Firebase list is not updating in set state - firebase

Question

The Flutter setState function not updating the list after retrieving from Firebase.
I am trying to develop a Flutter app. I am not getting updating the list in setState() function. The list is successfully retrieving from firebase. I have written the firebase connections in Services.dart file.
But my method _getList() is not getting the value in main.dart file.
main.dart
class DetailsPageState extends State<DetailsPage> {
List<Product> list;
#override
void initState() {
_checkUser(); // for getting user id from firebase auth
}
#override
Widget build(BuildContext context) {
return new Scaffold(
body: new Container(
child:new Text("data");
);
)
}
void _checkUser(){
debugPrint("Entering in _checkUser");
this.firebaseAuth.onAuthStateChanged.listen((firebaseUser)async{
_getList(firebaseUser.uid);
});
}
void _getList(String id)
debugPrint("Entering in _getList");
Services.retrieveItems(firestore, uid).then((onValue){
setState(() {
list=onValue;
debugPrint("items list:"+onValue.length.toString());
debugPrint("items list:"+listCart.length.toString());
});
});
}
}
Services.dart
static Future<List> retrieveItems(Firestore firestore, String userId) async {
List<Product> items = new List<Product>();
try {
firestore.collection("Items").document(userId)
.collection("ItemsMain").snapshots().listen((QuerySnapshot snapshot) {
List docList = snapshot.documents;
items = snapshot.documents.map((documentSnapshot) => Product.fromMap(documentSnapshot.data)).toList();
debugPrint("items:"+items.length.toString());
//return items;
});
} on Exception catch (e) {
print (e.toString());
}
debugPrint("items 2:"+items.length.toString());
return items;
}
Expected results:
Entering in _checkUser
Entering in _getList
items:6
items 2:6
items list:6
items list:6
Actual results:
Entering in _checkUser
Entering in _getList
items list:0
items list:0
items 2:0
items:6

Answer 1

You're returning the items before they are loaded. The simplest way to fix this is to use await in retrieveItems to wait for the data to be loaded from Firestore:
static Future<List> retrieveItems(Firestore firestore, String userId) async {
List<Product> items = new List<Product>();
var snapshot = await firestore.collection("Items").document(userId)
.collection("ItemsMain").getDocuments()
List docList = snapshot.documents;
items = snapshot.documents.map((documentSnapshot) => Product.fromMap(documentSnapshot.data)).toList();
debugPrint("items:"+items.length.toString());
return items;
}
You'll note that I:
Call get() instead of listen(). Since listen() starts actively monitoring the collection, it is impossible to say when it is "done". A get() on the other hand, returns the documents once, and is then done.
Removed the exception handling, just to make the code a bit more readable. But I also recommend only adding exception handlers in functional code like this if you're actually handling the exception. Leave "log and continue" handlers for higher-level code, such as your main method.