Cloud Messaging (push notifications) in Android Kotlin

Introduction

Push notifications let apps send messages to users even when the app is closed. This helps keep users informed and engaged.

To alert users about new messages or updates in a chat app.

To remind users about upcoming events or appointments.

To send promotional offers or news from a shopping app.

To notify users about important system alerts or security issues.

Syntax

Android Kotlin

class MyFirebaseMessagingService : FirebaseMessagingService() {
  override fun onMessageReceived(remoteMessage: RemoteMessage) {
    // Handle the received message here
  }

  override fun onNewToken(token: String) {
    // Send the new token to your server
  }
}

Extend FirebaseMessagingService to handle incoming messages.

Override onMessageReceived to react when a push notification arrives.

Examples

This example gets the notification text and shows it to the user.

Android Kotlin

override fun onMessageReceived(remoteMessage: RemoteMessage) {
  val message = remoteMessage.notification?.body
  showNotification(message ?: "You have a new message")
}

When the device gets a new token, send it to your server to keep notifications working.

Android Kotlin

override fun onNewToken(token: String) {
  sendTokenToServer(token)
}

Sample App

This service listens for push notifications and shows a simple notification with title and message.

Android Kotlin

class MyFirebaseMessagingService : FirebaseMessagingService() {
  override fun onMessageReceived(remoteMessage: RemoteMessage) {
    val notification = remoteMessage.notification
    val title = notification?.title ?: "App Notification"
    val message = notification?.body ?: "You have a new message"

    val notificationManager = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
    val channelId = "default_channel"

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
      val channel = NotificationChannel(channelId, "Default Channel", NotificationManager.IMPORTANCE_DEFAULT)
      notificationManager.createNotificationChannel(channel)
    }

    val builder = NotificationCompat.Builder(this, channelId)
      .setContentTitle(title)
      .setContentText(message)
      .setSmallIcon(android.R.drawable.ic_dialog_info)
      .setAutoCancel(true)

    notificationManager.notify(0, builder.build())
  }

  override fun onNewToken(token: String) {
    // Normally send token to your server here
  }
}

Important Notes

Make sure to add the service in your AndroidManifest.xml with the proper intent filter.

Push notifications require internet permission and Firebase setup in your project.

Test notifications using Firebase Console or your backend server.

Summary

Push notifications keep users updated even when the app is closed.

Use FirebaseMessagingService to receive and handle messages.

Always send the device token to your server to target notifications correctly.

Practice

1. What is the main purpose of Firebase Cloud Messaging (FCM) in Android apps?

easy

A. To manage app permissions and settings

B. To store user data locally on the device

C. To send push notifications to users even when the app is closed

D. To create user interfaces with buttons and text

Solution

1. Step 1: Understand FCM's role

   Firebase Cloud Messaging is designed to send messages or notifications to users' devices.

2. Step 2: Identify the correct purpose

   It works even if the app is closed, keeping users updated with push notifications.

3. Final Answer:

   To send push notifications to users even when the app is closed -> Option C

4. Quick Check:

   FCM purpose = push notifications [OK]

Hint: Remember: FCM = push notifications, not UI or storage [OK]

Common Mistakes:

• Confusing FCM with local data storage
• Thinking FCM manages UI elements
• Assuming FCM controls app permissions

2. Which Kotlin class should you extend to handle incoming Firebase push messages in your Android app?

easy

A. FirebaseMessagingService

B. FirebaseDatabase

C. FirebaseAuth

D. FirebaseStorage

Solution

1. Step 1: Identify the service for message handling

   FirebaseMessagingService is the class designed to receive and handle push notifications.

2. Step 2: Exclude unrelated Firebase classes

   FirebaseDatabase, FirebaseAuth, and FirebaseStorage serve different purposes like data storage, authentication, and file storage.

3. Final Answer:

   FirebaseMessagingService -> Option A

4. Quick Check:

   Message handler class = FirebaseMessagingService [OK]

Hint: Use FirebaseMessagingService to receive messages [OK]

Common Mistakes:

• Choosing FirebaseDatabase for message handling
• Confusing FirebaseAuth with messaging
• Using FirebaseStorage instead of messaging service

3. Given the following Kotlin code snippet inside a FirebaseMessagingService subclass, what will be logged when a message is received?

override fun onMessageReceived(remoteMessage: RemoteMessage) {
    val title = remoteMessage.notification?.title ?: "No Title"
    val body = remoteMessage.notification?.body ?: "No Body"
    Log.d("FCM", "Title: $title, Body: $body")
}

medium

A. Title: No Title, Body: No Body if notification is null

B. App crashes if notification is null

C. Title and Body are always empty strings

D. Logs only the body, not the title

Solution

1. Step 1: Analyze null safety with Elvis operator

   The code uses ?: to provide default strings if notification or its fields are null.

2. Step 2: Understand logging behavior

   If notification is null, title and body become "No Title" and "No Body" respectively, so the log shows these defaults.

3. Final Answer:

   Title: No Title, Body: No Body if notification is null -> Option A

4. Quick Check:

   Elvis operator handles nulls = default text logged [OK]

Hint: Elvis operator ?: gives default when null [OK]

Common Mistakes:

• Assuming app crashes on null notification
• Thinking title/body become empty strings
• Believing only body is logged

4. You wrote this code to get the device token for FCM but it never logs the token. What is the error?

FirebaseMessaging.getInstance().token.addOnCompleteListener {
    if (!it.isSuccessful) return@addOnCompleteListener
    val token = it.result
    Log.d("FCM", "Token: $token")
}

medium

A. You need to call addOnSuccessListener instead of addOnCompleteListener

B. You forgot to add internet permission in AndroidManifest.xml

C. You must call getInstance() with context parameter

D. You should use getToken() instead of token property

Solution

1. Step 1: Check the API for token retrieval

   FirebaseMessaging.getInstance().getToken() is the correct method; there is no .token property.

2. Step 2: Verify other options

   Internet permission is needed to run, but the code won't compile due to undefined token property. getInstance() without context works, and addOnCompleteListener is correct.

3. Final Answer:

   You should use getToken() instead of token property -> Option D

4. Quick Check:

   No token property = use getToken() [OK]

Hint: Use getToken() method to fetch FCM token [OK]

Common Mistakes:

• Forgetting internet permission
• Thinking context is required for getInstance()
• Using wrong listener type for token retrieval

5. You want to send a push notification only to devices that have a specific user preference enabled. Which step is essential to implement this targeting correctly?

hard

A. Use FirebaseDatabase to store notifications and query them on the device

B. Send the device token to your server and filter tokens based on user preferences

C. Send notifications to all devices and let the app ignore unwanted ones

D. Use FirebaseAuth to authenticate users before sending notifications

Solution

1. Step 1: Understand device token role

   Device tokens uniquely identify devices to receive push notifications.

2. Step 2: Implement server-side filtering

   Send tokens to your server and filter them by user preferences before sending notifications.

3. Final Answer:

   Send the device token to your server and filter tokens based on user preferences -> Option B

4. Quick Check:

   Token + server filtering = targeted notifications [OK]

Hint: Always send tokens to server for targeted notifications [OK]

Common Mistakes:

• Sending notifications to all devices without filtering
• Using FirebaseDatabase instead of server filtering
• Confusing authentication with notification targeting