An Updated Notification Example

MainActivity.kt

import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.view.Menu
import android.view.MenuItem
import kotlinx.android.synthetic.main.activity_main.*
import android.app.AlarmManager
import android.app.Notification
import android.os.SystemClock
import android.app.PendingIntent
import android.content.Context
import android.content.Intent
import android.app.NotificationManager
import android.app.NotificationChannel
import android.os.Build
import android.support.v4.app.NotificationCompat

class MainActivity : AppCompatActivity() {

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

        // Create a NotificationChannel if needed for API 26+
        createNotificationChannel()

        fab.setOnClickListener { startActivity(Intent(this, MessageActivity::class.java)) }
    }

    override fun onCreateOptionsMenu(menu: Menu): Boolean {
        menuInflater.inflate(R.menu.menu_main, menu)
        return true
    }

    override fun onOptionsItemSelected(item: MenuItem): Boolean {
        return when (item.itemId) {
            R.id.action_5 -> {
                scheduleNotification(getNotification(getString(R.string.delay_5_seconds)),
                        DELAY_5_SECONDS)
                return true
            }
            R.id.action_10 -> {
                scheduleNotification(getNotification(getString(R.string.delay_10_seconds)),
                        DELAY_10_SECONDS)
                return true
            }
            R.id.action_30 -> {
                scheduleNotification(getNotification(getString(R.string.delay_30_seconds)),
                        DELAY_30_SECONDS)
                return true
            }
            else -> super.onOptionsItemSelected(item)
        }
    }

    /**
     * Schedule a notification using an alarm
     *
     * @param notification Notification - used by the alarm to send a notification
     * @param delaySeconds Int - delay in seconds before sending the notification
     * @return Unit
     */
    private fun scheduleNotification(notification: Notification, delaySeconds: Int) {
        // create an intent for the broadcast receiver
        val notificationIntent = Intent(this, NotificationReceiver::class.java)
        notificationIntent.putExtra(NotificationReceiver.NOTIFICATION_ID, 1)
        notificationIntent.putExtra(NotificationReceiver.NOTIFICATION, notification)

        // create a pending intent for the alarm
        val pendingIntent = PendingIntent.getBroadcast(this, 0,
                notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT)

        val futureInMillis = SystemClock.elapsedRealtime() + delaySeconds * 1000
        val alarmManager = getSystemService(Context.ALARM_SERVICE) as AlarmManager
        alarmManager.set(AlarmManager.ELAPSED_REALTIME_WAKEUP, futureInMillis, pendingIntent)
    }

    /**
     * Get a Notification for the alarm
     *
     * @param message String - sent from the notification to the MessageActivity
     * @return Notification - used by the alarm to send a notification
     */
    private fun getNotification(message: String): Notification {
        // create a pending intent for the notification action
        // message will be added to the pending intent and received by MessageActivity
        val actionIntent = MessageActivity.notificationIntent(this, message)

        return NotificationCompat.Builder(this,
                MessageActivity.NOTIFICATION_CHANNEL_ID)
                // set the pending intent for the notification action
                // (used to launch MessageActivity when the notification is clicked)
                .setContentIntent(actionIntent)
                // remove the notification from the app bar when clicked
                .setAutoCancel(true)
                .setContentTitle(getString(R.string.notification_title))
                .setContentText(message)
                .setSmallIcon(R.drawable.ic_announcement_white_24dp)
                .build()
    }

    /**
     * Create a notification channel for versions >= Oreo
     */
    private fun createNotificationChannel() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            val name = getString(R.string.channel_name)
            val description = getString(R.string.channel_description)
            val importance = NotificationManager.IMPORTANCE_DEFAULT
            val channel = NotificationChannel(MessageActivity.NOTIFICATION_CHANNEL_ID, name, importance)
            channel.description = description
            // Register the channel with the system; you can't change the importance
            // or other notification behaviors after this
            val notificationManager = getSystemService(NotificationManager::class.java)
            notificationManager!!.createNotificationChannel(channel)
        }
    }
}

MessageActivity.kt

import android.app.PendingIntent
import android.content.Context
import android.content.Intent
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_message.*
import kotlinx.android.synthetic.main.content_message.*

class MessageActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_message)
        setSupportActionBar(toolbarTarget)

        val message: String? = intent.getStringExtra(MESSAGE_KEY)
        ?: getString(R.string.message_not_received)

        textViewMessage.text = String.format(resources.getString(R.string.message), message)
    }

    companion object {
        const val NOTIFICATION_CHANNEL_ID = "ntxdroid.com.notifications.MessageActivity"
        private const val PENDING_INTENT_ID = 2001
        const val MESSAGE_KEY = "MESSAGE_KEY"

        fun notificationIntent(context: Context, message: String): PendingIntent {
            val startActivityIntent = Intent(context, MessageActivity::class.java)
            startActivityIntent.putExtra(MESSAGE_KEY, message)
            return PendingIntent.getActivity(
                    context,
                    PENDING_INTENT_ID,
                    startActivityIntent,
                    PendingIntent.FLAG_UPDATE_CURRENT)
        }
    }
}

NotificationReceiver.kt

import android.app.Notification
import android.content.BroadcastReceiver
import android.content.Context
import android.content.Intent
import android.app.NotificationManager

class NotificationReceiver : BroadcastReceiver() {

    override fun onReceive(context: Context, intent: Intent) {

        val notificationManager =
                context.getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager

        val notification = intent.getParcelableExtra<Notification>(NOTIFICATION)
        val id = intent.getIntExtra(NOTIFICATION_ID, 0)
        notificationManager.notify(id, notification)

    }

    companion object {
        var NOTIFICATION_ID = "notification-id"
        var NOTIFICATION = "notification"
    }
}