Implement FCM push notification for Android APP in Kotlin.

Add Firebase GoogleService-Info.plist to the project app folder

Add FCM to dependencies

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dependencies {
    ...
    implementation 'com.google.firebase:firebase-messaging-ktx:23.0.8'
    implementation platform('com.google.firebase:firebase-bom:30.4.1')
    implementation 'com.google.firebase:firebase-analytics-ktx'
}

Add permission to Manifest

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<uses-permission android:name="android.permission.POST_NOTIFICATIONS"/>

Initialize FCM

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class MainActivity : BaseActivity() {

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

        ...
        Firebase.messaging.isAutoInitEnabled = true
        FirebaseMessaging.getInstance().token.addOnCompleteListener(OnCompleteListener { task ->
            if (!task.isSuccessful) {
                return@OnCompleteListener
            }
            //Handle FCM token
        })
    }
}

Implement FCMService

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class FCMService : FirebaseMessagingService() {

    override fun onMessageReceived(remoteMessage: RemoteMessage) {

        // TODO(developer): Handle FCM messages here.
        remoteMessage.notification?.let { noti ->
            noti.title?.let { title ->
                sendNotification(title, noti.body ?: "")
            }

            Log.d("TAG", "Message Notification Body: " + noti.title?:"title empty")
        }
    }

    override fun onNewToken(token: String) {
        super.onNewToken(token)
        println(token)
        
        //Handle FCM token
    }

    private fun sendNotification(title: String, body: String) {
        val notifID = System.currentTimeMillis().hashCode()
        val intent = Intent(this, NotificationListActivity::class.java)
        intent.addFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT)
        intent.putExtra("notificationId", notifID)
        val contentIntent = PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_ONE_SHOT)

        val defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION)
        val notificationBuilder = NotificationCompat.Builder(this, "default")
            .setSmallIcon(R.drawable.ic_launcher)
            .setContentTitle(title)
            .setContentText(body)
//            .setDefaults(Notification.DEFAULT_ALL)
            .setAutoCancel(true)
            .setContentIntent(contentIntent)
            .setTicker("PushNotificationTest APP")
            .setSound(defaultSoundUri)

        val notificationManager = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            val channelID = "pushnotificationtest";//applicationInfo.loadLabel(packageManager).toString()
            val channel = this.createNotificationChannel(
                notificationManager,
                channelID,
                channelID,
                NotificationManager.IMPORTANCE_HIGH
            )

            channel.setShowBadge(true)
            notificationManager?.createNotificationChannel(channel)
            notificationBuilder.setChannelId(channelID)
        }

        notificationManager.notify(notifID /* ID of notification */, notificationBuilder.build())
    }

    @TargetApi(26)
    private fun createNotificationChannel(
        notificationManager: NotificationManager,
        id: String,
        name: String,
        importance: Int
    ): NotificationChannel {
        return if (notificationManager.getNotificationChannel(id) != null) {
            notificationManager.getNotificationChannel(id)
        } else {
            val notificationChannel = NotificationChannel(id, name, importance)
            notificationChannel.enableVibration(true)
            notificationChannel.enableLights(true)
            notificationChannel
        }
    }
}

Add FCMService to Manifest

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<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.maochun.pushnotificationtest">
    ...
    <uses-permission android:name="android.permission.POST_NOTIFICATIONS"/>
    <application
        ...>
        ...
        <service
            android:name=".FCMService"
            android:exported="false">
            <intent-filter>
                <action android:name="com.google.firebase.MESSAGING_EVENT" />
            </intent-filter>
        </service>
    </application>