AlarmReceiver.kt

class AlarmReceiver : BroadcastReceiver() {
    private var notificationManager: NotificationManagerCompat? = null
    override fun onReceive(p0: Context?, p1: Intent?) {
        val taskInfo = p1?.getSerializableExtra("task_info") as? TaskInfo
            val tapResultIntent = Intent(p0, MainActivity::class.java)
            tapResultIntent.flags = Intent.FLAG_ACTIVITY_SINGLE_TOP
            val pendingIntent: PendingIntent = getActivity( p0,0,tapResultIntent,FLAG_UPDATE_CURRENT or FLAG_IMMUTABLE)
            
            // creating intent and pending intent to open OnCompletedBroadcastReceiver class
            val intentOnCompleted = Intent(p0, OnCompletedBroadcastReceiver::class.java).apply {
                putExtra("task_info", taskInfo)
            }
            val pendingIntentOnCompleted: PendingIntent? =
                taskInfo?.let { getBroadcast(p0, it.id,intentOnCompleted,FLAG_UPDATE_CURRENT or FLAG_IMMUTABLE) }
                
            // creating the action button
            val actionCompleted : NotificationCompat.Action = NotificationCompat.Action.Builder(0,"Completed",pendingIntentOnCompleted).build()

            val notification = p0?.let {
                NotificationCompat.Builder(it, "to_do_list")
                    .setContentTitle("Task Reminder")
                    .setContentText(taskInfo?.description)
                    .setSmallIcon(R.mipmap.ic_launcher)
                    .setAutoCancel(true)
                    .setPriority(NotificationCompat.PRIORITY_HIGH)
                    .setContentIntent(pendingIntent)
                    .addAction(actionCompleted) // adding the action button to notification
                    .build()
            }
            notificationManager = p0?.let { NotificationManagerCompat.from(it) }
            notification?.let { taskInfo?.let { it1 -> notificationManager?.notify(it1.id, it) } }
    }
}