給出下面的例子#app div {  padding: 10px;  border: 1px solid;  margin: 10px 0;}div:focus {  background: red;}div:focus:before {  content: "focused";  display: block;}<div id="app"></div><script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.6/umd/react.production.min.js"></script><script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.6/umd/react-dom.production.min.js"></script><script src="https://unpkg.com/babel-standalone@6.15.0/babel.min.js"></script><script type="text/babel">  let { useState, useRef, Fragment } = React;   let App = () => {     let [, forceUpdate] = useState();     let [num, setNum] = useState(0);     let history = useRef('null');         let keyPressHandler = () => {     history.current = num;         // just to force an update     forceUpdate(Date.now());  };     return (  <Fragment>    <button onClick={()=>{setNum(Date.now())}}>update state</button>    <div tabindex="0" onKeyPress={keyPressHandler}>this div has a keypress listener. <br/>for now whatever key you press it will save the state in a ref, that changes on the button click. <br/>Click the button to change the state then focus this div then press any key to save the state in the ref</div>    <div>State&nbsp;&nbsp;&nbsp;&nbsp;: {num}</div>    <div>History: {history.current}</div>  </Fragment>  ); }; ReactDOM.render(<App />, document.querySelector("#app"));</script>現(xiàn)在，我將把按鍵監(jiān)聽器移到窗口，應該做同樣的事情。但這并不num總是初始值。