如何使用 useReducer([state,dispatch]) 和 useContext 避免无用的重新渲染? [英] How to avoid useless rerender using useReducer([state,dispatch]) and useContext?

问题描述

当使用多个 useReducers 时，每个组件都会重新渲染一部分状态.

import React, { useContext } from 'react'从../store"导入商店从../actions/nameActions"导入 { setName }const 名称 = () =>{const { state: { nameReducer: { name } }, dispatch } = useContext(Store)const handleInput = ({ target: { value } }) =>{调度(设置名称(值))}console.log('如果状态的其他部分(不是名称)被改变，则无用的重新渲染');返回 
<p>{名称}</p><input value={name} onChange={handleInput}/>
}导出默认名称；

如何避免这种无用的重新渲染?

解决方案

如果 useState 或 useReducer 状态改变，则组件更新，没有办法阻止在组件本身中.

在依赖于部分状态的子组件中应该防止重新渲染，例如通过使其纯净:

const NameContainer = () =>{const { state: { nameReducer: { name } }, dispatch } = useContext(Store)return ;}const Name = React.memo(({ name, dispatch }) => {const handleInput = ({ target: { value } }) =>{调度(设置名称(值))}返回 
<p>{名称}</p><input value={name} onChange={handleInput}/>
});

NameContainer 可以重写为 HOC 并用于与 Redux connect 相同的目的，从存储中提取所需的属性并将它们映射到连接的组件 props.>

When using multiple useReducers every component using a part of the state rerenders.

import React, { useContext } from 'react'
import Store from '../store'
import { setName } from "../actions/nameActions"

const Name = () => {
    const { state: { nameReducer: { name } }, dispatch } = useContext(Store)
    const handleInput = ({ target: { value } }) => { dispatch(setName(value)) }
    console.log('useless rerender if other part (not name) of state is changed'); 
    return <div>
        <p>{name}</p>
        <input value={name} onChange={handleInput} />
    </div>
}

export default Name;

How to avoid this useless rerendering?

解决方案

If useState or useReducer state changes, the component is updated, there is no way to prevent this in the component itself.

Re-render should be prevented in child component that depends on partial state, e.g. by making it pure:

const NameContainer = () => {
    const { state: { nameReducer: { name } }, dispatch } = useContext(Store)
    return <Name name={name} dispatch={dispatch}/>;
}

const Name = React.memo(({ name, dispatch }) => {
    const handleInput = ({ target: { value } }) => { dispatch(setName(value)) }
    return <div>
        <p>{name}</p>
        <input value={name} onChange={handleInput} />
    </div>
});

NameContainer can be rewritten to a HOC and serve the same purpose as Redux connect, to extract needed properties from a store and map them to connected component props.

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