如何避免将一个变量结果更改为另一个变量结果

Question

I have a variable cart which is a list of cart items of type CartItem . Then a another variable selectedItem also of type CartItem . When i loop through cart and get the item of interest by comparing if its ID is equal to the ID of the selected item, I change its quantity. That works fine and expected. Challenge is it results into the selectedItem's quantity also changing. Also changing the selectedItem's quantity alone changes the quantity of the cart item. If I change both then a double increment occurs. Why is that happening and how can i prevent that. The aim is to change both. Obviously by changing only one I can get what I want but I need to understand what is going on;

var cart = List<CartItem>();

var selectedItem = CartItem();

First case

 for (CartItem item in cart) {
        if (item.product.id == selectedItem.product.id) {
          //this affects both selected item and item in cart's quantity
          item.quantity++;
          notifyListeners();
          printCart();
        }
      }

Second case

 for (CartItem item in cart) {
            if (item.product.id == selectedItem.product.id) {
              //this affects both selected item and item in cart's quantity
              selectedItem.quantity++;
              notifyListeners();
              printCart();
            }
          }

Third case

 for (CartItem item in cart) {
            if (item.product.id == selectedItem.product.id) {
              //this affects both selected item and item in cart's quantity
              //results in a double increment
              selectedItem.quantity++;
              item.quantity++;
              notifyListeners();
              printCart();
            }
          }

Answer 1

You are creating a new instance of CartItem() for your selectedCartItem instead of referring to one of your List<CartItem> . You need to do something like:

var cart = List<CartItem>();

var selectedItem = cart[index]; // where index is the position of the selected item

If you don't know which one is, you can also do something like:

var cart = List<CartItem>();

var selectedItem = cart.singleWhere((item) => item.product.id == selectedId);