如何在更新状态通知程序 Riverpod 中的状态之前获取旧状态？

问题描述

我正在尝试使用 Riverpod 的状态通知程序，这是我的代码

import 'package:Flutter_riverpod/Flutter_riverpod.dart';

class Citiesstate {
  List<String> cities = [];
  String selectedCity = "New York";

  Citiesstate({required this.cities,required this.selectedCity});
}

class ChooseCityPageModel extends StateNotifier<AsyncValue<Citiesstate>> {
  ChooseCityPageModel() : super(AsyncValue.loading());

  Future<void> updateCitiesData() async {
    state = AsyncValue.loading();

    try {
      final latestCities = await myAPI.getAvailableCities();
      state = AsyncValue.data(
        ChooseCityPageState(
          selectedCity: '??????',// <------ I need to get the prevIoUs selectedCity state
          cities: latestCities,// <---- because I just want to update available cities
        ),);
    } catch (error) {
      state = AsyncValue.error(error);
    }
  }
}

如您所见，我有一种方法可以从服务器更新可用城市。但是状态是不可变的，对吗？我需要再次创建实例。

但我需要保留之前状态中 selectedCity 的值，因为我只想更新 cities 属性

怎么做？抱歉，我是 Flutter 新手

解决方法

读取之前的状态并使用该值来定义新状态。

state = AsyncValue.data(
  CitiesState(
    selectedCity: state.data!.value.selectedCity,cities: latestCities,),);

使用像 freezed 这样的包可以让这一切变得更加简单。

@freezed
class CitiesState with _$CitiesState {
  const factory CitiesState({
    required List<String> cities,required String selectedCity,}) = _CitiesState;
}

state = AsyncValue.data(state.data!.value.copyWith(cities: latestCities));