为什么 react 中 effect cleanup 是在每次 re-render 之后再调用?

[ethusdt]

function Detail() {
  const x = Math.random();
  console.log('render: ', x);
  React.useEffect(() => {
    console.log('effect: ', x);
    return function() {
      console.log('cleanup: ', x);
    }
  })
  const [, forceUpdate] = React.useReducer(x=>x+1,0);
  return (
    <View style={{ flex: 1, justifyContent: 'center', alignItems: 'center' }}>
      <Text>Detail!</Text>
      <Button title="forceupdate" onPress={forceUpdate} />
    </View>
  );
}

页面进入-点击 forceupdate 按钮, 点击返回按钮页面结束, 日志:

render:  0.5817026745695149
effect:  0.5817026745695149
render:  0.7876494718531499
cleanup:  0.5817026745695149
effect:  0.7876494718531499
cleanup:  0.7876494718531499

实际上:

Mount -> Update -> Re-render -> cleanup -> Unmount -> cleanup

为什么不是:

Mount -> Update -> cleanup -> Re-render -> Unmount -> cleanup

React 官网的介绍:

When exactly does React clean up an effect? React performs the cleanup when the component unmounts. However, as we learned earlier, effects run for every render and not just once. This is why React also cleans up effects from the previous render before running the effects next time.

虽然说了 cleanup 是在下一次 effect 之前调用, 但也没有讲为什么是在下一次 render 之后调用.

[noe132]

你把 useEffect 理解成这样的模型就可以了。
useEffect = (effect) => {
if (clearUp) {
clearUp()
}
clearUp = effect()
}

[dumbass]

https://overreacted.io/zh-hans/a-complete-guide-to-useeffect/

[86co]

因为是 componentDidUpdate 啊

[XTTX]

&t=806s

这里解释得清楚。 你的代码有点绕，正常情况下 useEffect 要带 dependencies.

```
const [change, SetChange] = React.useState(false)
useEffect(() => {
console.log('fetch')
let isCancelled = false
setTimeout(() => {
if (!isCancelled) {
console.log('first')
}
}, 1000)
return () => {
isCancelled = true
}
}, [change])

```

[xzh654321]

写 useEffect 的时候要把 dependencies 写进去.


参考：
https://overreacted.io/zh-hans/a-complete-guide-to-useeffect/
https://www.zhihu.com/question/508780830/answer/2290031407

[charlie21]

useEffect 里面既能看到当前 render 的值，也能看到上一次 render 的值
https://overreacted.io/zh-hans/a-complete-guide-to-useeffect/

React 能做到在绘制后立即处理 effects — 并且默认情况下使你的应用运行更流畅。如果你的代码需要依然可以访问到老的 props

所以这篇文章给出的答案是 “使你的应用运行更流畅”，即使 cleanup 里面卡到了，它不会卡到 UI 渲染

[ethusdt]

@charlie21 嗯, 应该是这样的, UI 渲染第一位.