updateRange method
Implementation
void updateRange({DateTime? from, DateTime? to}) {
assert(from != null || to != null, 'from and to cannot be both null');
assert(_state != null, 'controller is not attached to any calendar');
// update the date time range of the calendar,
// if not provided, keep the old value
final range = DateTimeRange(
start: from ?? _state!.range.start,
end: to ?? _state!.range.end,
);
_state!.updateRange(range);
}