1. Observe("dateonly,dateFormat")
void changeDateFormat([_, __])

Source

@Observe("dateonly,dateFormat")
void changeDateFormat([_,__]) {
  format = new DateFormat(dateFormat);
  if (!dateonly) {
    format.add_Hm();
  }
  if (selectedDate!=null) {
    textDate = format.format(selectedDate);
  }
}