How to Integrate Google Sign-In Authentication in Android Apps

Add Google Services JSON

• Download the google-services.json configuration file from the Firebase Console after enabling Google Sign-In. Place this file in the app/ directory of your Android project (your module folder).

Update build.gradle Files

• Open the level-specific build.gradle (project level) and ensure you have the Google Services classpath dependency in the build script.

buildscript {
    dependencies {
        // ...
        classpath 'com.google.gms:google-services:4.3.15' // Check for the latest version
    }
}

• In the build.gradle file at the app level, apply the Google services plugin and include the dependency for Google Play Services Auth.

plugins {
    id 'com.android.application'
    id 'com.google.gms.google-services'
}

dependencies {
    // ...
    implementation 'com.google.android.gms:play-services-auth:20.5.0' // Update to latest version
}

Configure GoogleSignInClient

• Inside your MainActivity (or the activity where you're implementing the sign-in), configure the GoogleSignInOptions and create a GoogleSignInClient instance.

GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
        .requestIdToken(getString(R.string.default_web_client_id))
        .requestEmail()
        .build();

GoogleSignInClient mGoogleSignInClient = GoogleSignIn.getClient(this, gso);

Initiate Sign-In Process

• To start the sign-in process, create an intent and start it with startActivityForResult().

private void signIn() {
    Intent signInIntent = mGoogleSignInClient.getSignInIntent();
    startActivityForResult(signInIntent, RC_SIGN_IN);
}

Handle Sign-In Result

• Override the onActivityResult method to handle the result from the sign-in intent.

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (requestCode == RC_SIGN_IN) {
        Task<GoogleSignInAccount> task = GoogleSignIn.getSignedInAccountFromIntent(data);
        handleSignInResult(task);
    }
}

private void handleSignInResult(Task<GoogleSignInAccount> completedTask) {
    try {
        GoogleSignInAccount account = completedTask.getResult(ApiException.class);

        // Signed in successfully, show authenticated UI.
        updateUI(account);
    } catch (ApiException e) {
        // The ApiException status code indicates the detailed failure reason.
        updateUI(null);
    }
}

Update UI

• Create a helper function to update the UI based on the user’s sign-in status.

private void updateUI(@Nullable GoogleSignInAccount account) {
    if (account != null) {
        // User is signed in
        // Display user details like name, email, profile picture, etc.
    } else {
        // User is not signed in
        // Update UI accordingly
    }
}

Integrate Sign-Out Functionality

• To allow users to sign out, use the signOut method.

private void signOut() {
    mGoogleSignInClient.signOut()
        .addOnCompleteListener(this, task -> updateUI(null));
}

Integrate Revoke Access Functionality

• To allow users to revoke access, use the revokeAccess method.

private void revokeAccess() {
    mGoogleSignInClient.revokeAccess()
        .addOnCompleteListener(this, task -> updateUI(null));
}

Check for Existing Sign-In

• When the app starts, check if the user is already signed in by accessing the last signed-in account.

@Override
protected void onStart() {
    super.onStart();
    GoogleSignInAccount account = GoogleSignIn.getLastSignedInAccount(this);
    updateUI(account);
}

Handle User Information

• Once signed in, you can access user information from the GoogleSignInAccount object.

// Retrieve information like email, display name, and profile picture.
String displayName = account.getDisplayName();
String email = account.getEmail();
Uri profilePicUri = account.getPhotoUrl();

Additional Points to Consider

• Make sure you use the correct client ID from your google-services.json for the token request.
• Handle different error cases by examining the ApiException status codes appropriately.