[google_sign_in] Add GoogleSignInExceptionCode export (#9545)

`GoogleSignInException` and `GoogleSignInExceptionCode` were both intended to be exported to clients to allow for structured error handling, but the latter was missed. This adds the missing export, as well as example usage in the example app (which would have caught the mistake).

Fixes flutter/flutter#171462

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: stuartmorgan-g

packages/google_sign_in/google_sign_in/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 7.1.0
+
+* Adds the missing export of `GoogleSignInExceptionCode`, to allow for
+  structured error handling as intended by the API.
+
 ## 7.0.0
 
 * **BREAKING CHANGE**: Many APIs have changed or been replaced to reflect the

packages/google_sign_in/google_sign_in/example/lib/main.dart

@@ -109,7 +109,7 @@ class _SignInDemoState extends State<SignInDemo> {
       _currentUser = null;
       _isAuthorized = false;
       _errorMessage = e is GoogleSignInException
-          ? 'GoogleSignInException ${e.code}: ${e.description}'
+          ? _errorMessageFromSignInException(e)
           : 'Unknown error: $e';
     });
   }
@@ -208,7 +208,7 @@ class _SignInDemoState extends State<SignInDemo> {
       });
       unawaited(_handleGetContact(_currentUser!));
     } on GoogleSignInException catch (e) {
-      _errorMessage = 'GoogleSignInException ${e.code}: ${e.description}';
+      _errorMessage = _errorMessageFromSignInException(e);
     }
   }
 
@@ -228,7 +228,7 @@ class _SignInDemoState extends State<SignInDemo> {
         _serverAuthCode = serverAuth == null ? '' : serverAuth.serverAuthCode;
       });
     } on GoogleSignInException catch (e) {
-      _errorMessage = 'GoogleSignInException ${e.code}: ${e.description}';
+      _errorMessage = _errorMessageFromSignInException(e);
     }
   }
 
@@ -335,4 +335,14 @@ class _SignInDemoState extends State<SignInDemo> {
           child: _buildBody(),
         ));
   }
+
+  String _errorMessageFromSignInException(GoogleSignInException e) {
+    // In practice, an application should likely have specific handling for most
+    // or all of the, but for simplicity this just handles cancel, and reports
+    // the rest as generic errors.
+    return switch (e.code) {
+      GoogleSignInExceptionCode.canceled => 'Sign in canceled',
+      _ => 'GoogleSignInException ${e.code}: ${e.description}',
+    };
+  }
 }

packages/google_sign_in/google_sign_in/lib/google_sign_in.dart

@@ -12,7 +12,7 @@ import 'src/identity_types.dart';
 import 'src/token_types.dart';
 
 export 'package:google_sign_in_platform_interface/google_sign_in_platform_interface.dart'
-    show GoogleSignInException;
+    show GoogleSignInException, GoogleSignInExceptionCode;
 export 'src/event_types.dart';
 export 'src/identity_types.dart';
 export 'src/token_types.dart';

packages/google_sign_in/google_sign_in/pubspec.yaml

@@ -3,7 +3,7 @@ description: Flutter plugin for Google Sign-In, a secure authentication system
   for signing in with a Google account.
 repository: https://github.com/flutter/packages/tree/main/packages/google_sign_in/google_sign_in
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+google_sign_in%22
-version: 7.0.0
+version: 7.1.0
 
 environment:
   sdk: ^3.6.0