浅谈React

forwardRef和useImperativeHandle的联动使用

import React, { useImperativeHandle, useRef } from "react"
import { forwardRef } from "react"const CustomInput = forwardRef((props, ref) => {const inputRef = useRef<HTMLInputElement>(null)useImperativeHandle(ref, () => ({focus: () => {inputRef.current?.focus()}}))return <div><input ref={inputRef} /></div>
})export default CustomInput

巧用children

• 一般用法

父组件：import React from "react"
import Child from './Child'const CustomInput = () => {return <Child><div>hello 靓仔</div></Child>
}export default CustomInput子组件：import React from "react"const Child = ({children
}) => {return <div>{children}</div>
}export default Child

• 函数用法

父组件：import React from "react"
import Child from './Child'const CustomInput = () => {return <Child>{(arr)=><div>{arr.map((v,idx)=>{return <div key={idx}>{v}</div>})}</div>}</Child>
}export default CustomInput子组件：import React from "react"const Child = ({children
}) => {const arr = [1,2,4,5]return <div>{children(arr)}</div>
}export default Child

useEffect

• 没有依赖，类似于componentDidMount和componentDidUpdate

import React, { useEffect, useState } from "react"const Detail = () => {const [count, setCount] = useState(1)const [num,setNum] = useState(2)useEffect(()=>{console.log(count,'count',num,'num')})return <div><div onClick={() => setCount(count + 1)}>add count</div><div onClick={() => setNum(num + 1)}>add num</div><div>count: {count}</div><div>num: {num}</div></div>
}export default Detail

• 依赖是个空数组，相当于componentDidMount

import React, { useEffect, useState } from "react"const Detail = () => {const [count, setCount] = useState(1)const [num,setNum] = useState(2)useEffect(()=>{console.log(count,'count',num,'num')},[])return <div><div onClick={() => setCount(count + 1)}>add count</div><div onClick={() => setNum(num + 1)}>add num</div><div>count: {count}</div><div>num: {num}</div></div>
}export default Detail

• 有依赖，componentDidMount和对应依赖的componentDidUpdate

import React, { useEffect, useState } from "react"const Detail = () => {const [count, setCount] = useState(1)const [num,setNum] = useState(2)useEffect(()=>{console.log(count,'count',num,'num')},[num])return <div><div onClick={() => setCount(count + 1)}>add count</div><div onClick={() => setNum(num + 1)}>add num</div><div>count: {count}</div><div>num: {num}</div></div>
}export default Detail

useEffect和useLayoutEffect

useEffect在渲染后执行，而useLayouEffect是在渲染之前执行

最典型的例子就是实现一个tooltip组件，在性能比较差的情况下，useEffect会先渲染初始状态再更新，而useLayoutEffect会阻塞UI的更新即不会出现组件闪烁的情况~

阻塞代码：

let now = performance.now();
while (performance.now() - now < 100) {
}

useEffect在性能差的情况下会出现以下效果

useContext

依赖注入

父级：
export const ThemeContext = createContext({});
const App = ()=>{return  <ThemeContext.Provider value={{name:"real hot"}}>......</ThemeContext.Provider>
}子级：const context = useContext(ThemeContext)console.log(context)

useState的变动

在react管辖下(react17.x.x)

• 函数形式

状态能够更改多次，只渲染一次

const handleCount= ()=>{setCount(count=>count+1)setCount(count=>count+1)
}

• 对象形式

多次更改状态会被合并成一次更改，即一次生效其他无效，只渲染一次

const handleCount= ()=>{setCount(count+1)setCount(count+1)
}

在异步任务/原生事件下(react17.x.x)

const handleCount= ()=>{
setTimeout(()=>{setCount(count=>count+1)setCount(count=>count+1)
})
}

版本的演变（React18）

react18之后在异步操作或者react事件中都是批量更新，即多个状态更新合成一次渲染，若需要多次渲染可使用flushsync

const handleCount= ()=>{flushSync(()=>{setCount(count=>count+1)})flushSync(()=>{setCount(count=>count+1)})}

探索生命周期函数

父子组件生命周期执行顺序

挂载： 

更新： 

卸载： 

错误处理

生命周期方法：

getDerivedStateFromError
componentDidCatch

缺点（没办法捕获）：

• 异步操作
• 事件处理函数报错
• 错误边界自己报错 

class ErrorBoundary extends React.Component {constructor(props) {super(props);this.state = { hasError: false };}static getDerivedStateFromError(error) {// Update state so the next render will show the fallback UI.return { hasError: true };}componentDidCatch(error, errorInfo) {// You can also log the error to an error reporting servicelogErrorToMyService(error, errorInfo);}render() {if (this.state.hasError) {// You can render any custom fallback UIreturn <h1>Something went wrong.</h1>;}return this.props.children; }
}

实现一个简单的Message

import ReactDOM from 'react-dom';
import { Modal } from 'antd';
const ToastFn = () => {
let parent = null;
return {
open: function ({ el, container = document.body }) {
// this.destroy();
parent = document.createElement('div');
document.body.appendChild(parent);
ReactDOM.render(<Modal open onCancel={this.destroy}>{el}</Modal>, parent)
},
destroy: function () {
ReactDOM.unmountComponentAtNode(parent);
},
};
};
const Toast = ToastFn();
export default Toast;