Masamune Auth Google for Firebase

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Masamune Auth Google for Firebase

Usage

masamune_auth_google_firebase bridges Google Sign In credentials to Firebase Authentication using the Masamune/Katana stack. Use it alongside:

• katana_auth – core authentication abstractions
• katana_auth_firebase – Firebase Authentication integration
• masamune_auth_google – native Google sign-in flow

Installation

Install the base packages first:

flutter pub add katana_auth
flutter pub add katana_auth_firebase

Then add Google sign-in support:

flutter pub add masamune_auth_google
flutter pub add masamune_auth_google_firebase

Register Adapters

Configure adapters near the top of your widget tree so Google tokens can be exchanged for Firebase credentials.

// lib/adapter.dart

/// Masamune adapters used in the application.
final masamuneAdapters = <MasamuneAdapter>[
  const UniversalMasamuneAdapter(),

  const GoogleAuthMasamuneAdapter(
    clientId: "YOUR_GOOGLE_CLIENT_ID",
  ),
  FirebaseGoogleAuthMasamuneAdapter(
    functionsAdapter: const FunctionsMasamuneAdapter(),
  ),
];

Note: This package requires katana_auth and katana_auth_firebase to be installed separately. Those packages provide the core authentication infrastructure.

FirebaseGoogleAuthMasamuneAdapter expects a backend (Cloud Functions or your own server) that validates Google ID tokens and returns Firebase credentials to the app.

Authenticate Users

Use Authentication from katana_auth to start the Google sign-in flow.

class SignInPage extends PageScopedWidget {
  @override
  Widget build(BuildContext context, PageRef ref) {
    final auth = ref.app.controller(Authentication.query());

    // Initialize on page load
    ref.page.on(
      initOrUpdate: () {
        auth.initialize();
      },
    );

    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            if (auth.isSignedIn)
              Column(
                children: [
                  Text("Signed in with Google"),
                  Text("User ID: ${auth.userId}"),
                  Text("Email: ${auth.userEmail}"),
                  TextButton(
                    onPressed: () => auth.signOut(),
                    child: const Text("Sign Out"),
                  ),
                ],
              )
            else
              ElevatedButton.icon(
                icon: Icon(Icons.g_mobiledata),
                label: const Text("Sign in with Google"),
                onPressed: () async {
                  try {
                    await auth.signIn(GoogleAuthQuery.signIn());
                  } catch (e) {
                    print("Sign in failed: $e");
                  }
                },
              ),
          ],
        ),
      ),
    );
  }
}

After completion, Firebase Authentication holds the signed-in user and auth.isSignedIn becomes true.

Backend Token Exchange

Implement FirebaseGoogleAuthFunctionsAction to exchange Google credentials:

1. Receive the ID token and optional server auth code from the client.
2. Verify the token with Google’s OAuth endpoints.
3. Exchange tokens if necessary (server auth code flow).
4. Mint a Firebase custom token or use signInWithCredential.
5. Return the Firebase ID token/custom token to the client.

Linking Providers

• Call auth.link(GoogleAuthQuery.link()) to add Google as another sign-in method for an existing user.
• Supports upgrading anonymous accounts to Google sign-in without losing data.

Tips

• Configure OAuth consent screen, package names, SHA certificates, and reversed client IDs in Google Cloud Console.
• Test on real devices; some emulators lack Google Play Services support.
• Provide fallback authentication methods for users without Google accounts.
• Log sign-in attempts using AuthLoggerAdapter for analytics and debugging.

GitHub Sponsors

Sponsors are always welcome. Thank you for your support!

https://github.com/sponsors/mathrunet