SharedPreferences web example

main.dart

import 'package:flutter/foundation.dart' show ValueListenable;
import 'package:shared_preferences/shared_preferences.dart'
    show SharedPreferences;
import 'package:flutter/material.dart';

void main() async {
  final counter = await SharedCounter.instance();
  runApp(
    MaterialApp(
      home: Scaffold(
        body: Center(
          child: ValueListenableBuilder<int>(
            valueListenable: counter,
            builder: (context, count, _) => Text('$count'),
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () => counter.value++,
          tooltip: 'Increment',
          child: const Icon(Icons.add),
        ),
      ),
    ),
  );
}

abstract class Counter extends ValueListenable<int> with ChangeNotifier {
  @override
  int get value;
  set value(int value);
}

final class SharedCounter extends Counter {
  SharedCounter._(
    this._prefs,
    String key,
    this.initialValue,
  ) : _key = key;

  final SharedPreferences _prefs;
  final String _key;
  final int initialValue;

  static Future<SharedCounter> instance({
    String key = 'COUNTER',
    initialValue = 0,
  }) async =>
      SharedCounter._(
        await SharedPreferences.getInstance(),
        key,
        initialValue,
      );

  @override
  int get value => _prefs.getInt(_key) ?? initialValue;

  @override
  set value(int value) {
    _prefs.setInt(_key, value);
    notifyListeners();
  }
}