addListener method
Adds a listener to this notifier and triggers an asynchronous refreshes the data, if necessary.
Data will only be refreshed by this method if it hasn't been loaded yet (i.e. isInitial).
Implementation
@override
void addListener(VoidCallback listener) {
super.addListener(listener);
Future.microtask(() {
if (_active && isInitial) refresh();
});
}