javascript - 当第一次在 useEffect 上获取数据时，为什么 React 的 useReducer 返回空状态？

Question

我正在使用 React hooks 和 contextAPI 开发一个简单的 MERN stack todo 应用程序，我使用 axios 从 mongodb 数据库中获取数据，并设置 initialState 数组变量useEffect来模拟componentDidMount，然后将initialState作为参数传给useReducer。我的问题是子组件没有收到 initialState 而是一个空数组，但是所有其他 Prop (例如调度和加载变量)都可以通过。为什么 useReducer 返回空状态数组？

父组件:

import React, { useEffect, useState, useReducer } from 'react';
import Header from './components/header/Header';
import Main from './components/main/Main';
import AppContext from './context/AppContext';
import axios from 'axios';
import './stylesheets/index.scss';

function App() {
  const [initialState, setInitialState] = useState([]);
  const [isError, setIsError] = useState(false);
  const [isLoading, setIsLoading] = useState(false);

  /* Fetch data */
  const fetchData = async () => {
    setIsError(false);
    setIsLoading(true);
    try {
      const response = await axios.get('/list');
      setInitialState(response.data);
    } catch (error) {
      setIsError(true);
      console.log(`Unable to get: ${error}`);
    }
    setIsLoading(false);
  };

  /* Post new data */
  const postNewData = async (parameter) => {
    setIsError(false);
    try {
      const response = await axios.post('/list', parameter);
      console(response.data);
    } catch (error) {
      setIsError(true);
      console.log(`Unable to post: ${error}`);
    }
  };

  /* Delete data */
  const deleteData = async (id) => {
    setIsError(false);
    try {
      const response = await axios.delete(`/list/${id}`);
      console(response.data);
    } catch (error) {
      setIsError(true);
      console.log(`Unable to delete: ${error}`);
    }
  };

  useEffect(() => {
    /* On load fetch data from the database */
    fetchData();
  }, []);

  useEffect(() => {
    console.log(initialState);
  }, [initialState]); 

  const appReducer = (state = initialState, action) => {
    switch (action.type) {
      case 'ADD_NEW_LIST':
        return postNewData(action.new_list);
      case 'DELETE_LIST':
        return deleteData(action.list_id);
      default:
        return state;
    }
  };

  const [state, dispatch] = useReducer(appReducer, initialState);

  return (
    <AppContext.Provider value={{ state, dispatch, isError, isLoading }}>
      <div className='App'>
        <Header />
        <Main />
      </div>
    </AppContext.Provider>
  );
}

export default App;

这是主组件内的子组件:

import React, { useContext, useEffect, useState } from 'react';
import AppContext from '../../context/AppContext';

function Listarea() {
  const { state, isError, isLoading, dispatch } = useContext(AppContext);

    console.log(state);
    console.log(isError);
    console.log(isLoading);

  return <div className='list-area'>list area here</div>;
}

export default Listarea;

这是 console.log 输出

正如您所看到的，数据存在于父组件中，但子组件中不存在，除了能够进入的其他 Prop 之外。

Answer 1

如果你更新你的initialState值，它不会改变你的useReducer的状态。

您可以简单地使用一种状态并在获取数据时更新该状态，您不需要initialState 和state 两者，因此您可以删除initialState。

const appReducer = (state = initialState, action) => {
    switch (action.type) {
      case 'SET_DATA':
        return action.data;
      case 'ADD_NEW_LIST':
        return postNewData(action.new_list);
      case 'DELETE_LIST':
        return deleteData(action.list_id);
      default:
        return state;
    }
  };

const [state, dispatch] = useReducer(appReducer, []);

const fetchData = async () => {
    setIsError(false);
    setIsLoading(true);
    try {
      const response = await axios.get('/list');
      dispatch({ type: 'SET_DATA', data: response.data });
    } catch (error) {
      setIsError(true);
      console.log(`Unable to get: ${error}`);
    }
    setIsLoading(false);
  };