private void addListener(String sid, int intervalMs) {
   Field field;
   field = MainActivity.fields.getBySID(sid);
   if (field != null) {
     field.addListener(this);
     MainActivity.device.addActivityField(field, intervalMs);
     subscribedFields.add(field);
   } else {
     MainActivity.toast("sid " + sid + " does not exist in class Fields");
   }
 }