flutter - Variable that changed the value using sharedPreferences does not work properly -Has a simple example code

I created a button that allows users to change the size of letters.
I hope that the font size value will remain changed even if I run the app again.
But in my code, it goes back to its original state.I made a simple code of the problem I faced. I made three files.

1.HomePage.dart

import 'dart:async';
import 'dart:convert';
// import 'package:google_mobile_ads/google_mobile_ads.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:provider/provider.dart';
import 'package:shared_preferences/shared_preferences.dart';
import './main.dart';
import './count_page.dart';
class HomePage extends StatefulWidget {
const HomePage({Key? key}) : super(key: key);
@override
State<HomePage> createState() => _HomePageState();
}
class _HomePageState extends State<HomePage> {
late CountPage _countPage;
@override
Widget build(BuildContext context) {
String prefsFont = "prefs_font";
_countPage = Provider.of<CountPage>(context, listen: true);
return  Scaffold(
appBar: AppBar(
backgroundColor: Colors.green,
title: Text('Test_Code'),
centerTitle: true, // 중앙 정렬
elevation: 0.0,
),
body: Container(
color: Colors.white,
child: Text('Font_size',style: TextStyle(fontSize: _countPage.font),),
),
bottomNavigationBar: BottomAppBar(
color: Colors.lime,
child: Container(
height: 200,
child: Row(
children: [
IconButton(
onPressed: () async {
SharedPreferences prefs = await SharedPreferences.getInstance();
await prefs.setDouble('prefs_font', 10);
setState(() {
_countPage.font = (prefs.getDouble('prefs_font') ?? 40) ;
});
},
icon: Icon(Icons.looks_one_outlined)),
IconButton(
onPressed: () async {
SharedPreferences prefs = await SharedPreferences.getInstance();
await prefs.setDouble('prefs_font', 40);
setState(() {
_countPage.font = (prefs.getDouble('prefs_font') ?? 40) ;
});
},
icon: Icon(Icons.looks_two_outlined)),
IconButton(
onPressed: () async {
SharedPreferences prefs = await SharedPreferences.getInstance();
await prefs.setDouble('prefs_font', 80);
setState(() {
_countPage.font = (prefs.getDouble('prefs_font') ?? 40) ;
});
},
icon: Icon(Icons.looks_3_outlined)),
],
),
),
),
);
}
}

2.main.dart

import 'dart:convert';
// import 'package:google_mobile_ads/google_mobile_ads.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:provider/provider.dart';
import 'package:shared_preferences/shared_preferences.dart';
import './HomePage.dart';
import 'count_page.dart';
void main()  {
// WidgetsFlutterBinding.ensureInitialized();
// MobileAds.instance.initialize();
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
@override
Widget build(BuildContext context) {
return ChangeNotifierProvider(
create: (BuildContext context) => CountPage(),
child: MaterialApp(
debugShowCheckedModeBanner: false,
theme: ThemeData(
primarySwatch: Colors.blue,
splashColor: Colors.transparent,
highlightColor: Colors.transparent,
),
home: HomePage(),
),
);
}
}

3.count_page.dart

import 'dart:ui';
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
class CountPage extends ChangeNotifier {
double _font = 40;
double get font => _font;
set font(double font) {
_font = font;
}
}

I made it small so that the composition is similar to the project I am working on.
The value changes even if you press the button, but it goes back to the beginning when you run the app again.
What is the problem?

So basically what's happening here is, you're only storing the font inside your SharedPrefrence. But you're not fetching it back when the app starts. Your CountPage class isn't storing the font size. It's shared preference that's storing it. So you have to just fetch the data from SharedPrefrence on the app start. And then use it in your code. A small example will be

1. Initialize SharedPrefrence so you can use later.

void main()  async {
  WidgetsFlutterBinding.ensureInitialized();
  final sharedPref = await SharedPrefrence.getInstance(); // Instance will be created.

  runApp(const MyApp());
}

2.Then make your class use SharedPrefrence instance by default

class CountPage extends ChangeNotifier {
final SharedPrefrence pref;

CountPage({required this.pref});

  double get font => pref.getDouble('prefs_font') ?? 40;
  setFont(double font) async {
    await prefs.setDouble('prefs_font', font);
    notifyListeners(); //This is necessary so that when the value changes the UI gets an update.
  }

}

3. Then pass the SharedPrefrence instance to that class, so it can use it.

class MyApp extends StatelessWidget {
final SharedPrefrence pref;
  const MyApp({Key? key, required this.pref}) : super(key: key); // Getting the instance from main.
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      create: (BuildContext context) => CountPage({pref: pref}),//Passing the instance.
      child: MaterialApp(
        debugShowCheckedModeBanner: false,
        theme: ThemeData(
          primarySwatch: Colors.blue,
          splashColor: Colors.transparent,
          highlightColor: Colors.transparent,
        ),
        home: HomePage(),
      ),
    );
  }
}

Also on your save button it looks like you're using a getter to set the value, which shouldn't work. But now you can just call it like this

onPressed: () async {
setState(() { // Now this setState is optional bcz you're saying the notifier to notify it's listeners, whenever you're setting the value.
await _countPage.setFont(\* font size for this button */);
});
                  },