Simple example of using Firebase Cloud Messaging in Android | Kotlin

Firebase Cloud Messaging (FCM) is a cross-platform messaging solution that lets you reliably deliver messages at no cost.

You can send notification messages to drive user re-engagement and retention.

To write your Firebase Cloud Messaging Android client app, use the FirebaseMessaging API and Android Studio 1.4 or higher with Gradle.

FCM clients require devices running Android 4.0 or higher that also have the Google Play Store app installed, or an emulator running Android 4.0 with Google APIs. Note that you are not limited to deploying your Android apps through Google Play Store.

Set up Firebase and the FCM SDK

* If you haven't already, add Firebase to your Android project.
* In Android Studio, add the FCM dependency to your app-level build.gradle file:

implementation 'com.google.firebase:firebase-messaging:17.1.0'

Create a Service that extends FirebaseMessagingService.

Here is the full code of FCMService.kt

class FCMService : FirebaseMessagingService() {

    override fun onNewToken(s: String?) {
        super.onNewToken(s)

        // If you want to send messages to this application instance or
        // manage this apps subscriptions on the server side, send the
        // Instance ID token to your app server.
    }

    override fun onMessageReceived(remoteMessage: RemoteMessage?) {
        super.onMessageReceived(remoteMessage)

        // [START_EXCLUDE]
        // There are two types of messages data messages and notification messages. Data messages are handled
        // here in onMessageReceived whether the app is in the foreground or background. Data messages are the type
        // traditionally used with GCM. Notification messages are only received here in onMessageReceived when the app
        // is in the foreground. When the app is in the background an automatically generated notification is displayed.
        // When the user taps on the notification they are returned to the app. Messages containing both notification
        // and data payloads are treated as notification messages. The Firebase console always sends notification
        // messages. For more see: https://firebase.google.com/docs/cloud-messaging/concept-options
        // [END_EXCLUDE]

        // TODO(developer): Handle FCM messages here.
        // Not getting messages here? See why this may be: https://goo.gl/39bRNJ
        Log.d(TAG, "From: " + remoteMessage!!.from!!)

        // Check if message contains a data payload.
        if (remoteMessage.data.size > 0) {
            Log.d(TAG, "Message data payload: " + remoteMessage.data)
        }

        // Check if message contains a notification payload.
        if (remoteMessage.notification != null) {
            Log.d(TAG, "Message Notification Body: " + remoteMessage.notification!!.body!!)
        }

        // Also if you intend on generating your own notifications as a result of a received FCM
        // message, here is where that should be initiated. See sendNotification method below.
    }

    companion object {

        private val TAG = "FCMService"
    }
}

Define this service in Manifest as follows.

<service android:name=".FCMService">
    <intent-filter>
        <action android:name="com.google.firebase.MESSAGING_EVENT" />
    </intent-filter>
    <!-- Set custom default icon. This is used when no icon is set for incoming notification messages.
         See README(https://goo.gl/l4GJaQ) for more. -->
    <meta-data
        android:name="com.google.firebase.messaging.default_notification_icon"
        android:resource="@mipmap/ic_launcher_round" />
    <!-- Set color used with incoming notification messages. This is used when no color is set for the incoming
         notification message. See README(https://goo.gl/6BKBk7) for more. -->
    <meta-data
        android:name="com.google.firebase.messaging.default_notification_color"
        android:resource="@color/colorAccent" />
</service>

Get firebase tokan as follows in your MainActivity.

FirebaseInstanceId.getInstance().instanceId
        .addOnCompleteListener(OnCompleteListener { task ->
            if (!task.isSuccessful) {
                Log.w(TAG, "getInstanceId failed", task.exception)
                return@OnCompleteListener
            }

            // Get new Instance ID token
            val token = task.result.token

            // Log and toast
            val msg = getString(R.string.msg_token_fmt, token)
            Log.d(TAG, msg)
            Toast.makeText(this@MainActivity, msg, Toast.LENGTH_SHORT).show()
        })

By default, your app can be notified by package id, If you want to get notified by Topic something like, News, Cinema then subscribe for a topic like follows,

//Subscription to the topic News
FirebaseMessaging.getInstance().subscribeToTopic("News")
        .addOnCompleteListener { task ->
            var msg = getString(R.string.msg_subscribed)
            if (!task.isSuccessful) {
                msg = getString(R.string.msg_subscribe_failed)
            }
            Log.d(TAG, msg)
            Toast.makeText(this@MainActivity, msg, Toast.LENGTH_SHORT).show()
        }

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Source code on GitHub

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