruppiList;
}
Map<String, dynamic> toJson() =>
<String, dynamic>{'result': result, 'message': message, 'gruppo': gruppo};
}

Answer 1

If you json string look like this
{"result":"success","message":"Query Successful","gruppo":
[{"utente":null,"G":"utente1"}]}
you can parse with
final eventObject = eventObjectFromJson(jsonString);
print('${eventObject.gruppo[0].g}');
related class
// To parse this JSON data, do
//
// final eventObject = eventObjectFromJson(jsonString);
import 'dart:convert';
EventObject eventObjectFromJson(String str) => EventObject.fromJson(json.decode(str));
String eventObjectToJson(EventObject data) => json.encode(data.toJson());
class EventObject {
String result;
String message;
List<Gruppo> gruppo;
EventObject({
this.result,
this.message,
this.gruppo,
});
factory EventObject.fromJson(Map<String, dynamic> json) => EventObject(
result: json["result"] == null ? null : json["result"],
message: json["message"] == null ? null : json["message"],
gruppo: json["gruppo"] == null ? null : List<Gruppo>.from(json["gruppo"].map((x) => Gruppo.fromJson(x))),
);
Map<String, dynamic> toJson() => {
"result": result == null ? null : result,
"message": message == null ? null : message,
"gruppo": gruppo == null ? null : List<dynamic>.from(gruppo.map((x) => x.toJson())),
};
}
class Gruppo {
dynamic utente;
String g;
Gruppo({
this.utente,
this.g,
});
factory Gruppo.fromJson(Map<String, dynamic> json) => Gruppo(
utente: json["utente"],
g: json["G"] == null ? null : json["G"],
);
Map<String, dynamic> toJson() => {
"utente": utente,
"G": g == null ? null : g,
};
}
full code
import 'package:flutter/material.dart';
// To parse this JSON data, do
//
// final eventObject = eventObjectFromJson(jsonString);
import 'dart:convert';
EventObject eventObjectFromJson(String str) => EventObject.fromJson(json.decode(str));
String eventObjectToJson(EventObject data) => json.encode(data.toJson());
class EventObject {
String result;
String message;
List<Gruppo> gruppo;
EventObject({
this.result,
this.message,
this.gruppo,
});
factory EventObject.fromJson(Map<String, dynamic> json) => EventObject(
result: json["result"] == null ? null : json["result"],
message: json["message"] == null ? null : json["message"],
gruppo: json["gruppo"] == null ? null : List<Gruppo>.from(json["gruppo"].map((x) => Gruppo.fromJson(x))),
);
Map<String, dynamic> toJson() => {
"result": result == null ? null : result,
"message": message == null ? null : message,
"gruppo": gruppo == null ? null : List<dynamic>.from(gruppo.map((x) => x.toJson())),
};
}
class Gruppo {
dynamic utente;
String g;
Gruppo({
this.utente,
this.g,
});
factory Gruppo.fromJson(Map<String, dynamic> json) => Gruppo(
utente: json["utente"],
g: json["G"] == null ? null : json["G"],
);
Map<String, dynamic> toJson() => {
"utente": utente,
"G": g == null ? null : g,
};
}
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',
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: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
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;
String jsonString = '{"result":"success","message":"Query Successful","gruppo":[{"utente":null,"G":"utente1"}]}';
void _incrementCounter() {
final eventObject = eventObjectFromJson(jsonString);
print('${eventObject.gruppo[0].g}');
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.
);
}
}
Output
I/flutter (19854): utente1