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Discover a seamless method to verify if a user already exists in Firebase using Google Sign-In, ensuring a smooth user experience in your Flutter app.
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This video is based on the question https://stackoverflow.com/q/76360715/ asked by the user 'Cidiko Andorem' ( https://stackoverflow.com/u/21897241/ ) and on the answer https://stackoverflow.com/a/76427760/ provided by the user 'Roman Maydana' ( https://stackoverflow.com/u/21990317/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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The Problem: Identifying Existing Users during Google Sign-In with Firebase

When developing applications that utilize Google authentication, one common challenge developers face is determining if a user already exists in the Firebase database. This is particularly crucial for providing a seamless experience. If a user mistakenly attempts to sign up again, they should be redirected automatically without experiencing confusion. In this post, we will delve into how to check if a user already exists in Firebase when they attempt to sign in with Google.

The Sign-In Process with Firebase

To start, let's recap the typical Google sign-in process using Firebase within a Flutter application. Here’s the basic structure of the sign-in method:

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In this example, the signInWithGoogle method handles Google sign-in. However, it does not provide feedback about whether the user is new or returning. To enhance user experience, we need a way to verify if the user exists before proceeding with any additional setups.

The Solution: Check If the User Exists

Fortunately, Firebase provides a straightforward method for checking if an email is already registered. Here’s how you can incorporate that into your sign-in process:

Step 1: Implement the Existence Check

Add the following method to your authentication logic. This method checks whether a user exists in Firebase Authentication by fetching the sign-in methods available for a given email:

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Step 2: Integrate with Your Sign-In Logic

You’ll want to call the above method after successfully acquiring the user's email during the sign-in process. Here’s how you can modify your existing signInWithGoogle method to include this check:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Handling User Redirection

Once you determine that the user already exists, you can redirect them to the home page or another suitable location in your app. You can implement this redirection easily within your Flutter navigation logic.

Conclusion

By implementing the checkExistUser method and integrating it into your Google sign-in flow, you can significantly enhance user experience in your Flutter application. This code not only verifies if a user exists during the sign-in process but also helps manage user expectations by automatically redirecting them if they unknowingly attempt to sign up again.

Now you have a better grasp of how to check for existing users in Firebase when using Google Authentication, taking another step toward building a smoother and more efficient user experience within your application.