如何获取 Flutter firestore 中的值的总数？

Question

I have a Firestore Collection 'Cart' contains a List of Products and every Product has a price so I want to get all of them and calculate the sum of prices: FireStore Collection

I tried a some of code and this one is working but how I can show the sum on the Screen

  Future getTotal() async {
int sum = 0;
FirebaseFirestore.instance.collection('Users/$userid/Cart').get().then(
  (querySnapshot) {
    querySnapshot.docs.forEach((result) {
      for (var i = 1; i < result.data()['price'].toString().length; i++) {
        sum = sum + result.data()['price'];
      }
    });
    print('total : $sum');
  },
);
}

Capture from Cart Screen

Update: I Tried this is Working but when I delete a product from the cart the value didn't change Except when I Close The screen and then come back again,

  double sum = 0;
   double total = 0;
  final String userid = FirebaseAuth.instance.currentUser.uid;

 @override
 void initState() {
   FirebaseFirestore.instance.collection('Users/$userid/Cart').get().then(
    (querySnapshot) {
      querySnapshot.docs.forEach((result) {
      sum = sum + result.data()['price'];
      });
    setState(() {
      total = sum;
      });
    },
   );
  super.initState();
 }

any help will be appreciated.

Answer 1

this Works for me:

 final cartitems = snapshot.data;
            final List<CartModel> products = cartitems;
            double getPrice() {
              double price = 0;
              cartitems.forEach((x) {
                price += x.price * x.numofitem;
              });
              return price;
            }