Flutter add value to another class list - flutter

Question

I want to add the date picker value to the List.
When user after select the date, I will calling function [addToDateList] in _AddFieldDynamicItem.
For this function, it will add to list:
List <DateTime> dateList=[];
Which is in _AddFieldDynamicTest
After click "Add another", I don't know why that it only can record the latest picker value.
Did I need to using another storage method such as sql lite or shared preferences?
import 'dart:convert';
import 'package:flutter/material.dart';
class AddFieldDynamicTest extends StatefulWidget {
#override
_AddFieldDynamicTest createState() => _AddFieldDynamicTest();
}
class _AddFieldDynamicTest extends State<AddFieldDynamicTest> {
Map<String, String> _formdata = {};
var _myPets = List<Widget>();
List <DateTime> dateList=[];
int _index = 1;
void _add() {
_myPets = List.from(_myPets)
..add(AddFieldDynamicItem(_index));
setState(() {
_index += 1;
});
}
#override
void initState() {
// TODO: implement initState
super.initState();
_add();
}
addToDateList(DateTime d){
dateList.add(d);
}
#override
Widget build(BuildContext context) {
return Scaffold(
floatingActionButton: FloatingActionButton(
onPressed: () => getDateList(),
child: Text('Save'),
),
appBar: AppBar(
title: Text('Add Test 2'),
actions: <Widget>[
FlatButton(
child: Text('Add another'),
onPressed: _add,
),
],
),
body: ListView(
children: _myPets,
),
);
}
getDateList(){
print(dateList.length);
}
}
class AddFieldDynamicItem extends StatefulWidget {
AddFieldDynamicItem(this._index);
final int _index;
#override
_AddFieldDynamicItem createState() => _AddFieldDynamicItem(_index);
}
class _AddFieldDynamicItem extends State<AddFieldDynamicItem> {
_AddFieldDynamicItem(this._index);
String _value = '';
final int _index;
List<DateTime> d=[];
Future _selectDate() async {
DateTime picked = await showDatePicker(
context: context,
initialDate: new DateTime.now(),
firstDate: new DateTime(2000),
lastDate: new DateTime(2100)
);
if(picked != null)
_AddFieldDynamicTest().addToDateList(picked);
setState(() {
_value = picked.toString();
});
}
#override
Widget build(BuildContext context) {
return Column(
children: <Widget>[
new Column(
children: <Widget>[
new Text('$_index . Current Date'),
new Text(_value),
new RaisedButton(onPressed: _selectDate, child: new Text('Date picker'),)
],
)
],
);
}
}

Answer 1

"_AddFieldDynamicTest().addToDateList(picked);" is make new instance.
So, fix like this.
from
_myPets = List.from(_myPets)
..add(AddFieldDynamicItem(_index));
to
_myPets = List.from(_myPets)
..add(AddFieldDynamicItem(_index, addToDateList));
from
AddFieldDynamicItem(this._index);
final int _index;
to
AddFieldDynamicItem(this._index, this.addToDateList);
final int _index;
final Function(DateTime) addToDateList;
from
if(picked != null)
_AddFieldDynamicTest().addToDateList(picked);
to
if (picked != null) widget.addToDateList(picked);