Need assistance with Providers in Flutter - flutter

Question

I'm trying to get my head around the Providers in Flutters... but after following some tutorials, I'm still facing some issue.
When I try to run this code, it gives me an error
Error: Could not find the correct Provider above this MyHomePage Widget
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:provider_way/MyHomePageViewModel.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
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> {
#override
Widget build(BuildContext context) {
return ChangeNotifierProvider(
create: (context) => MyHomePageViewModel(),
child: Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Consumer<MyHomePageViewModel>(
builder: (context, viewModel, child) {
return Text(viewModel.text);
},
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: () =>
Provider.of<MyHomePageViewModel>(context, listen: false)
.onClicked(),
tooltip: 'Increment',
child: Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
),
);
}
}
import 'package:flutter/foundation.dart';
class MyHomePageViewModel extends ChangeNotifier {
String text = 'Initial text';
void onClicked() {
text = 'Something was clicked';
notifyListeners();
}
}
The website where I found this example use it as
Provider.of<MainViewModel>(context, listen: false).onClicked(),
But that doesn't work either...

Answer 1

Before a widget that needs a provider is presented, it is required you create that particular provider before the page is built.
Checkout the working sample of your code below.
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:provider_way/MyHomePageViewModel.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return ChangeNotifierProvider(
create: (_) => MyHomePageViewModel(),
builder: (_, __) => MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
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> {
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Consumer<MyHomePageViewModel>(
builder: (context, viewModel, child) {
return Text(viewModel.text);
},
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: () =>
Provider.of<MyHomePageViewModel>(context, listen: false)
.onClicked(),
tooltip: 'Increment',
child: Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}
import 'package:flutter/foundation.dart';
class MyHomePageViewModel extends ChangeNotifier {
String text = 'Initial text';
void onClicked() {
text = 'Something was clicked';
notifyListeners();
}
}

Answer 2

In your provider code, do you have a class defined like:
class MyHomePageViewModel extends ChangeNotifier {
// your stuff here, like getter and setters, methods, etc
}
That class will deal with all the centralisation of your states, essentially acting as you'd hope - the provider.