Integrating Google Sign-In and OAuth 2.0 in Android with Firebase Auth

1. Why Credential Manager is the New Standard

Credential Manager unifies all authentication methods under a single bottom-sheet bottom dialog: Passkeys, Google Sign-In, and saved passwords. Users tap once with their biometric fingerprint to authenticate instantly.

2. Android Implementation with Credential Manager

import androidx.credentials.*
import com.google.android.libraries.identity.googleid.GetGoogleIdOption
import com.google.android.libraries.identity.googleid.GoogleIdTokenCredential

suspend fun triggerGoogleSignIn(context: Context, serverClientId: String) {
    val credentialManager = CredentialManager.create(context)

    val googleIdOption = GetGoogleIdOption.Builder()
        .setFilterByAuthorizedAccounts(false)
        .setServerClientId(serverClientId)
        .setAutoSelectEnabled(true)
        .build()

    val request = GetCredentialRequest.Builder()
        .addCredentialOption(googleIdOption)
        .build()

    try {
        val result = credentialManager.getCredential(context = context, request = request)
        val credential = result.credential
        if (credential is CustomCredential && credential.type == GoogleIdTokenCredential.TYPE_GOOGLE_ID_TOKEN_CREDENTIAL) {
            val googleIdToken = GoogleIdTokenCredential.createFrom(credential.data).idToken
            // Send googleIdToken to your backend or Firebase Auth
        }
    } catch (e: GetCredentialException) {
        // Handle error or user cancellation
    }
}