// AuthenticationActivity.kt
import android.content.ContentValues.TAG
import android.content.Intent
import android.os.Bundle
import android.util.Log
import android.widget.Button
import androidx.appcompat.app.AppCompatActivity
import com.google.android.gms.auth.api.identity.BeginSignInRequest
import com.google.android.gms.auth.api.identity.Identity
import com.google.android.gms.auth.api.identity.SignInClient
import com.google.firebase.auth.FirebaseAuth
import com.google.firebase.auth.GoogleAuthProvider

class AuthenticationActivity : AppCompatActivity() {

    private lateinit var signInRequest: BeginSignInRequest
    private lateinit var oneTapClient: SignInClient
    private lateinit var auth: FirebaseAuth

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_login)

        auth = FirebaseAuth.getInstance()

        // Check if the user is already authenticated
        val currentUser = auth.currentUser
        if (currentUser != null) {
            // User is already signed in, navigate to MainActivity
            navigateToMainActivity()
        }

        // Set click listener for Gmail login button
        findViewById<Button>(R.id.btnSignIn).setOnClickListener {
            // Initialize one-tap sign-in client
            oneTapClient = Identity.getSignInClient(this)

            // Configure sign-in request
            signInRequest = BeginSignInRequest.builder()
                .setGoogleIdTokenRequestOptions(
                    BeginSignInRequest.GoogleIdTokenRequestOptions.builder()
                        .setSupported(true)
                        .setServerClientId(getString(R.string.your_web_client_id))
                        .setFilterByAuthorizedAccounts(true)
                        .build())
                .setAutoSelectEnabled(true)
                .build()

            // Start the sign-in flow
            oneTapClient.beginSignIn(signInRequest)
                .addOnSuccessListener(this) {
                    val googleCredential = oneTapClient.getSignInCredentialFromIntent(intent)
                    val idToken = googleCredential.googleIdToken

                    // Sign in with Firebase using Google ID token
                    if (idToken != null) {
                        val credential = GoogleAuthProvider.getCredential(idToken, null)
                        auth.signInWithCredential(credential)
                            .addOnCompleteListener(this) { task ->
                                if (task.isSuccessful) {
                                    // Sign in success, navigate to MainActivity
                                    val user = auth.currentUser
                                    navigateToMainActivity()
                                } else {
                                    // Sign in failed
                                    // Handle sign-in failure
                                    Log.w(TAG, "signInWithCredential:failure", task.exception)
                                }
                            }
                    }
                }
        }
    }

    private fun navigateToMainActivity() {
        val intent = Intent(this, MainActivity::class.java)
        startActivity(intent)
        finish()
    }
}

Kotlin online compiler

Write, Run & Share Kotlin code online using OneCompiler’s Kotlin online compiler for free. It’s a modern and fast online playground for Kotlin, supporting the latest version and ideal for learning, experimenting, and sharing code instantly.

About Kotlin

Kotlin is a statically typed, modern programming language developed by JetBrains. It runs on the JVM and is fully interoperable with Java. Kotlin is concise, expressive, and safe, and it’s officially supported by Google for Android app development.

Sample Code

The following is a simple Kotlin program that prints a greeting:

fun main() {
    println("Hello, OneCompiler!")
}

Taking inputs (stdin)

OneCompiler’s Kotlin editor supports stdin. You can provide input using the I/O tab. Here's a sample program that reads a line of input and prints a greeting:

fun main() {
    print("Enter your name: ")
    val name = readLine()
    println("Hello, $name")
}

Syntax Basics

Variables

val name: String = "OneCompiler"  // Immutable
var age: Int = 25                 // Mutable

Kotlin supports type inference, so explicit types are optional:

val city = "Hyderabad"
var count = 10

Conditionals

val score = 85
if (score >= 50) {
    println("Pass")
} else {
    println("Fail")
}

Loops

For loop

for (i in 1..5) {
    println(i)
}

While loop

var i = 1
while (i <= 5) {
    println(i)
    i++
}

Do-While loop

var j = 1
do {
    println(j)
    j++
} while (j <= 5)

Functions

fun add(a: Int, b: Int): Int {
    return a + b
}

fun greet(name: String) = "Hello, $name"

Collections

val items = listOf("apple", "banana", "cherry")
for (item in items) {
    println(item)
}

This guide provides a quick reference to Kotlin programming syntax and features. Start coding in Kotlin using OneCompiler’s Kotlin online compiler today!