Flutter Google Sign-in: Android Setup Guide

After extensive testing and debugging with various Google API configurations, I've found a straightforward approach to implement Google sign-in for Android apps. Here's the most efficient way to set it up:

The solution

Here's the most efficient way to initialize the "google_sign_in" package for Android:

1. Open the Auth Clients page of the Google Cloud Console

2. Create an "Android" client and fill out the form (package name + SHA-1)

3. Create a "Web application" client and enter a name

4. Use the Client ID of the web client to initialize GoogleSignIn:

   final googleSignIn = GoogleSignIn(
     clientId: WEB_CLIENT_ID,
     scopes: ['email'],
   );

Why this works

Here's the key insight about this approach:

Even if the Android Client ID isn't directly used in the app, registering it in the Google Cloud Console (with the correct package name and SHA-1 certificate) ensures that Google's authentication services recognize the app.

This might seem counterintuitive at first, but it's a reliable solution. During testing, I found that removing the Android client from the console immediately broke the authentication. Adding it back restored functionality.

The reason this works is because Google's authentication infrastructure allows different OAuth clients (Web, Android, iOS) under the same Google Cloud project to work together, as they're all tied to the same project identity.