访问作为对象的 reducer 有效负载的元素

Question

这是一个基本的天气应用程序，我正在做它来学习 Redux。 API 没有提供搜索的城市名称，所以我必须通过 Redux 传递它。

我有以下容器：

import React, { Component } from "react";
import { connect } from "react-redux";

class WeatherList extends Component {
  renderWeather = cityData => {
    const conditions =
      cityData.forecast.simpleforecast.forecastday[0].conditions;
    const fHigh =
      cityData.forecast.simpleforecast.forecastday[0].high.fahrenheit;
    return (
      <tr>
        {/* <td>{cityData.city}</td> */}
        <td>{cityData.meta.city}</td>
        <td>{conditions}</td>
        <td>{fHigh}</td>
      </tr>
    );
  };
  render() {
    return (
      <table className="table table-hover">
        <thead>
          <tr>
            <th>City</th>
            <th>Conditions</th>
            <th>High (F)</th>
            <th>Humidity</th>
          </tr>
        </thead>
        {/* <tbody>{this.props.weather.map(this.renderWeather)}</tbody> */}
        <tbody>{this.props.weather.data.map(this.renderWeather)}</tbody>
      </table>
    );
  }
}

const mapStateToProps = ({ weather }) => ({
  weather
});

export default connect(mapStateToProps)(WeatherList);

this.props.weather.data.map 抛出“无法读取未定义的属性映射”的错误。

提供“天气”状态的reducer是：

import { FETCH_WEATHER } from "../actions/index";

export function WeatherReducer(state = [], action) {
  switch (action.type) {
    case FETCH_WEATHER:
      console.log(action.payload.data);
      console.log(action.meta.city);
      return { data: [action.payload.data, ...state], meta: action.meta.city };
    // return [action.payload.data, ...state];
  }
  return state;
}

最后是相关的动作创建者：

import axios from "axios";

const API_KEY = "e95fb12f6c69ae61";
const ROOT_URL = `http://api.wunderground.com/api/${API_KEY}/forecast/q/`;

export const FETCH_WEATHER = "FETCH_WEATHER";

export function fetchWeather(searchData) {
  const url = `${ROOT_URL}${searchData.stateName}/${searchData.city}.json`;
  const request = axios.get(url);

  return {
    type: FETCH_WEATHER,
    payload: request,
    meta: { city: searchData.city }
  };
}

您可以从注释掉的代码中看到，如果我只传递一个数组进行迭代，我就可以让它工作。但我需要传递更多信息才能获得一个人搜索的城市名称。如何读取状态对象的第一个元素、数组并消除未定义的错误？

非常感谢您的任何想法！

Answer 1

由于 WeatherReducer 返回一个具有数据和元属性的对象，因此您必须在 initialState 中将其声明为对象。你的减速器必须看起来像

const initialState = {
    data: [],
    meta: ''
}
export function WeatherReducer(state = initialState, action) {
  switch (action.type) {
    case FETCH_WEATHER:
      console.log(action.payload.data);
      console.log(action.meta.city);
      return { data: [action.payload.data, ...state.data], meta: action.meta.city };
  }
  return state;
}

错误可能是因为在触发fetchWeather 操作之前最初返回一个空数组作为reducer 值，因此this.props.weather.data 将是undefined。在这种情况下要遵循的另一件事是有条件地使用在特定时间点可能未定义的所有此类值