Existing state editing in Riverpod / Flutter - flutter

Question

How I can manipulate the existing state in Riverpod. 'm a beginner about Flutter and Riverpod. When I try add one to Order error pops up and says:
Error: A value of type 'int' can't be assigned to a variable of type
'List'.
final OrderPaperProvider = StateNotifierProvider<OrderPaper, List<Order>>((ref) {
return OrderPaper();
});
#immutable
class Order {
final String product_id;
final String product_name;
final int product_price;
final int product_count;
Order({required this.product_id, required this.product_name, required this.product_price, required this.product_count});
Order copyWith({product_id, product_name, product_price, product_count}) {
return Order(
product_id: product_id,
product_name: product_name,
product_price: product_price,
product_count: product_count,
);
}
}
class OrderPaper extends StateNotifier<List<Order>> {
OrderPaper() : super([]);
void addOrder(Order order) {
for (var x = 0; x < state.length; x++) {
if (state[x].product_id == order.product_id) {
addOneToExistingOrder(x, order);
return;
}
}
state = [...state, order];
}
void removeOrder(String product_id) {
state = [
for (final order in state)
if (order.product_id != order) order,
];
}
void addOneToExistingOrder(index, Order order) {
state = state[index].product_count + 1; // <--------- Error in this line
}
void clearOrderPaper() {
state = [];
}
}

Answer 1

What is happening in the code you posted is basically this:
You are telling it to update the state which is a List<Order> with an integer.
This is because state[index].product_count + 1 actually equals a number, for example 1+2 = 3.
So you are telling it, state = 3, and this causes your error.
What you need to do, is create a new state with the list of items and the edited item, like this (you don't need to pass the index, you can get it in the function):
void addOrder(Order order) {
//find if the product is in the list. The index will not be -1.
final index = state.indexWhere((entry) => entry.product_count == order. product_count);
if (index != -1) {
//if the item is in the list, we just edit the count
state = [
...state.sublist(0, index),
order.copyWith(product_count: state[index].product_count + 1),
...state.sublist(index + 1),
];
}else {
//otherwise, just add it as a new entry.
state = [...state, order],
}
}