Firebase FCM Data Message — Complete Guide

class MyFcmService : FirebaseMessagingService() {
    override fun onMessageReceived(message: RemoteMessage) {
        // Expecting this to fire for ALL messages
        showNotification(message.notification?.title ?: "", message.notification?.body ?: "")
    }
}

class MyFcmService : FirebaseMessagingService() {
    override fun onMessageReceived(message: RemoteMessage) {
        // Check for data payload (always received)
        val data = message.data
        val title = data["title"] ?: message.notification?.title ?: "Message"
        val body = data["body"] ?: message.notification?.body ?: ""
        val type = data["type"] ?: "general"

        // Show notification with data
        showNotification(title, body, type)

        // Handle special data actions
        when (type) {
            "new_message" -> handleNewMessage(data)
            "call" -> handleIncomingCall(data)
            "silent_update" -> performSilentUpdate(data)
        }
    }

    // This also fires when app is in foreground for notification messages
    // For background notifications, use a helper:

    private fun createNotificationChannel() { /* ... */ }

    private fun showNotification(title: String, body: String, type: String) {
        val intent = Intent(this, MainActivity::class.java).apply {
            putExtra("type", type)
            flags = FLAG_ACTIVITY_NEW_TASK or FLAG_ACTIVITY_CLEAR_TOP
        }
        val pendingIntent = PendingIntent.getActivity(this, type.hashCode(), intent, FLAG_IMMUTABLE)

        val notification = NotificationCompat.Builder(this, FCM_CHANNEL_ID)
            .setContentTitle(title)
            .setContentText(body)
            .setSmallIcon(R.drawable.ic_notification)
            .setContentIntent(pendingIntent)
            .setAutoCancel(true)
            .build()

        NotificationManagerCompat.from(this).notify(System.currentTimeMillis().toInt(), notification)
    }
}

// For high-priority data messages (FCM priority: high)
// Send data-only messages from server (no notification payload)
// { "data": { "title": "...", "body": "..." }, "priority": "high" }