Flutter: How to change the state of a variable in another dart file? - flutter

Question

I am currently working on an app; I want to change the value of a String which is declared in another dart file and then pass that changed state to the stateful widget.
I.E;
I create a file called as "Body.dart" file where I have declared a String called as 'scale' who's value initially is "Empty".
Later when a button in another dart file "scale_button" is pressed, I want to assign the string scale = "Hello" in my Body.dart file. So that the stateful widget also displays the same on the screen.

Answer 1

You can use provider(or any other state management) package in that case. In yaml file add, provider: ^4.3.2+4
class HomeApp extends StatefulWidget {
#override
_HomeAppState createState() => _HomeAppState();
}
class _HomeAppState extends State<HomeApp> {
StringProvider _stringProvider;
#override
void initState() {
super.initState();
_stringProvider = Provider.of<StringProvider>(context, listen: false);
}
void updateString() {
_stringProvider.setString('hai');
}
#override
Widget build(BuildContext context) {
StringProvider _stringProvider = Provider.of<StringProvider>(context);
return Scaffold(
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Container(
child: Text(
_stringProvider.str,
style: TextStyle(
fontSize: 22,
),
),
),
RaisedButton(
onPressed: updateString,
child: Text('Click'),
),
],
),
),
);
}
}
// class for storing data(StringProvider.dart)
import 'package:flutter/material.dart';
class StringProvider extends ChangeNotifier { // create a common file for data
String _str = 'hello';
String get str => _str;
void setString(String st) {
_str = st;
notifyListeners();
}
}

Answer 2

When you create a new Flutter project the sample code of the counter shows you how to do this. Check out the comments in the next code:
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
// here is passing a String to MyHomePage.
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
// And here you can see how to make the widget wait for a variable
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
Full code of Sample Counter App
import 'package:flutter/material.dart';
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(
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> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
}