How to pass selected item from dropdown to firebase? - flutter

Question

I need to pass the selected value from the dropdown to my Firestore, but the value passed to the Firebase is empty.
I used this dropdown_search package: dropdown_search version 3.0.1
So, how to pass selected data from the dropdown correctly?
This is my code:
class _AddSellerState extends State<AddSeller> {
final controller = TextEditingController();
#override
Widget build(BuildContext context) {
Size size = MediaQuery.of(context).size;
final seller_name = controller.text;
return StreamBuilder<QuerySnapshot>(
stream: FirebaseFirestore.instance.collection('sellers').snapshots(),
builder: (context, snapshot) {
if (snapshot.hasData) {
return Scaffold(
appBar: AppBar(
backgroundColor: const Color.fromARGB(255, 135, 176, 255),
title: const Text(
"Evaluate Seller",
style: TextStyle(fontSize: 24, color: Colors.black),
),
),
body: SingleChildScrollView(
child: Column(
children: <Widget>[
Container(
margin: const EdgeInsets.fromLTRB(0, 10, 0, 25),
child: DropdownSearch<String>(
mode: Mode.MENU,
items: snapshot.data!.docs
.map((DocumentSnapshot document) {
Map<String, dynamic> data =
document.data()! as Map<String, dynamic>;
return data["seller_name"];
})
.toList()
.cast<String>(),
showSelectedItems: true,
showSearchBox: true,
onChanged: ((value) => seller_name),
),
),
Container(
margin: const EdgeInsets.only(top: 20),
child: TextButton(
child: Container(
color: const Color.fromARGB(255, 43, 115, 255),
padding: const EdgeInsets.symmetric(
vertical: 13, horizontal: 20),
child: const Text(
'Add Seller',
style:
TextStyle(fontSize: 16.0, color: Colors.white),
),
),
onPressed: () {
addSeller(seller_name: seller_name);
},
),
),
],
),
),
);
}
if (snapshot.connectionState == ConnectionState.waiting) {
return Center(
child: CircularProgressIndicator(),
);
}
return Text('Error');
});
}
Future addSeller({required String seller_name}) async {
final docSeller = FirebaseFirestore.instance.collection('sellers').doc();
final json = {
'seller_name': seller_name,
};
await docSeller.set(json);
}
}
I am not sure how to initialize a variable that gets the selected value and passes it to my firebase after clicking the Add Seller button.
This is my output, which receives an empty value:
Photo