学习：卡颂 https://react.iamkasong.com/process/reconciler.html#%E9%80%92-%E9%98%B6%E6%AE%B5
    render阶段开始于performSyncWorkOnRoot或performConcurrentWorkOnRoot方法的调用。这取决于本次更新是同步更新还是异步更新。

    1. // performSyncWorkOnRoot会调用该方法
    2. function workLoopSync() {
    3.   while (workInProgress !== null) {
    4.     performUnitOfWork(workInProgress);
    5.   }
    6. }
    8. // performConcurrentWorkOnRoot会调用该方法
    9. function workLoopConcurrent() {
    10.   while (workInProgress !== null && !shouldYield()) {
    11.     performUnitOfWork(workInProgress);
    12.   }
    13. }

    区别：shouldYield。如果当前浏览器帧没有剩余时间，shouldYield会中止循环，直到浏览器有空闲时间后再继续遍历。
    performUnitOfWork——创建下一个Fiber节点，并赋值给workInProgress,并将workInProgress与创建的Fiber节点连接构成Fiber树。

    注：只有单一文本子节点的Fiber，React会特殊处理。优化点。
    demo

    1. import logo from "./logo.svg"
    2. import "./App.css"
    3. import { useState } from "react"
    4. function App() {
    5.   const [num, setNum] = useState(0)
    6.   return (
    7.     <div className="App">
    8.       <header className="App-header">
    9.         <img src={logo} className="App-logo" alt="logo" />
    10.         <p onClick={() => setNum(num++)}>
    11.           Edit <code>{num}</code> and save to reload.
    12.         </p>
    13.         <a className="App-link" href="https://reactjs.org" target="_blank" rel="noopener noreferrer">
    14.           Learn React
    15.         </a>
    16.       </header>
    17.     </div>
    18.   )
    19. }
    21. export default App
    1. import React from 'react';
    2. import ReactDOM from 'react-dom/client';
    3. import './index.css';
    4. import App from './App';
    5. import reportWebVitals from './reportWebVitals';
    7. const root = ReactDOM.createRoot(document.getElementById('root'));
    8. root.render(
    9.   <React.StrictMode>
    10.     <App />
    11.   </React.StrictMode>
    12. );
    14. // If you want to start measuring performance in your app, pass a function
    15. // to log results (for example: reportWebVitals(console.log))
    16. // or send to an analytics endpoint. Learn more: https://bit.ly/CRA-vitals
    17. reportWebVitals();