chore(ml-opensource#80): Introduce AuthTokenStorage

lib/data/api/new/auth_token_storage/auth_token_storage.dart

@@ -0,0 +1,19 @@
+import 'package:flutter_template/data/model/auth/auth_tokens.dart';
+
+/// A storage for authentication tokens.
+abstract class AuthTokenStorage {
+  // Future<String?> get accessToken;
+  // Future<String?> get refreshToken;
+  // Future<double?> get tokenExpiryTime;
+
+  // Future<bool> hasValidAccessToken();
+
+  // Future setAccessToken(String token);
+  // Future setRefreshToken(String token);
+  // Future setRefreshTokenExpiry(double expiresIn);
+
+  Future<AuthTokens?> get();
+  Future<void> set(AuthTokens tokens);
+
+  Future<void> clear();
+}

lib/data/api/new/auth_token_storage/secure_auth_token_storage.dart

@@ -0,0 +1,44 @@
+import 'dart:convert';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+import 'package:flutter_template/data/api/new/auth_token_storage/auth_token_storage.dart';
+import 'package:flutter_template/data/model/auth/auth_tokens.dart';
+import 'package:injectable/injectable.dart';
+
+/// A storage for auth tokens that uses [FlutterSecureStorage].
+@LazySingleton(as: AuthTokenStorage)
+class SecureAuthTokenStorage extends AuthTokenStorage {
+  static const String _prefix = 'auth';
+
+  @visibleForTesting
+  static const String tokenKey = '${_prefix}_token';
+
+  SecureAuthTokenStorage(this._storage);
+
+  final FlutterSecureStorage _storage;
+
+  @override
+  Future<AuthTokens?> get() async {
+    final json = await _storage.read(key: tokenKey);
+
+    if (json == null) {
+      return null;
+    }
+
+    final map = jsonDecode(json) as Map<String, dynamic>;
+
+    return AuthTokens.fromJson(map);
+  }
+
+  @override
+  Future<void> set(AuthTokens tokens) {
+    final json = jsonEncode(tokens.toJson());
+    return _storage.write(key: tokenKey, value: json);
+  }
+
+  @override
+  Future<void> clear() {
+    return _storage.delete(key: tokenKey);
+  }
+}