为什么我在React中的clearInterval不起作用？

Question

当我点击更改代码时，我只看到console.log。我试着理解它，但我找不到答案..

function App() {
  const [laps, setLaps] = useState(0);
  const [running, setRunning] = useState(false);
  const startTime = Date.now() - laps;

   useEffect(() => {
    function interval() {
      setInterval(() => {
        setLaps(Date.now() - startTime);
      }, 100);
    }

    if (!running) {
      clearInterval(interval);
      console.log('ok');
    } else {
      interval();
      console.log('no');
    }
    console.log(running);
  }, [running]);

  return (
    <div className="App">
        <label>Count: {laps}</label>
        <button onClick={() => setRunning(!running)}>{running ? 'stop' : 'start'}</button>
        <button>Clear</button>
    </div>
  );
}

在干净的JavaScript中，这段代码应该正常工作（当然没有JSX）？

Answer 1

clearInterval期望从setInterval返回一个数字作为参数，但是你给它interval函数作为参数。

如果running为真，你可以改为创建间隔，并从useEffect返回一个清理函数，该函数将在下次运行效果时运行。

const { useState, useEffect } = React;

function App() {
  const [laps, setLaps] = useState(0);
  const [running, setRunning] = useState(false);
  const startTime = Date.now() - laps;

  useEffect(
    () => {
      if (running) {
        const interval = setInterval(() => {
          setLaps(Date.now() - startTime);
        }, 100);
        return () => clearInterval(interval);
      }
    },
    [running]
  );

  return (
    <div className="App">
      <label>Count: {laps}</label>
      <button onClick={() => setRunning(!running)}>
        {running ? "stop" : "start"}
      </button>
      <button>Clear</button>
    </div>
  );
}

ReactDOM.render(<App />, document.getElementById("root"));

<script src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>

<div id="root"></div>