@State<CartStateModel>({
  name: 'cart',
  defaults: {},
  children: [CartItemsState]
})
export class CartState {

  @Action(Feed)
  feed({ state, setState, dispatch }: StateContext<MyStateModel>, { payload }: Feed) {
    setState({ ...state, feeding: true });

    return this.feedService.feed(payload)
      .pipe(map((animals) => {
        setState({ ...state, animals });
        return dispatch(new Feed());
      }));
  }

  @Action(AddTodo)
  addTodo({ state, setState }: StateContext<MyStateModel>, { payload }: AddTodo) {
    setState([
      ...state,
      payload
    ]);
  }

}

@State<CartItemsStateModel>({
  name: 'items',
  defaults: []
})
export class CartItemsState {}