chore: create a new example for secure persistent auth store

packages/nhost_flutter_auth/example/lib/secure_persistent_auth_example.dart

@@ -0,0 +1,104 @@
+/// This example demonstrates the ability to store authentication tokens between
+/// restarts of your application, so that the user is logged in automatically.
+///
+/// This example depends on the `flutter_secure_storage` package, which is used to
+/// implement the [SecureAuthStore] class.
+///
+/// To try it out, run the application, log in, then restart the app. You should
+/// see the contents of [ExampleProtectedScreen] without having to log in a
+/// second time.
+library secure_persistent_auth_example;
+
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+import 'package:nhost_flutter_auth/nhost_flutter_auth.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+
+import 'config.dart';
+import 'simple_auth_example.dart';
+
+void main() {
+  runApp(const SecurePersistentAuthExample());
+}
+
+class SecurePersistentAuthExample extends StatefulWidget {
+  const SecurePersistentAuthExample({
+    super.key,
+  });
+
+  @override
+  SecurePersistentAuthExampleState createState() => SecurePersistentAuthExampleState();
+}
+
+class SecurePersistentAuthExampleState extends State<SecurePersistentAuthExample> {
+  late NhostClient nhostClient;
+
+  @override
+  void initState() {
+    super.initState();
+    // Create a new Nhost client using your project's subdomain and region.
+    nhostClient = NhostClient(
+      subdomain: Subdomain(
+        subdomain: subdomain,
+        region: region,
+      ),
+      // Instruct the client to store tokens using shared preferences.
+      authStore: const SecureAuthStore(),
+    );
+    // this will fetch refresh token and will sign user in!
+    nhostClient.auth
+        .signInWithStoredCredentials()
+        .then((value) => null)
+        .catchError(
+      (e) {
+        // ignore: avoid_print
+        print(e);
+      },
+    );
+  }
+
+  @override
+  void dispose() {
+    super.dispose();
+    nhostClient.close();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return NhostAuthProvider(
+      auth: nhostClient.auth,
+      child: const MaterialApp(
+        title: 'Nhost.io Persistent Flutter Authentication Example',
+        home: Scaffold(
+          body: ExampleProtectedScreen(),
+        ),
+      ),
+    );
+  }
+}
+
+/// An Nhost [AuthStore] implementation backed by the `flutter_secure_storage`
+/// plugin, so authentication information is retained between runs of the
+/// application.
+class SecureAuthStore implements AuthStore {
+  const SecureAuthStore();
+
+  @override
+  Future<String?> getString(String key) {
+    const FlutterSecureStorage storage = FlutterSecureStorage();
+    return storage.read(key: key);
+  }
+
+  @override
+  Future<void> setString(String key, String value) {
+    const FlutterSecureStorage storage = FlutterSecureStorage();
+    return storage.write(key: key, value: value);
+  }
+
+  @override
+  Future<void> removeItem(String key) {
+    const FlutterSecureStorage storage = FlutterSecureStorage();
+    return storage.delete(key: key);
+  }
+}