public class MyFirebaseInstanceIdService extends FirebaseInstanceIdService {
        private static final String TAG = "MyFirebaseIIDService";

        /**
         * Called if InstanceID token is updated. This may occur if the security of
         * the previous token had been compromised. Note that this is called when the InstanceID token
         * is initially generated so this is where you would retrieve the token.
         */
        @Override
        public void onTokenRefresh() {
            // Get updated InstanceID token.
            String refreshedToken = FirebaseInstanceId.getInstance().getToken();
            sendRegistrationToServer(refreshedToken);
        }


        /**
         * Persist token to third-party servers.
         *
         * Modify this method to associate the user's FCM InstanceID token with any server-side account
         * maintained by your application.
         *
         * @param token The new token.
         */
        private void sendRegistrationToServer(String token) {
            DatabaseReference reference = FirebaseDatabase.getInstance().getReference();
            reference
                    .child(FirebaseAuth.getInstance().getCurrentUser().getUid())
                    .setValue(token);
        }
    }

public class MyFirebaseMessagingService extends FirebaseMessagingService {

    private static final String TAG = "MyFirebaseMsgService";
    private static final int BROADCAST_NOTIFICATION_ID = 1;

    @Override
    public void onDeletedMessages() {
        super.onDeletedMessages();
    }

    /**
     * Called when message is received.
     *
     * @param remoteMessage Object representing the message received from Firebase Cloud Messaging.
     */
    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {

        String notificationBody = "";
        String notificationTitle = "";
        String notificationData = "";
        try{
            notificationData = remoteMessage.getData().toString();
            notificationTitle = remoteMessage.getNotification().getTitle();
            notificationBody = remoteMessage.getNotification().getBody();
        }catch (NullPointerException e){

        }


        String dataType = remoteMessage.getData().get("direct_message");
        if(dataType.equals("direct_message")){
            String title = remoteMessage.getData().get("title");
            String message = remoteMessage.getData().get("message");
            sendMessageNotification(title, message);
        }
    }

    /**
     * Build a push notification for a chat message
     * @param title
     * @param message
     */
    private void sendMessageNotification(String title, String message){
        //get the notification id


        int notificationId = (int) System.currentTimeMillis();

        // Instantiate a Builder object.
        NotificationCompat.Builder builder = new NotificationCompat.Builder(this, "default");
        // Creates an Intent for the Activity
        Intent pendingIntent = new Intent(this, ChatActivity.class);
        // Sets the Activity to start in a new, empty task
        pendingIntent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        // Creates the PendingIntent
        PendingIntent notifyPendingIntent =
                PendingIntent.getActivity(
                        this,
                        0,
                        pendingIntent,
                        PendingIntent.FLAG_UPDATE_CURRENT
                );

        //add properties to the builder
        builder.setSmallIcon(R.drawable.ic_notif)
                .setLargeIcon(BitmapFactory.decodeResource(getApplicationContext().getResources(),
                        R.drawable.ic_notif))
                .setSound(RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION))
                .setContentTitle(title)
                .setAutoCancel(true)
                //.setSubText(message)
                .setStyle(new NotificationCompat.BigTextStyle().bigText(message))
                .setOnlyAlertOnce(true);

        builder.setContentIntent(notifyPendingIntent);
        NotificationManager mNotificationManager =
                (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);

        mNotificationManager.notify(notificationId, builder.build());

    }




}

exports.rentPaid = functions.database.ref('/Rent_Paid_Status/{UserID}/status').onWrite((change, context) => {

  const current_status = change.after.val();

  const User_ID = context.params.UserID;

//  if(current_status.toString().trim() === "yes")
  //{

      const token_ref = admin.database().ref('/Tokens/User_ID');
      token_ref.once("value", function(data){

        //construct single value event listener.
            const token = data.val();
            console.log("token: ", token);

            //we have everythin g we need
            //Build the message payload and send the message
            console.log("Construction the notification message.");
            const payload = {
              data: {
                data_type: "direct_message",
                title: "Rent Paid",
                message: "Thank you for paying your rent!",
              }
            };

            return admin.messaging().sendToDevice(token, payload)
                  .then(function(response) {
                    console.log("Successfully sent message:", response);
                    })
                    .catch(function(error) {
                    console.log("Error sending message:", error);
                    });
            //send notif saying 'thank you for paying your rent!'


      });
  }

return null;

})