Getting sum of amounts from firestore

Question

    class totalDS extends StatefulWidget {
  const totalDS({
    Key? key,
  }) : super(key: key);

  @override
  _totalDSState createState() => _totalDSState();
}

double _total = 0.00;

class _totalDSState extends State<totalDS> {
  Future getTotal() async {
    final _firestore = FirebaseFirestore.instance.collection('DailySale').get();

    _firestore.then((querySnapshot) => {
          querySnapshot.docs.forEach((element) {
            for (var i = 1; i < element.data().length; i++) {
              _total = _total + element.data()['invAmountDS'];
            }
          })
        });
  }

 @override
  Widget build(BuildContext context) {
    return Container(child: Text('$_total'.toString()));
  }
}

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I m getting the sum as 0.0

anybody can help me regarding this? i want to display total sum of the invoices.

Answer 1

void main() {
  final myList = [1, 3, 5, 8, 7, 2, 11];
  final result = myList.fold(0, (sum, element) => sum + element);
  
  print(result);
}

Use Fold Method

Answer 2

In code you provided, you never even call getTotal() function. You can call it in initState() method like this:

@override
void initState() {
  super.initState();
  getTotal();
}

And you should also add setState() function after you finish your for loop

for (var i = 1; i < element.data().length; i++) {
  _total = _total + element.data()['invAmountDS'];
}
setState((){});

This will make the widget to rebuild when the function counted _total value

Answer 3

This is best method to add ,

var sum = [1, 2, 3].reduce((a, b) => a + b);

but if the list is empty than to handle this empty condition we prefer fold() over .reduce()

int sum = [1, 2, 3].fold(0, (a, b) => a + b);