State not updating with Map using StateNotifier of Riverpod-CodePudding

I have an itemCount map with an ItemDescription keys and int values.

Map<ItemDescription, int> itemCount = {
  ItemDescription(
    name: 'Pepsi',
    price: 10,
  ): 1,
  ItemDescription(
    name: 'Coca-Cola',
    price: 12,
  ): 1,
};

I want to update the int value of the map in various places throughout my code, so I have defined the functions increment and decrement in a StateNotifier:

class ItemCountQuantity extends StateNotifier<Map<ItemDescription, int>> {
  ItemCountQuantity() : super(itemCount);

  void decrement(int index) {
    state.update(
      state.keys.elementAt(index),
      (value) {
        return --value;
      },
    );
  }

  void increment(int index) {
    state.update(
      state.keys.elementAt(index),
      (value) {
        return   value;
      },
    );
  }
}

The StateNotifierProvider for the above is defined as such:

final itemCountQuantity =
    StateNotifierProvider<ItemCountQuantity, Map<ItemDescription, int>>(
  (ref) => ItemCountQuantity(),
);

When I am calling ref.read(itemCountQuantity.notifier).decrement(index); or ref.read(itemCountQuantity.notifier).increment(index); in the onPressed parameter of a button, I can see that the value is updating but the state is not. Is there some error in my StateNotifier?

EDIT:

I tried setting the StateNotifier as such:

class ItemCountQuantity extends StateNotifier<Map<ItemDescription, int>> {
  ItemCountQuantity() : super(itemCount);

  void decrement(int index) {
    state = {
      ...state,
      state.keys.elementAt(index):
          state[state.keys.elementAt(index)]! - 1
    };
  }

  void increment(int index) {
    state = {
      ...state,
      state.keys.elementAt(index):
          state[state.keys.elementAt(index)]!   1
    };
  }
}

But neither did this work.

CodePudding user response：

Your issue is that you’re missing:

state = Map.of(state.update(…));
// instead of just
state.update(…);

This is because you need to assign the state to a new value so the StateNotifier knows something was updated. (state is implemented as a getter and setter under the hood.)

However, I’d like to note that doing state.keys.elementAt(index) feels very wrong when using a map, as it relies on the implementation of the map. If you must use the index to access an element, use a list of tuples. Or, just reference the map using its keys (and not an index).

CodePudding user response：

When you access your provider have you tried using the watch() method?

ref.read(itemCountQuantity.notifier).decrement(index)

ref.watch(itemCountQuantity.notifier).decrement(index)

read() will only get the value of/in provider just once (one-time read)

watch() get the value of/in provider the first time and every time the value changes