why setstate works on second button click flutter - flutter

Question

I am calculating pregnancy weeks .but the set state only update on second click.
ElevatedButton (
onPressed: () {
setState(() {
_selectDate(context);
sDate = selectedDate;
dueDate = sDate.add(Duration(days: DAYS_IN_PREGNANCY));
DateTime today = DateTime.now();
remaining = today.difference(dueDate).inDays.abs();
daysIn = DAYS_IN_PREGNANCY - remaining;
weekPart = daysIn % 7;
weekValue = daysIn / 7;
week = weekValue + "." + weekPart;
});
},
child: const Text("Choose Date"),
),
_selectDate(BuildContext context) async {
final DateTime? selected = await showDatePicker(
context: context,
initialDate: selectedDate,
firstDate: DateTime(2010),
lastDate: DateTime(2025),
);
setState(() {});
selectedDate = selected!;
}

Answer 1

As you can see, your _selectDate function is async and you need to await for it and also you don't need to call setState inside _selectDate, so change you click to this:
ElevatedButton (
onPressed: () async{ //<--- add this
var result = await _selectDate(context); //<--- add this
if(result != null){ //<--- add this
setState(() {
sDate = result; //<--- add this
dueDate = sDate.add(Duration(days: DAYS_IN_PREGNANCY));
DateTime today = DateTime.now();
remaining = today.difference(dueDate).inDays.abs();
daysIn = DAYS_IN_PREGNANCY - remaining;
weekPart = daysIn % 7;
weekValue = daysIn / 7;
week = weekValue + "." + weekPart;
});
}
},
child: const Text("Choose Date"),
),
and change your _selectDate to this:
DateTime? _selectDate(BuildContext context) async {
final DateTime? selected = await showDatePicker(
context: context,
initialDate: selectedDate,
firstDate: DateTime(2010),
lastDate: DateTime(2025),
);
return selected;
}