✨ Add Feature oauth2 pkce flow

lib/src/common.dart

@@ -9,6 +9,7 @@ export 'package:github_flutter/src/common/util/device_flow.dart';
 export 'package:github_flutter/src/common/util/errors.dart';
 export 'package:github_flutter/src/common/util/json.dart';
 export 'package:github_flutter/src/common/util/oauth2.dart';
+export 'package:github_flutter/src/common/util/oauth2_pkce.dart';
 export 'package:github_flutter/src/common/util/pagination.dart';
 export 'package:github_flutter/src/common/util/service.dart';
 export 'package:github_flutter/src/common/util/utils.dart';

lib/src/common/util/oauth2_pkce.dart

@@ -0,0 +1,89 @@
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:github_flutter/src/common.dart';
+import 'package:http/http.dart' as http;
+
+class OAuth2PKCE {
+  /// OAuth2 Client ID
+  final String clientId;
+
+  /// Requested Scopes
+  final List<String> scopes;
+
+  /// Redirect URI
+  final String? redirectUri;
+
+  /// State
+  final String? state;
+
+  /// OAuth2 Base URL
+  final String baseUrl;
+
+  /// Confidential Intermediary Server URL
+  final String confidentialIntermediaryServer;
+
+  GitHub? github;
+
+  /// Constructor to initialize [OAuth2PKCE] with required parameters.
+  OAuth2PKCE(
+    this.clientId,
+    this.confidentialIntermediaryServer, {
+    String? redirectUri,
+    this.scopes = const [],
+    this.state,
+    this.github,
+    this.baseUrl = 'https://github.com/login/oauth',
+  }) : redirectUri =
+           redirectUri == null ? null : _checkRedirectUri(redirectUri);
+
+  static String _checkRedirectUri(String uri) {
+    return uri.contains('?') ? uri.substring(0, uri.indexOf('?')) : uri;
+  }
+
+  /// Generates an Authorization URL
+  ///
+  /// This should be displayed to the user.
+  String createAuthorizeUrl(String codeChallenge) {
+    return '$baseUrl/authorize${buildQueryString({'client_id': clientId, 'scope': scopes.join(','), 'redirect_uri': redirectUri, 'state': state, 'code_challenge': codeChallenge, 'code_challenge_method': 'S256'})}';
+  }
+
+  Future<ExchangeResponse> exchange(
+    String code,
+    String codeVerifier, [
+    String? origin,
+  ]) async {
+    final headers = <String, String>{
+      'Accept': 'application/json',
+      'content-type': 'application/json',
+    };
+    if (origin != null) {
+      headers['Origin'] = origin;
+    }
+
+    final body = GitHubJson.encode(<String, dynamic>{
+      'client_id': clientId,
+      'code_verifier': codeVerifier,
+      'code': code,
+      'redirect_uri': redirectUri,
+    });
+
+    return (github == null ? http.Client() : github!.client)
+        .post(
+          Uri.parse(confidentialIntermediaryServer),
+          body: body,
+          headers: headers,
+        )
+        .then((response) {
+          final json = jsonDecode(response.body) as Map<String, dynamic>;
+          if (json['error'] != null) {
+            throw Exception(json['error']);
+          }
+          return ExchangeResponse(
+            json['access_token'],
+            json['token_type'],
+            (json['scope'] as String).split(','),
+          );
+        });
+  }
+}