The issue is in product price

Question

i dont know what the issue in my code but when i remain 2 quantity and 200 price and go back and go to cart and i press add icon it show 400 price and 2 quantity but i want 3quantity and 300 price I think issue is in updating product price and quantity .I don't know what the error.Please anyone who solve the issue ? Here is my code

Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            Text(
              snapshot.data!.docs[index]["Productname"],
              style: TextStyle(
                fontWeight: FontWeight.bold,
                fontSize: 16,
              ),
            ),
            SizedBox(height: 8),
            Row(
              children: [
                Text(

           "${int.parse(snapshot.data!.docs[index]["productprice"].toString())*snapshot.data!.docs[index]["quantity"]}",
                  style: TextStyle(
                    fontWeight: FontWeight.bold,
                    fontSize: 16,
                  ),
                ),
                SizedBox(width: 16),
                IconButton(
                  icon: Icon(Icons.remove),
                  onPressed: ()async{
                    setState(() {
                      if (_quantity>1) {
                            _quantity--;
                      }
                    });
                    await FirebaseFirestore.instance
                        .collection('Cart')
                        .doc(snapshot.data!.docs[index].id) // Use the document ID as the unique identifier
                        .update({'quantity': _quantity});
                  },
                ),
                SizedBox(width: 8),
                Text(
                  "${snapshot.data!.docs[index]["quantity"]}",
                  style: TextStyle(
                    fontWeight: FontWeight.bold,
                    fontSize: 16,
                  ),
                ),
                SizedBox(width: 8),
                IconButton(
                  icon: Icon(Icons.add),
                  onPressed: ()async{
                    setState(() {
                      _quantity++;
                    });
                    await FirebaseFirestore.instance
                        .collection('Cart')
                        .doc(snapshot.data!.docs[index].id) // Use the document ID as the unique identifier
                        .update({'quantity': _quantity});
                    var newPrice = int.parse(snapshot.data!.docs[index]["productprice"].toString()) * snapshot.data!.docs[index]["quantity"];
                    await FirebaseFirestore.instance
                        .collection('Cart')
                        .doc(snapshot.data!.docs[index].id)
                        .update({'productprice': newPrice});
                  },
                ),
              ],
            ),
          ],
        ),

Answer 1

You are updating the productPrice as well in the place of updating only quantity

You should remove following two lines in add button

var newPrice = int.parse(snapshot.data!.docs[index]["productprice"].toString()) * snapshot.data!.docs[index]["quantity"];
                    await FirebaseFirestore.instance
                        .collection('Cart')
                        .doc(snapshot.data!.docs[index].id)
                        .update({'productprice': newPrice});

Your add button code should be as following

IconButton(
                  icon: Icon(Icons.add),
                  onPressed: ()async{
                    setState(() {
                      _quantity++;
                    });
                    await FirebaseFirestore.instance
                        .collection('Cart')
                        .doc(snapshot.data!.docs[index].id) // Use the document ID as the unique identifier
                        .update({'quantity': _quantity});
                  },
                )