React常见知识点

1. setCount(10)与setCount(preCount => preCount + 10) 的区别：

import React, { useState } from 'react';
export default function CounterHook() {const [count, setCount] = useState(() => 10);console.log('CounterHook渲染');function handleBtnClick() {// 下面这种只会加10setCount(count + 10);setCount(count + 10);setCount(count + 10);setCount(count + 10);// 下面这种会加到40// setCount((prevCount) => prevCount + 10);// setCount((prevCount) => prevCount + 10);// setCount((prevCount) => prevCount + 10);// setCount((prevCount) => prevCount + 10);}return (<div><h2>当前计数: {count}</h2><button onClick={handleBtnClick}>+10</button></div>);
}

2. 如何在父组件中引用子组件的某个元素? forwardRef

// 父组件
import React, { useEffect, useRef } from 'react';
import Product from './product';
export default function App(props) {const liRef = useRef();useEffect(() => {console.log('ref', liRef.current)})return (<><Product ref={liRef} /></>);
}

// 子组件 - 函数组件
import React from 'react';
import { forwardRef } from 'react';
export default forwardRef((props, ref) => {const getProdInfo = function (e) {console.log(e.target.innerText);};return (<ul onClick={getProdInfo}><li ref={ref}>商品1</li><li>商品2</li></ul>);
});

// 子组件 - class组件
import React, { forwardRef } from 'react';
class Product extends React.Component {constructor(props) {super(props);}getProdInfo = function (e) {console.log(e.target.innerText);};render() {console.log(1, this.props)return (<ul onClick={this.getProdInfo}><li ref={this.props.innerRef}>商品1</li><li>商品2</li></ul>);}
}
export default forwardRef((props, ref) => <Product innerRef={ref} {...props} />);

联想：
ref的作用：

1. 通过ref操作dom，分为 1.作用在DOM节点上；2作用在组件上(此时为组件的实例)；
2. 用ref引用一个值；参考官网useRef

3. useImperativeHandle

import { forwardRef } from 'react';const MyInput = forwardRef(function MyInput(props, ref) {return <input {...props} ref={ref} />;
});