Flutter – local state vs App-wide state

Local State (sometimes called UI state or Ephemeral State): It is a state which can be manageable in a single widget. There is no need for state management techniques(Bloc, redux, etc ) on this kind of state a Stateful Widget would suffice.

Examples:

Current page in a page view, current tab in the navigation bar.

Dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  List<Widget> _spaces = <Widget>[
    Text('Home Page',
        style: TextStyle(fontSize: 25, fontWeight: FontWeight.bold)),
    Text('Search Page',
        style: TextStyle(fontSize: 25, fontWeight: FontWeight.bold)),
    Text('Account Page',
        style: TextStyle(fontSize: 25, fontWeight: FontWeight.bold)),
  ];

  // local variable
  int _index = 0;
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Geeks For Geeks',
      theme: ThemeData(
        primarySwatch: Colors.green,
      ),
      darkTheme: ThemeData.dark(),
      home: Scaffold(
        appBar: AppBar(
          leading: Icon(Icons.backspace),
          title: Text("Geeks for Geeks"),
          centerTitle: true,
        ),
        bottomNavigationBar: BottomNavigationBar(
          currentIndex: _index,
          onTap: (newIndex) {
            setState(() {

               // changing in widget
              _index = newIndex;
              print('$_index');
            });
          },
          items: [
            BottomNavigationBarItem(
              label: "Home",
              icon: Icon(Icons.home),
            ),
            BottomNavigationBarItem(
              label: "Search",
              icon: Icon(Icons.search),
            ),
            BottomNavigationBarItem(
              label: "Account",
              icon: Icon(Icons.person),
            ),
          ],
        ),
        body: Center(child: _spaces.elementAt(_index)),
      ),
    );
  }
}

Output:

App-wide State: State that is not local and is accessible across our application, is what is known as App-wide state or Shared state.

Examples: Notifications in a social networking app.

Shopping cart state in an e-commerce app.

Step 1: First add Provider dependency in pubspec.yaml file in the lib folder.

Dart

dev_dependencies:
    provider: ^4.3.2

We have added the dependency for Provider in our pubspec.yaml file located in the lib folder in dependencies as shown above:

Now click pub get or save the file so the package gets fetched.

Step 2: Using provider we notify changes and update the state of counter :

Dart

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

void main() {
  runApp(
    ChangeNotifierProvider(
      create: (context) => Counter(),
      child: MyApp(),
    ),
  );
}

class Counter with ChangeNotifier {
  int value = 0;

  void increment() {
    value += 1;
    notifyListeners();
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Geeks For Geeks',
      theme: ThemeData(
        primarySwatch: Colors.green,
      ),
      darkTheme: ThemeData.dark(),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        leading: Icon(Icons.backspace),
        title: Text("Geeks for Geeks"),
        centerTitle: true,
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('You have pushed the button this many times:'),
            Consumer<Counter>(
              builder: (context, counter, child) => Text(
                '${counter.value}',
                style: TextStyle(fontSize: 26),
              ),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          var counter = context.read<Counter>();
          counter.increment();
          // calling increment on tap
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

Output:

To better understand the difference between an App state and Ephemeral State look into the following diagram: