The CustomerCart widget displays the customer’s photo,
name, total, and item count based on a Customer instance.

To update a customer’s cart when a menu item is dropped,
add the dropped item to the associated Customer object.

The _itemDroppedOnCustomerCart method is invoked in
onAccept() when the user drops a menu item on a
CustomerCart widget. By adding the dropped item to the 
customer object, and invoking setState() to cause a
layout update, the UI refreshes with the new customer’s
price total and item count.

Congratulations! You have a drag-and-drop interaction
that adds food items to a customer’s shopping cart.

Interactive example

Run the app:

Scroll through the food items.

Press and hold on one with your
finger or click and hold with the
mouse.

While holding, the food item’s image
will appear above the list.