Flutter/Dart: Passing this.variable as parameter - function

Question

I have a list of profile fields that I would like to create using a single widget, but I'm new to Flutter and can't seem to work out one single thing: passing a variable through a parameter. I have been able to create many widgets that work just fine, using this.variable = value, but now that I'm trying to convert it to a single widget as not to repeat myself, that's where I'm having the problem.
I have the following code (of course removing all that I believe to be unnecessary). Here it currently shows an error of The setter 'listType' isn't defined for the class '_ProfileDataState'.
class _ProfileDataState extends State<ProfileData> {
final _countries = DropDownLists.countries;
String _country;
var listType; //<-- added this per comments
Widget profileDropDown(var list, var listType) {
return Card(
onTap: () async {
AlertDialog(
content: DropdownButtonFormField<String>(
isExpanded: true,
items: list.map((String value) {
return DropdownMenuItem<String>(
value: value,
child: Text(value),
);
}).toList(),
isDense: true,
value: listType,
onChanged: (value) {
FocusScope.of(context).requestFocus(FocusNode());
setState(() {
this.listType = value;
});
},
)
)
}
)
}
#override
Widget build(BuildContext context) {
return profileDropDown(_countries, _country),
...

Answer 1

class _ProfileDataState extends State<ProfileData> {
String _country;
var listType; // declare a variable
#override
void initState() {
super.initState();
listType = widget.listType; // assign it a value here
}
Widget profileDropDown(var listType) {
return Card(
onTap: () async {
AlertDialog(
onChanged: (value) {
FocusScope.of(context).requestFocus(FocusNode());
setState(() {
this.listType = value; // works
});
},
)
}
)
}
// other methods
}