<p>我試圖傳遞一個在非同步函數內部產生的狀態，該函數本身位於 useState 內部。我了解到useRef 可能是解決此問題的最佳方法，因為它可以引用可變狀態，並且在這個過程中了解了React-useStateRef，它最終解決了我的主組件內狀態永遠不會更新的另一個問題（會不斷得到「渲染太多」錯誤）。因此它本質上充當 useRef 和 useState 的合而為一。 </p> <p>但是，雖然我的狀態最終更新了，但它仍然沒有傳遞到我的 Canvas 元件。我正在嘗試根據從資料集中獲得的溫度來更新畫布的 BG 圖形。 </p> <pre class="brush:php;toolbar:false;">import { useEffect } from 'react'; import useState from 'react-usestateref'; import { getServerData } from './serviceData'; import "./App.css" import Canvas from './components/Canvas'; function App() { const [dataset, setDataset] = useState(null); const [units, setUnits] = useState('metric'); // canvas background graphic stuff var [currentBG, setCurrentBG, currentBGref] = useState(null); useEffect(() => { const fetchServerData = async () => { const data = await getServerData(city, units); setDataset(data); function updateBG() { const threshold = units === "metric" ? 20 : 60; if (data.temp <= threshold) { setCurrentBG('snow'); } else { setCurrentBG('sunny'); } } updateBG(); } fetchServerData(); console.log(currentBG) }, [city, units, currentBGref.current, currentFGref.current]) const isCelsius = currentUnit === "C"; button.innerText = isCelsius ? "°F" : "°C"; setUnits(isCelsius ? "metric" : "imperial"); }; return ( <div className="app"> { dataset && ( <Canvas width={640} height={480} currentBG={currentBGref.current}></Canvas> )} </div> ); } export default App;</pre> <p>我只能傳遞初始值，而且它永遠不會更新超過該值，儘管 useEffect 內部的 console.log 顯示它肯定正在更新。那為什麼它沒有傳遞給我的組件呢？ </p>

function App({ width, height }) {
  const canvas = React.useRef()
  const [color, setColor] = React.useState("white")

  // when color changes..
  React.useEffect(() => {
    if (canvas.current) {
      const context = canvas.current.getContext('2d');
      context.fillStyle = color
      context.fillRect(0, 0, width, height)
    }
  }, [color, width, height])

  return <div>
    <canvas ref={canvas} width={width} height={height} />
    <button onClick={_ => setColor("blue")} children="blue" />
    <button onClick={_ => setColor("green")} children="green" />
    <button onClick={_ => setColor("red")} children="red" />
  </div>
}

ReactDOM.createRoot(document.querySelector("#app")).render(<App width={200} height={150} />)

canvas { display: block; border: 1px solid black; }

<script crossorigin src="https://unpkg.com/react@18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@18/umd/react-dom.development.js"></script>
<div id="app"></div>

function App({ width, height }) {
  const canvas = React.useRef()
  const setColor = color => event => {
    if (canvas.current) {
      const context = canvas.current.getContext('2d');
      context.fillStyle = color
      context.fillRect(0, 0, width, height)
    }
  }
  return <div>
    <canvas ref={canvas} width={width} height={height} />
    <button onClick={setColor("blue")} children="blue" />
    <button onClick={setColor("green")} children="green" />
    <button onClick={setColor("red")} children="red" />
  </div>
}

ReactDOM.createRoot(document.querySelector("#app")).render(<App width={200} height={150} />)

canvas { display: block; border: 1px solid black; }

<script crossorigin src="https://unpkg.com/react@18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@18/umd/react-dom.development.js"></script>
<div id="app"></div>