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10
        setCount(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} />;
});