Add auth provider

lib/providers/authentication_provider.dart

@@ -0,0 +1,74 @@
+import 'dart:convert';
+import 'package:flutter/foundation.dart';
+import 'package:frosty/constants.dart';
+import 'package:frosty/models/user.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+import 'package:http/http.dart' as http;
+
+class Authentication extends ChangeNotifier {
+  final _storage = new FlutterSecureStorage();
+  final _secret = const String.fromEnvironment('SECRET');
+  final _clientId = const String.fromEnvironment('CLIENT_ID');
+
+  var isLoggedIn = false;
+  var tokenIsValid = false;
+
+  String? token;
+  Map<String, String>? authHeaders;
+  UserTwitch? user;
+
+  Authentication() {
+    print('test');
+    initAuth();
+  }
+
+  /// Initialize by retrieving a token if it does not already exist.
+  void initAuth() async {
+    debugPrint('Creating auth provider');
+    token = await _storage.read(key: 'USER_TOKEN') ?? await getDefaultToken();
+    authHeaders = {'Authorization': 'Bearer $token', 'Client-Id': _clientId};
+
+    if (isLoggedIn) {
+      await getUserInfo();
+    }
+    notifyListeners();
+  }
+
+  /// Returns a token for an anonymous user.
+  Future<String> getDefaultToken() async {
+    final url = Uri(
+      scheme: 'https',
+      host: 'id.twitch.tv',
+      path: '/oauth2/token',
+      queryParameters: {
+        'client_id': _clientId,
+        'client_secret': _secret,
+        'grant_type': 'client_credentials',
+      },
+    );
+
+    final response = await http.post(url);
+    final token = jsonDecode(response.body)['access_token'];
+    _storage.write(key: 'USER_TOKEN', value: token);
+    return token;
+  }
+
+  Future<void> validateToken() async {
+    final response = await http.get(Uri.parse(twitchValidateUrl), headers: {'Authorization': 'Bearer $token'});
+    if (response.statusCode == 200) {
+      tokenIsValid = true;
+      notifyListeners();
+      return;
+    }
+
+    print('Token invalidated');
+    tokenIsValid = false;
+    notifyListeners();
+  }
+
+  Future<UserTwitch> getUserInfo() async {
+    final response = await http.get(Uri.parse(twitchUsersUrl), headers: authHeaders);
+    final userData = jsonDecode(response.body)['data'];
+    return UserTwitch.fromJson(userData);
+  }
+}