Update Foreground Notification Text in Xamarin.Android

Question

I would like to update the ContentText of a Foreground Notification so that the ContentText displays the date of the latest product.

I am using two global variables to keep a reference to the original Builder and notificationManager. UpdateNotificationContent() is called from another class, and I've supplied the updated text to SetContentText and called notificationManager.notify to try to update the builder.

I have an error with the following line (last line of UpdateNotificationContent): notificationManager.Notify(NOTIFICATION_SERVICE_ID, notificationBuilder.Build());

Java.Lang.NullPointerException: 'Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference'

Full code here.

class MyService : Service
{
        private Handler handler;
        private Action runnable;
        private bool isStarted;
        private int DELAY_BETWEEN_LOG_MESSAGES = 5000;
        private int NOTIFICATION_SERVICE_ID = 1001;
        private int NOTIFICATION_AlARM_ID = 1002;
        private string NOTIFICATION_CHANNEL_ID = "1003";
        private string NOTIFICATION_CHANNEL_NAME = "Update Notifications";
        private NotificationCompat.Builder notificationBuilder;
        private NotificationManager notificationManager

        private void DispatchNotificationThatServiceIsRunning()
        {
            Intent intent = new Intent(this, typeof(CustomReceiver));
            PendingIntent pendingIntent = PendingIntent.GetBroadcast(
                    this,
                    1,
                    intent,
                    PendingIntentFlags.UpdateCurrent
            );

            string contextText = App.latestProduct.ProductSaveTime.ToString();
            }

            var notification = new Notification.Builder(this, default)
           .SetChannelId("location_notification")
           .SetContentTitle("AppName")
           .SetContentText(contextText)
           .SetSmallIcon(Resource.Drawable.icon)
           .SetVisibility(NotificationVisibility.Secret)
           .SetContentIntent(pendingIntent)
           .SetOngoing(true)
           .Build();

            // Enlist this instance of the service as a foreground service
            StartForeground(NOTIFICATION_SERVICE_ID, notification);

            var notificationManager = (NotificationManager)GetSystemService(NotificationService);
            notificationManager.Notify(NOTIFICATION_SERVICE_ID, notification);
        }

        // every 5 seconds push a notificaition
        private void DispatchNotificationThatAlarmIsGenerated()
        {
            Intent intent = new Intent(this, typeof(CustomReceiver));
            PendingIntent pendingIntent = PendingIntent.GetBroadcast(
                    this,
                    1,
                    intent,
                    PendingIntentFlags.UpdateCurrent
            );

           string contextText = App.latestProduct.ProductSaveTime.ToString();

            notificationBuilder = new NotificationCompat.Builder(this, "default")
                .SetAutoCancel(false)
                .SetSmallIcon(Resource.Drawable.icon)
                .SetContentIntent(pendingIntent)
                .SetContentTitle("AppName")
                .SetContentText(contextText);

            notificationManager = (NotificationManager)GetSystemService(NotificationService);
            notificationManager.Notify(NOTIFICATION_AlARM_ID, notificationBuilder.Build());
        }


        public void UpdateNotificationContent()
        {  
            string contextText = App.latestProduct.ProductSaveTime.ToString();

            notificationBuilder = new NotificationCompat.Builder(this, "default")
                .SetContentText(contextText);

            notificationManager.Notify(NOTIFICATION_SERVICE_ID, notificationBuilder.Build());
        }
}

Answer 1

notificationManager must also be declared only once

notification_Manager = (NotificationManager)GetSystemService(NotificationService);
if (Build.VERSION.SdkInt >  BuildVersionCodes.O)
{
    '
    '
    notification_Manager.CreateNotificationChannel(channel);
}

then use the same variable notification_Manager.notify() and notificationBuilder for every notification update you make.

Answer 2

Do not create a new NotificationCompat.Builder. If you just want to update the content, use the same NotificationCompat.Builder seetings with same id(NOTIFICATION_SERVICE_ID) to cover.

Change:

 string contextText = App.latestProduct.ProductSaveTime.ToString();

        notificationBuilder = new NotificationCompat.Builder(this, "default")
            .SetContentText(contextText);

        notificationManager.Notify(NOTIFICATION_SERVICE_ID, notificationBuilder.Build());

To:

  string contextText = "update";//I do not have you contextText, so i use string instead

            notificationBuilder .SetContentText(contextText);

            notificationManager.Notify(NOTIFICATION_SERVICE_ID, notificationBuilder .Build());