change theme and persisting theme

lib/main.dart

@@ -1,20 +1,32 @@
 import 'package:flutter/material.dart';
-
+import 'package:light_dark_theme_flutter/models/ThemeNotifier.dart';
+import 'package:provider/provider.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 import 'screens/Home.dart';
 
-void main() {
-  runApp(MainWidget());
+void main() async {
+  WidgetsFlutterBinding
+      .ensureInitialized(); //required to use platform channels to call native code.
+  SharedPreferences prefs = await SharedPreferences.getInstance();
+  bool themeBool = prefs.getBool("isDark") ?? false; //null check
+  runApp(
+    ChangeNotifierProvider(
+      create: (BuildContext context) => ThemeProvider(isDark: themeBool),
+      child: MainWidget(),
+    ),
+  );
 }
 
 class MainWidget extends StatelessWidget {
-  const MainWidget({Key? key}) : super(key: key);
-
   @override
   Widget build(BuildContext context) {
-    return MaterialApp(
-      title: 'Flutter Theme',
-      home: HomeScreen(),
-      routes: {},
-    );
+    return Consumer<ThemeProvider>(builder: (context, themeProvider, child) {
+      return MaterialApp(
+        title: 'Flutter Theme',
+        home: HomeScreen(),
+        theme: themeProvider.getTheme,
+        routes: {},
+      );
+    });
   }
 }

lib/models/ThemeNotifier.dart

@@ -0,0 +1,27 @@
+import 'package:flutter/material.dart';
+import '/themes/Themes.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+class ThemeProvider extends ChangeNotifier {
+  late ThemeData _selectedTheme;
+  late SharedPreferences prefs;
+  ThemeProvider({bool isDark: false}) {
+    this._selectedTheme = isDark ? darkTheme : lightTheme;
+  }
+
+  ThemeData get getTheme => _selectedTheme;
+
+  Future<void> changeTheme() async {
+    prefs = await SharedPreferences.getInstance();
+
+    if (_selectedTheme == darkTheme) {
+      _selectedTheme = lightTheme;
+      await prefs.setBool("isDark", false);
+    } else {
+      _selectedTheme = darkTheme;
+      await prefs.setBool("isDark", true);
+    }
+
+    notifyListeners();
+  }
+}

lib/screens/Home.dart

@@ -1,4 +1,6 @@
 import 'package:flutter/material.dart';
+import 'package:light_dark_theme_flutter/models/ThemeNotifier.dart';
+import 'package:provider/provider.dart';
 
 class HomeScreen extends StatefulWidget {
   const HomeScreen({Key? key}) : super(key: key);
@@ -12,7 +14,20 @@ class _HomeScreenState extends State<HomeScreen> {
   Widget build(BuildContext context) {
     return Scaffold(
       body: Center(
-        child: Text("Hello World!"),
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            Text("Hello World!"),
+            IconButton(
+              onPressed: () {
+                ThemeProvider themeProvider =
+                    Provider.of<ThemeProvider>(context, listen: false);
+                themeProvider.changeTheme();
+              },
+              icon: Icon(Icons.brightness_4_outlined),
+            ),
+          ],
+        ),
       ),
     );
   }

lib/themes/Themes.dart

@@ -0,0 +1,30 @@
+import 'package:flutter/material.dart';
+
+//LightTheme
+ThemeData lightTheme = ThemeData(
+  brightness: Brightness.light,
+  primaryColor: Colors.white,
+  textTheme: lightTextTheme,
+);
+
+TextStyle lightTextStyle = TextStyle(
+  fontSize: 20,
+  color: Colors.black,
+);
+TextTheme lightTextTheme = TextTheme(
+  bodyText1: lightTextStyle,
+);
+
+//DarkTheme
+ThemeData darkTheme = ThemeData(
+  brightness: Brightness.dark,
+  primaryColor: Colors.black,
+);
+
+TextStyle darkTextStyle = TextStyle(
+  fontSize: 20,
+  color: Colors.white,
+);
+TextTheme darkTextTheme = TextTheme(
+  bodyText1: lightTextStyle,
+);