shared_preferences Flutter Add items - flutter

Question

I expect that when I click the button I will add a single item that has brand and date in it in the Save Data function however I have no idea how to do that.Currently after each click they add tems however they are multiplied by themselves and that completely misses the point . I tried to add a single item however I get information about unsupported Error . In conclusion Please help
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return const MaterialApp(
title: 'SharedPreferences Demo',
home: Test(),
);
}
}
class Test extends StatefulWidget {
const Test({Key? key}) : super(key: key);
#override
State<Test> createState() => _TestState();
}
class _TestState extends State<Test> {
List<Map<String, dynamic>> listItems = [];
List<String> cart = [];
final Map<String, dynamic> item = Map<String, String>();
saveData() async {
var date = DateTime.now();
print('save data ');
final Map<String, dynamic> item = Map<String, dynamic>();
item['brand'] = 'item';
item['date'] = 'date';
SharedPreferences prefs = await SharedPreferences.getInstance();
if (cart == null) cart = [];
setState(() {
cart = prefs.getStringList("cart") ?? [];
cart.add(jsonEncode(item));
listItems.add(jsonDecode(item));
//<-- when i click button i want add one item card. on but idk how
prefs.setStringList("cart", cart);
});
}
getUser() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
var cart = prefs.getStringList("cart");
setState(() {
if (cart == null) {
print('null');
} else {
cart.forEach((item) {
listItems.add(jsonDecode(item));
});
}
});
}
#override
void initState() {
super.initState();
getUser();
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: Column(
children: <Widget>[
Expanded(
child: listItems.isNotEmpty
? ListView.builder(
itemCount: listItems.length, itemBuilder: buildList)
: Center(
child: Text('EMPTY LIST'),
)),
Center(
child: FloatingActionButton(
onPressed: saveData,
tooltip: 'Increment',
child: const Icon(Icons.add),
),
)
],
),
);
}
Widget buildList(BuildContext context, int index) {
return Container(
margin: EdgeInsets.all(4),
decoration: BoxDecoration(
border: Border.all(
color: Colors.black,
width: 2,
),
borderRadius: BorderRadius.circular(5)),
child: ListTile(
title: Text(listItems[index]['brand']),
subtitle: Text(listItems[index]['date'] ?? "null"),
),
);
}
}
}