Google Signing using Firebase Authentication in Kotlin

Firebase is a mobile and web application development platform. It provides services that a web application or mobile application might require. Firebase provides email and password authentication without any overhead of building the backend for user authentication. Google Sign-In is a secure way to authenticate users in your apps. It reduces the hassle of dealing with and handling those extra passwords by the user to get authenticated to the app. Firebase offers a great number of options to implement Login in your app like Email, Phone number, Google, Facebook, etc.

Step by Step Implementation

Step 1: Create a New Project

To create a new project in Android Studio please refer to How to Create/Start a New Project in Android Studio. Note that select Java as the programming language.

Step 2: Connect your app to Firebase

After creating a new project in Android Studio. Connect your app to Firebase. For connecting your app to firebase. Navigate to Tools on the top bar. After that click on Firebase. A new window will open on the right side. Inside that window click on Authentication and then email and password authentication. 

After clicking on email and password authentication you will get to see the below screen. Inside this screen click on the first option connect to firebase and after that click on the second option to add Firebase authentication to your app.  

Add the following dependencies if it is not already present there.

implementation ‘com.google.firebase:firebase-auth:16.0.5’

implementation ‘com.google.android.gms:play-services-auth:18.1.0’

Step 3: Enable Google Sign In

Step 4: Working with the activity_main.xml file

Navigate to the app > res > layout > activity_main.xml and add the below code to that file. Below is the code for the activity_main.xml file. 

XML

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/Signin"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="20dp"
        android:padding="20dp"
        android:text="Google Sign In"
        android:textColor="#000"
        android:textSize="22sp" />

</LinearLayout>

Step 5: Working with the MainActivity.kt file

Go to the MainActivity.kt file and refer to the following code. Below is the code for the MainActivity.kt file. Comments are added inside the code to understand the code in more detail.

Kotlin

import android.content.Intent
import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import com.google.android.gms.auth.api.signin.GoogleSignIn
import com.google.android.gms.auth.api.signin.GoogleSignInAccount
import com.google.android.gms.auth.api.signin.GoogleSignInClient
import com.google.android.gms.auth.api.signin.GoogleSignInOptions
import com.google.android.gms.common.api.ApiException
import com.google.android.gms.tasks.Task
import com.google.firebase.FirebaseApp
import com.google.firebase.auth.FirebaseAuth
import com.google.firebase.auth.GoogleAuthProvider
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    lateinit var mGoogleSignInClient: GoogleSignInClient
    val Req_Code: Int = 123
    private lateinit var firebaseAuth: FirebaseAuth

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

        FirebaseApp.initializeApp(this)

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

        mGoogleSignInClient = GoogleSignIn.getClient(this, gso)
        firebaseAuth = FirebaseAuth.getInstance()

        Signin.setOnClickListener { view: View? ->
            Toast.makeText(this, "Logging In", Toast.LENGTH_SHORT).show()
            signInGoogle()
        }
    }

    private fun signInGoogle() {
        val signInIntent: Intent = mGoogleSignInClient.signInIntent
        startActivityForResult(signInIntent, Req_Code)
    }

    // onActivityResult() function : this is where
    // we provide the task and data for the Google Account
    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        if (requestCode == Req_Code) {
            val task: Task<GoogleSignInAccount> = GoogleSignIn.getSignedInAccountFromIntent(data)
            handleResult(task)
        }
    }

    private fun handleResult(completedTask: Task<GoogleSignInAccount>) {
        try {
            val account: GoogleSignInAccount? = completedTask.getResult(ApiException::class.java)
            if (account != null) {
                UpdateUI(account)
            }
        } catch (e: ApiException) {
            Toast.makeText(this, e.toString(), Toast.LENGTH_SHORT).show()
        }
    }

    // this is where we update the UI after Google signin takes place
    private fun UpdateUI(account: GoogleSignInAccount) {
        val credential = GoogleAuthProvider.getCredential(account.idToken, null)
        firebaseAuth.signInWithCredential(credential).addOnCompleteListener { task ->
            if (task.isSuccessful) {
                SavedPreference.setEmail(this, account.email.toString())
                SavedPreference.setUsername(this, account.displayName.toString())
                val intent = Intent(this, DashboardActivity::class.java)
                startActivity(intent)
                finish()
            }
        }
    }

    override fun onStart() {
        super.onStart()
        if (GoogleSignIn.getLastSignedInAccount(this) != null) {
            startActivity(
                Intent(
                    this, DashboardActivity
                    ::class.java
                )
            )
            finish()
        }
    }
}

Step 6: Create a new empty activity

Please refer to Create New Activity in Android Studio and name the activity as DashboardActivity.

Step 7: Working with the activity_dashboard.xml file

Go to the activity_dashboard.xml file and refer to the following code. Below is the code for the activity_dashboard.xml file. Create a Button which when clicked, logs out the user from the app. 

XML

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".DashboardActivity">

    <Button
        android:id="@+id/logout"
        android:layout_width="300dp"
        android:layout_height="100dp"
        android:padding="10dp"
        android:text="Logout"
        android:textColor="#000"
        android:textSize="40sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

Step 8: Working with the DashboardActivity.kt file

Go to the DashboardActivity.kt file and refer to the following code. Below is the code for the DashboardActivity.kt file. Comments are added inside the code to understand the code in more detail.

Kotlin

import android.content.Intent
import android.os.Bundle
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import com.google.android.gms.auth.api.signin.GoogleSignIn
import com.google.android.gms.auth.api.signin.GoogleSignInClient
import com.google.android.gms.auth.api.signin.GoogleSignInOptions
import com.google.firebase.auth.FirebaseAuth
import kotlinx.android.synthetic.main.activity_dashboard.*

class DashboardActivity : AppCompatActivity() {

    // declare the GoogleSignInClient
    lateinit var mGoogleSignInClient: GoogleSignInClient

    private val auth by lazy {
        FirebaseAuth.getInstance()
    }

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

        // call requestIdToken as follows
        val gso = GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestIdToken(getString(R.string.default_web_client_id))
            .requestEmail()
            .build()
        mGoogleSignInClient = GoogleSignIn.getClient(this, gso)

        logout.setOnClickListener {
            mGoogleSignInClient.signOut().addOnCompleteListener {
                val intent = Intent(this, MainActivity::class.java)
                Toast.makeText(this, "Logging Out", Toast.LENGTH_SHORT).show()
                startActivity(intent)
                finish()
            }
        }
    }
}

Output: