对useEffect中的异步函数的React Hook警告:useEffect函数必须返回清除函数，否则不返回任何内容 [英] React Hook Warnings for async function in useEffect: useEffect function must return a cleanup function or nothing

问题描述

我正在尝试useEffect示例，如下所示:

I was trying the useEffect example something like below:

useEffect(async () => {
    try {
        const response = await fetch(`https://www.reddit.com/r/${subreddit}.json`);
        const json = await response.json();
        setPosts(json.data.children.map(it => it.data));
    } catch (e) {
        console.error(e);
    }
}, []);

，我在控制台中收到此警告.但是对于我认为的异步调用，清理是可选的.我不确定为什么会收到此警告.链接沙箱的示例. https://codesandbox.io/s/24rj871r0p

and I get this warning in my console. But the cleanup is optional for async calls I think. I am not sure why I get this warning. Linking sandbox for examples. https://codesandbox.io/s/24rj871r0p

推荐答案

我建议看看 Dan Abramov(React核心维护者之一)在这里回答:

我认为您正在使它变得比所需的复杂.

I think you're making it more complicated than it needs to be.

function Example() {
  const [data, dataSet] = useState<any>(null)

  useEffect(() => {
    async function fetchMyAPI() {
      let response = await fetch('api/data')
      response = await response.json()
      dataSet(response)
    }

    fetchMyAPI()
  }, [])

  return <div>{JSON.stringify(data)}</div>
}

从长远来看，我们将不鼓励这种模式，因为它会鼓励比赛条件.例如-在通话开始和结束之间可能会发生任何事情，并且您可能会获得新的道具.相反，我们建议使用Suspense进行数据提取，看起来更像

Longer term we'll discourage this pattern because it encourages race conditions. Such as — anything could happen between your call starts and ends, and you could have gotten new props. Instead, we'll recommend Suspense for data fetching which will look more like

const response = MyAPIResource.read();

，没有任何影响.但是与此同时，您可以将异步内容移到一个单独的函数中并调用它.

and no effects. But in the meantime you can move the async stuff to a separate function and call it.

您可以在此处阅读有关实验性悬念的更多信息.

You can read more about experimental suspense here.

如果您想在eslint之外使用其他功能.

If you want to use functions outside with eslint.

 function OutsideUsageExample() {
  const [data, dataSet] = useState<any>(null)

  const fetchMyAPI = useCallback(async () => {
    let response = await fetch('api/data')
    response = await response.json()
    dataSet(response)
  }, [])

  useEffect(() => {
    fetchMyAPI()
  }, [fetchMyAPI])

  return (
    <div>
      <div>data: {JSON.stringify(data)}</div>
      <div>
        <button onClick={fetchMyAPI}>manual fetch</button>
      </div>
    </div>
  )
}

---

使用useCallback useCallback . 沙盒.

---

With useCallback useCallback. Sandbox.

import React, { useState, useEffect, useCallback } from "react";

export default function App() {
  const [counter, setCounter] = useState(1);

  // if counter is changed, than fn will be updated with new counter value
  const fn = useCallback(() => {
    setCounter(counter + 1);
  }, [counter]);

  // if counter is changed, than fn will not be updated and counter will be always 1 inside fn
  /*const fnBad = useCallback(() => {
      setCounter(counter + 1);
    }, []);*/

  // if fn or counter is changed, than useEffect will rerun
  useEffect(() => {
    if (!(counter % 2)) return; // this will stop the loop if counter is not even

    fn();
  }, [fn, counter]);

  // this will be infinite loop because fn is always changing with new counter value
  /*useEffect(() => {
    fn();
  }, [fn]);*/

  return (
    <div>
      <div>Counter is {counter}</div>
      <button onClick={fn}>add +1 count</button>
    </div>
  );
}

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