在React JS中使用上下文API时，使用reducer的初始状态不会更新

编程问答 2022-08-11

问题描述

我在使用上下文API时非常陌生，并在反应中加入了钩子。遇到一个问题，即即使尝试将对象传递给它，尝试通过引用useReducer传递动作的派发时初始状态也不会更新。代码如下： reducer.js 

export const initialState={
    basket:[ ],}  

const reducer= (state= initialState,action) => {
    
   console.log(action);
    switch(action.type) {
        
        case 'ADD_TO_BASKET':
          return {
               ...state,basket:[...state.basket,action.item]
       };
        
        case 'REMOVE_FROM_BASKET':
             //we cloned the basket   
        let newBasket = [...state.basket];
        const index= state.basket.findindex((basketItem) => basketItem.id === action.id);
           if (index >= 0) {
               //item exist in basket,remove it
         newBasket.splice(index,1);
           } else {
               console.warn (
                   'cant remove product (id: ${action.id}) as its not in basket' 
               );
           }
           return {...state,basket: newBasket,};

        break;
        default:
            return state;
    }
}

export default reducer;

上面的console.log(action)给出了item: {…} ​​ id: "101" ​​ image: "https://images.pexels.com/photos/534/animal-countryside-agriculture-farm.jpg" ​​ price: 11.96 ​​ rating: 5 ​​ title: "dress1" ​​ <prototype>: Object { … } ​ type: "ADD_TO_BASKET" ​ <prototype>: Object { … }，因此该对象被传递给化简器，但初始状态未更新。只有 case：ADD_TO_BASKET 不能正常工作，否则REMOVE_FORM_BASKET可以正常工作。如果有人可以帮助我，我将不胜感激。

这是其他要参考的文件。 stateProvider.js 

//we need this to track the basket
//setup data layer
import React,{ createContext,useContext,useReducer } from "react";
import reducer,{initialState} from './reducer';

//this is the data Layer
export const StateContext = createContext();


//BUILD a provider

export const StateProvider = ({children}) =>{

    const state = useReducer(reducer,initialState);
    return (
      <StateContext.Provider value={state}>
        {children}
      </StateContext.Provider>
    );
}
//this is how we use it inside of a component
export const useStateValue = () =>useContext(StateContext);

product.js 

import { useStateValue } from './StateProvider';
import "./product.css";


function Product ({id,title,image,price,rating}){
   

    const [{},dispatch]  = useStateValue();

  const  addToBasket =() => {
       console.log("am clicked");
        
      dispatch({
          type: 'ADD_TO_BASKET',item: {
              id: id,title: title,image: image,price: price,rating: rating
          },});

    };


    return (......
......
.....

header.js 

import "./Header.css";
import {Link} from "react-router-dom"
import { useStateValue } from './StateProvider';
function Header() {
//const [state,dispach] = useStateValue();
const [{basket,user}]  = useStateValue();
console.log(basket);

return(
<div>
.
.
.
<Link to="/checkout" className="header_link">
       <div className="header_optionbasket">
      <ShoppingBasketIcon/>
      <span className="header_optiontwo basketcount">{basket.length}</span>
       </div>
       </Link>
.
.
.
</div>
)

初始状态更新后， header.js 中的 basket.length 被更新，但是它仍然为0，但是如果我在中硬编码一个对象initialState> basket ，则 basket.length 会被更新。但是对象没有被添加到 InitialState>篮子

解决方法

export const initialState={
    basket:[ ],}  

const reducer= (state=initialState,action) => {
    
   console.log(action);
    switch(action.type) {
        
        case 'ADD_TO_BASKET':
          return {
               ...state,basket:[...state.basket,action.item]
       };

您必须在reducer中初始化状态。另外，您不需要在reducer中添加Break语句。

actiondispatchcontext-apijavascriptreactjsreactjsuse-reducer