Integrate shared preferences for persistent key-value storage.

finlog_app/fluttery/lib/src/preferences/preferences_impl.dart

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+import 'package:flutter/material.dart';
+import 'package:fluttery/preferences/preferences.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+class PreferencesImpl implements Preferences {
+  final SharedPreferences _prefs;
+
+  /// Constructor
+  PreferencesImpl({required SharedPreferences instance}) : _prefs = instance;
+
+  @override
+  Future<void> setString(String key, String value) async {
+    await _prefs.setString(key, value);
+  }
+
+  @override
+  Future<String?> getString(String key) async {
+    return _prefs.getString(key);
+  }
+
+  @override
+  Future<void> setInt(String key, int value) async {
+    await _prefs.setInt(key, value);
+  }
+
+  @override
+  Future<int?> getInt(String key) async {
+    return _prefs.getInt(key);
+  }
+
+  @override
+  Future<void> setBool(String key, bool value) async {
+    await _prefs.setBool(key, value);
+  }
+
+  @override
+  Future<bool?> getBool(String key) async {
+    return _prefs.getBool(key);
+  }
+
+  @override
+  Future<void> setDouble(String key, double value) async {
+    await _prefs.setDouble(key, value);
+  }
+
+  @override
+  Future<double?> getDouble(String key) async {
+    return _prefs.getDouble(key);
+  }
+
+  @override
+  Future<void> setStringList(String key, List<String> value) async {
+    await _prefs.setStringList(key, value);
+  }
+
+  @override
+  Future<List<String>?> getStringList(String key) async {
+    return _prefs.getStringList(key);
+  }
+
+  @override
+  Future<void> remove(String key) async {
+    await _prefs.remove(key);
+  }
+
+  @override
+  Future<void> clear() async {
+    await _prefs.clear();
+  }
+}