Add shared_preferences (#15)

packages/shared_preferences/example/lib/main.dart

@@ -0,0 +1,88 @@
+// Copyright 2021 Sony Group Corporation. All rights reserved.
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ignore_for_file: public_member_api_docs
+
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+import 'package:shared_preferences_elinux/shared_preferences_elinux.dart';
+
+void main() {
+  runApp(MyApp());
+}
+
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'SharedPreferences Demo',
+      home: SharedPreferencesDemo(),
+    );
+  }
+}
+
+class SharedPreferencesDemo extends StatefulWidget {
+  SharedPreferencesDemo({Key? key}) : super(key: key);
+
+  @override
+  SharedPreferencesDemoState createState() => SharedPreferencesDemoState();
+}
+
+class SharedPreferencesDemoState extends State<SharedPreferencesDemo> {
+  final prefs = SharedPreferencesELinux.instance;
+  late Future<int> _counter;
+
+  Future<void> _incrementCounter() async {
+    final values = await prefs.getAll();
+    final int counter = (values['counter'] as int? ?? 0) + 1;
+
+    setState(() {
+      _counter = prefs.setValue('Int', 'counter', counter).then((bool success) {
+        return counter;
+      });
+    });
+  }
+
+  @override
+  void initState() {
+    super.initState();
+    _counter = prefs.getAll().then((Map<String, Object> values) {
+      return (values['counter'] as int? ?? 0);
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text("SharedPreferences Demo"),
+      ),
+      body: Center(
+          child: FutureBuilder<int>(
+              future: _counter,
+              builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
+                switch (snapshot.connectionState) {
+                  case ConnectionState.waiting:
+                    return const CircularProgressIndicator();
+                  default:
+                    if (snapshot.hasError) {
+                      return Text('Error: ${snapshot.error}');
+                    } else {
+                      return Text(
+                        'Button tapped ${snapshot.data} time${snapshot.data == 1 ? '' : 's'}.\n\n'
+                        'This should persist across restarts.',
+                      );
+                    }
+                }
+              })),
+      floatingActionButton: FloatingActionButton(
+        onPressed: _incrementCounter,
+        tooltip: 'Increment',
+        child: const Icon(Icons.add),
+      ),
+    );
+  }
+}