Firebase Authentication on Android (Kotlin)

This guide walks you through using Firebase Authentication in your Android app using Kotlin DSL (build.gradle.kts) and Kotlin code.

1, Enable Authentication via CLI

Before adding dependencies in your app, make sure you enable the Auth service in your Firebase Project using the Firebase CLI:

npx -y firebase-tools@latest init auth

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2. Add Dependencies

In your module-level build.gradle.kts (usually app/build.gradle.kts), add the dependency for Firebase Authentication:

dependencies {
    // [AGENT] Fetch the latest available BoM version from https://firebase.google.com/support/release-notes/android before adding this
    implementation(platform("com.google.firebase:firebase-bom:<latest_bom_version>"))

    // Add the dependency for the Firebase Authentication library
    // When using the BoM, you don't specify versions in Firebase library dependencies
    implementation("com.google.firebase:firebase-auth")
}

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3. Initialize FirebaseAuth

In your Activity or Fragment, initialize the FirebaseAuth instance:

import com.google.firebase.auth.FirebaseAuth
import com.google.firebase.auth.ktx.auth
import com.google.firebase.ktx.Firebase

class MainActivity : AppCompatActivity() {

    private lateinit var auth: FirebaseAuth

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val auth = Firebase.auth
        
        setContent {
            MaterialTheme {
                Text("Auth initialized!")
            }
        }
    }
}

Jetpack Compose (Modern)

Initialize inside a ComponentActivity using setContent:

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Text
import com.google.firebase.Firebase
import com.google.firebase.auth.auth

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val auth = Firebase.auth
        
        setContent {
            MaterialTheme {
                Text("Auth initialized!")
            }
        }
    }
}

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4. Check Current Auth State

You should check if a user is already signed in when your activity starts:

public override fun onStart() {
    super.onStart()
    // Check if user is signed in (non-null) and update UI accordingly.
    val currentUser = auth.currentUser
    if (currentUser != null) {
        // User is signed in, navigate to main screen or update UI
    } else {
        // No user is signed in, prompt for login
    }
}

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5. Sign Up New Users (Email/Password)

Use createUserWithEmailAndPassword to register new users:

fun signUpUser(email: String, password: String) {
    auth.createUserWithEmailAndPassword(email, password)
        .addOnCompleteListener(this) { task ->
            if (task.isSuccessful) {
                // Sign up success, update UI with the signed-in user's information
                val user = auth.currentUser
                // Navigate to main screen
            } else {
                // If sign up fails, display a message to the user.
                Toast.makeText(baseContext, "Authentication failed.", Toast.LENGTH_SHORT).show()
            }
        }
}

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6. Sign In Existing Users (Email/Password)

Use signInWithEmailAndPassword to log in existing users:

fun signInUser(email: String, password: String) {
    auth.signInWithEmailAndPassword(email, password)
        .addOnCompleteListener(this) { task ->
            if (task.isSuccessful) {
                // Sign in success, update UI with the signed-in user's information
                val user = auth.currentUser
                // Navigate to main screen
            } else {
                // If sign in fails, display a message to the user.
                Toast.makeText(baseContext, "Authentication failed.", Toast.LENGTH_SHORT).show()
            }
        }
}

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7. Sign Out

To sign out a user, call signOut() on the FirebaseAuth instance:

auth.signOut()
// Navigate to login screen