Firebase Auth Google Sign-In — Complete Guide

// Missing SHA-1 or web client ID configuration
val gso = GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
    .requestIdToken(getString(R.string.default_web_client_id)) // Wrong ID!
    .requestEmail()
    .build()

// Not handling ActivityResult properly
private fun signIn() {
    val intent = googleSignInClient.signInIntent
    startActivityForResult(intent, RC_SIGN_IN) // Deprecated! Breaks on config change
}

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
    // May not be called on configuration change
}

class MainActivity : AppCompatActivity() {
    private lateinit var auth: FirebaseAuth
    private lateinit var googleSignInClient: GoogleSignInClient

    // Activity Result API
    private val googleSignInLauncher = registerForActivityResult(
        ActivityResultContracts.StartActivityForResult()
    ) { result ->
        val task = GoogleSignIn.getSignedInAccountFromIntent(result.data)
        try {
            val account = task.getResult(ApiException::class.java)
            firebaseAuthWithGoogle(account.idToken!!)
        } catch (e: ApiException) {
            Log.w("GoogleSignIn", "Sign in failed", e)
            showError("Google sign in failed (${e.statusCode})")
        }
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        auth = FirebaseAuth.getInstance()

        val gso = GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestIdToken(getString(R.string.default_web_client_id)) // From google-services.json
            .requestEmail()
            .build()

        googleSignInClient = GoogleSignIn.getClient(this, gso)
    }

    private fun signIn() {
        googleSignInLauncher.launch(googleSignInClient.signInIntent)
    }

    private fun firebaseAuthWithGoogle(idToken: String) {
        val credential = GoogleAuthProvider.getCredential(idToken, null)
        auth.signInWithCredential(credential)
            .addOnCompleteListener(this) { task ->
                if (task.isSuccessful) {
                    val user = auth.currentUser
                    updateUI(user)
                } else {
                    showError("Authentication failed")
                }
            }
    }

    // Link Google account to existing email account
    private fun linkGoogleAccount(idToken: String) {
        val credential = GoogleAuthProvider.getCredential(idToken, null)
        auth.currentUser?.linkWithCredential(credential)
            ?.addOnCompleteListener { task ->
                if (task.isSuccessful) {
                    // Account linked!
                } else {
                    // Credential already in use by another account
                    val error = task.exception as? FirebaseAuthUserCollisionException
                    error?.let { handleMergeConflict(it) }
                }
            }
    }
}