Flutter: Unable to update variable state - flutter

Question

In my application, there is a section to add payment. To add a payment, admin will:
select a user
enter amount
select from date
select to date
And save it.
Goal: I want to update from date dynamically, based on previous date.
Example Case:
we added first payment of a user, from 1st June 2019 to 1st September 2019
when we try to make second payment, when the customer is selected; I want to get last payment to date as from date, populated automatically. (to date from db to from date in form)
It can't be done at initialization, because the date varies for each user, so we can do it upon user selection.
What I've done:
I've almost done everything, but having issue in updating field value.
I've done these steps:
created a function which executes, when the user is selected
function queries db, and pulls last payment detail
and returns to date field value of previous payment
the from date field value is set inside the same function like this:
setState(() {
_fromDate = DateTime.parse(toDate);
});
Problem:
Printing _fromDate inside setState function prints correct value, but it's null outside.
Please tell me, how can I update value properly and get rid off this issue?
Update: The Widget code is below: (the state is set as mentioned above, after getting data from db)
DateTimeField(
onSaved: (val) => setState(() => _fromDate = val),
keyboardType: TextInputType.datetime,
onChanged: (DateTime newValue) {
setState(() {
_fromDate = newValue;
});
},
onShowPicker: (context, currentValue) {
return showDatePicker(
context: context,
firstDate: DateTime.now(),
initialDate: currentValue ?? DateTime.now(),
);
},
);

Answer 1

Use TextEditingController class to handle run time text field changes.
Whenever the user modifies a text field with an associated TextEditingController, the text field updates value and the controller notifies its listeners. Listeners can then read the text and selection properties to learn what the user has typed or how the selection has been updated.
The process is pretty simple, just follow these steps:
Create controller:
TextEditingController _fromDateController = new TextEditingController();
Assign it to your DateTimeField like this:
DateTimeField(
...
controller: _fromDateController,
);
Update onSaved method of your field:
DateTimeField(
...
onSaved: (val) => _fromDateController.text = val.toString(),
);
Update controller when you get data from db:
setState(() {
_fromDateController.text = toDate;
});
Lastly, don't forget to dispose it:
Remember to dispose of the TextEditingController when it is no longer needed. This will ensure we discard any resources used by the object.
#override
void dispose() {
_fromDateController.dispose();
...
super.dispose();
}
Hope it helps!