I want to use Shared Preferences in the model and save the data, but it doesn't work.
For example, in the example below, I would like to save the counter of the default count-up app in the model so that the previous counter appears on the screen when I close the app and start it again, but when I restart it, it says zero.Press the plus button to display the previous number plus 1.
In this case, please let me know what the problem is.
import'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:shared_preferences/shared_preferences.dart';
void main() {
runApp(
ChangeNotifierProvider<MyHomePageModel>(
create:(_)=>MyHomePageModel(),
child —MyApp(),
)
);
}
class MyApp extensions StatelessWidget {
// This widget is the root of your application.
@override
Widget build (BuildContext context) {
US>return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwitch:Colors.blue,
visualDensity —VisualDensity.adaptivePlatformDensity,
),
home —MyHomePage(),
);
}
}
class MyHomePage extensions StatelessWidget{
@override
Widget build (BuildContext context) {
return Consumer<MyHomePageModel>(builder:(context,model,child){
int_counter=model.getCounter();
US>return Scaffold(
appBar —AppBar(
title:Text('FlutteR Demo Home Page'),
),
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—Button());
});
}
}
class Button extensions StatelessWidget {
@override
Widget build (BuildContext context) {
final model=Provider.of<MyHomePageModel>(context,listen:false);
return FloatingActionButton(
onPressed: ( ) = > model.addCounter(),
tooltip: 'Increment',
child —Icon(Icons.add),
);
}
}
class MyHomePageModel extensions ChangeNotifier {
int_counter = 0;
void_setPrefItems()async {
SharedPreferences prefs = wait SharedPreferences.getInstance();
prefs.setInt('counter',_counter);
}
void_getPrefItems()async {
SharedPreferences prefs = wait SharedPreferences.getInstance();
_counter=prefs.getInt('counter')??0;
}
void addCounter() {
_counter++;
_setPrefItems();
notifyListeners();
}
intgetCounter(){
_getPrefItems();
return_counter;
}
}
Explain why the behavior is as described in the questionnaire.
0 appears on reboot.
Consider the first build of MyHomePage.where model.getCounter(); returns 0.Why is that?
_getPrefItems is called in getCounter of MyHomePageModel, but _getPrefItems is an asynchronous function. _counter=prefs.getInt('counter') ??0; is called return> before .At this point _counter is the initial value of 0.The MyHomePageModel _counter is then set to prefs.getInt('counter'), but MyHomePage is not redrawn because there is no notifyListeners.
Press the plus button to display the previous number plus 1.
Press the plus button to call the MyHomePageModel addCounter._counter++ runs, and then notifyListeners is called. When notifyListeners is called, MyHomePage is redrawn, and getCounter returns "previous number plus 1".
An easy solution would be to add notifyListeners to _getPrefItems.
void_getPrefItems()async{
SharedPreferences prefs = wait SharedPreferences.getInstance();
_counter=prefs.getInt('counter')??0;
// addition
notifyListeners();
}
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