javascript - React.StrictMode : SetState function in useEffect is run multiple times when effect is run once

Question

当oldRunIn为undefined时，下面代码的输出与触发效果时的预期一致:

Effect is running

setState is running

但是，下次 useEffect 运行时定义了状态变量 runInArrow(在 setState 函数中称为 oldRunInArrow)，输出为:

Effect is running

setState is running

setState is running

setState is running

怎么可能effect只运行了一次而setState运行了3次？

const [runInArrow, setRunInArrow] = useState<mapkit.PolylineOverlay[] | undefined>(undefined);


useEffect(() => {
      const trueRunIn = settings.magneticRunIn + magVar;

      const boundingRegion = region.toBoundingRegion();
      const style = new mapkit.Style({
        lineWidth: 5,
        lineJoin: 'round',
        strokeColor: settings.runInColor,
      });
      const newRunInArrow = createArrow(boundingRegion, trueRunIn, style);
      console.log('Effect is running');

      setRunInArrow(oldRunIn => {

        // This runs too many times when oldRunIn aka runInArrow is defined

        console.log('setState is running');
        if (oldRunIn) map.removeOverlays(oldRunIn);
        return newRunInArrow;
      });
      map.addOverlays(newRunInArrow);
  }, [magVar, map, mapkit, region, settings.magneticRunIn, settings.runInColor, settings.showRunIn]);

要了解为什么我使用函数来设置状态，请参阅此 post

<小时/>

编辑:

这很奇怪。如果我删除 if (oldRunIn) map.removeOverlays(oldRunIn); 它会按预期工作。但是，如果我将其更改为 if (oldRunIn) console.log('oldRunIn is Defined'); ，它仍然会运行多次。我彻底糊涂了。

if (oldRunIn) map.addAnnotations([]); 不会运行多次。

if (oldRunIn) console.log('test'); 确实运行了多次。

它总是运行多次(每个使用效果 6 次):

  setRunInArrow(oldRunIn => {
    console.log('setState is running');
    console.log('test');
    return newRunInArrow;
  });

这不会运行多次:

  setRunInArrow(oldRunIn => {
    console.log('setState is running');
    return newRunInArrow;
  });

<小时/>

编辑2:

可重现的示例。单击按钮。您甚至不需要 if (oldState) console.log('Old state'); 您只需放入第二个 console.log 即可更改行为。

import { FunctionComponent, useState, useEffect } from 'react';

export const Test: FunctionComponent<> = function Test() {
  const [state, setState] = useState<number | undefined>(undefined);
  const [triggerEffect, setTriggerEffect] = useState(0);

  useEffect(() => {
    console.log('effect is running');
    setState(oldState => {
      console.log('setState is runnning');
      if (oldState) console.log('Old state');
      return 1;
    });
  }, [triggerEffect]);

  return <>
    <button onClick={() => setTriggerEffect(triggerEffect + 1)} type="button">Trigger Effect</button>
  </>;
};

<小时/>

编辑3:

我通过将此代码放入另一个 nextJS 项目中来重现此内容。我猜这与 nextJS 有关。

<小时/>

编辑4:

这不是 NextJS。它将它包装在 React.StrictMode 中。这是sandbox 。为什么？

<小时/>

编辑5:

正如答案所指出的，问题是由于 StrictMode 故意运行代码两次造成的。它不应该根据文档运行 useReducer 两次(这是我的其他问题中的“react-hooks/exhaustive-deps”的问题)。这是一个 UseReducer 演示，尝试使用和不使用 StrictMode。它还运行两次。看来它也需要纯净:

CodeSandbox

import { useState, useEffect, useReducer } from 'react';

function reducer(state, data) {
  console.log('Reducer is running');
  if (state) console.log(state);
  return state + 1;
}

export const Test = function Test() {
  const [state, dispatch] = useReducer(reducer, 1);
  const [triggerEffect, setTriggerEffect] = useState(0);

  useEffect(() => {
    dispatch({});
  }, [triggerEffect]);

  return (
    <>
      <button onClick={() => setTriggerEffect(triggerEffect + 1)} type="button">
        Trigger Effect
      </button>
    </>
  );
};

const Home = () => (
  <React.StrictMode>
    <Test></Test>
  </React.StrictMode>
);

export default Home;

Answer 1

正如您所发现的，当您使用 React 严格模式时就会发生这种情况，并且这是故意的。

如 this article 中所述:

It runs code TWICE

Another thing that React Strict Mode does is run certain callbacks/methods twice (in DEV mode ONLY). You read that right! The following callbacks/methods will be run twice in Strict Mode (in DEV mode ONLY):

• Class component constructor method
• The render method (includes function components)
• setState updater functions (the first argument)
• The static getDerivedStateFromProps lifecycle
• The React.useState state initializer callback function
• The React.useMemo callback

Checkout this codesandbox which logs to the console in hook callbacks and class methods to show you that certain things happen twice.

React 这样做是因为它无法可靠地警告您在这些方法中产生的副作用。但如果这些方法是幂等的，那么多次调用它们不会造成任何麻烦。如果它们不是幂等的，那么您应该注意到一些有趣的事情，您应该能够注意到并修复这些事情。

Note that useEffect and useLayoutEffect callbacks are not called twice even in dev mode + strict mode because the entire point of those callbacks is to perform side-effects.

Note that I also observed that the reducer you pass to React.useReducer is not called twice in dev mode. I'm not sure why this is because I feel like that could also benefit from this kind of warning.

如上所示，包含此行为是为了帮助您查找代码中的错误。由于更新程序函数应该是幂等且纯粹的，因此运行它们两次而不是一次应该不会对应用程序的功能产生影响。如果确实如此，那么这就是您的代码中的错误。

听起来像 map.removeOverlays()您调用的方法是一个有效的函数，因此不应在状态更新器函数中调用它。根据 your other question 的答案，我可以明白为什么您要以这种方式实现它，但我认为使用 chitova263 的答案可以解决这个问题。