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useEffect与useLayoutEffect.md

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Understanding useEffect and useLayoutEffect

useEffect and useLayoutEffect can be collectively referred to as the Effect Hook. The Effect Hook allows you to perform side effects in a functional component. Side effects refer to when the behavior of a function or expression depends on the external environment, or in this case, when modifying a certain state can affect other states. Data fetching, setting subscriptions, and manual manipulation of the DOM within a React component all fall under side effects.

useEffect

The useEffect Hook can be seen as a combination of the componentDidMount, componentDidUpdate, and componentWillUnmount lifecycle methods. Using multiple Effects achieves separation of concerns, meaning that the granularity of useEffect is lower, allowing you to separate the handling of side effects in different locations.

Since useEffect combines componentDidMount, componentDidUpdate, and componentWillUnmount, we can also separate them using useEffect. First, for componentDidMount and componentWillUnmount, which are meant to run only once (i.e., execute the effect only when the component mounts and unmounts), since there are no dependencies, the second parameter would be an empty array. If the second parameter is omitted, the effect runs during component initialization and updates. In general, this is not desirable because of the way Hooks are designed; each setState invocation re-executes the component function, leading to frequent execution of the effect function. Therefore, it's usually better not to omit the second parameter. In terms of lifecycle, if a timer is set up when the component is created, it's desired to destroy the timer when the component is unmounted to avoid memory leaks. This can be achieved by returning a function call within useEffect. This way, we can focus our attention in one place, without needing to handle two different lifecycles separately.

In terms of componentDidUpdate, if accomplishing the same functionality in a class component required embedding a lot of logic within this lifecycle, using useEffect allows us to separate different concerns and handle their side effects separately. Of course, if there is still a need to clean up things such as subscriptions or timers, a cleanup function can still be returned.

The documentation also points out the importance of ensuring that the array contains all the variables from the outer scope that are used in the effect and may change over time. Otherwise, your code would reference old variables from the previous render. If you pass an empty array [], the props and state within the effect will always have their initial values. In the example below, a bug would occur if the count were not included in the dependencies array, as the effect closure created when the effect is run would hold the value of count from the initial render and would not change. To resolve this issue, we can include count in the dependencies array. React has provided an exhaustive-deps ESLint rule as part of the eslint-plugin-react-hooks package, which helps identify components that do not handle updates consistently.

import { useEffect, useState } from "react";
import "./styles.css";

export default function App() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    const id = setInterval(() => {
      setCount(count + 1);
      console.log(count + 1);
    }, 1000);
    return () => clearInterval(id);
  }, []); // `count` has not been specified as a dependency

  return (
    <div>
      <div>{count}</div>
      <button onClick={() => setCount(count + 1)}>count + 1</button>
    </div>
  );
}

It looks very much like Vue's watch, but it's not exactly the same. The main syntax difference is that useEffect can monitor changes in multiple properties, which watch cannot. Of course, watch can achieve this indirectly, but the concepts are different. Vue listens for value changes while React is used to handle side effects. The reason for mentioning this is that I used to write more Vue before, and I always tend to compare various implementations of React from the perspective of Vue. I feel this approach has both advantages and disadvantages. The advantage is that it's easy to get started quickly, but the disadvantage is that it's easy to get stuck or overthink. As a great developer once said, you need to forget about Vue and then learn about Hooks, although it's not absolute, it makes a lot of sense.

When a function component refreshes and renders, the entire process including the component containing useEffect is as follows:

  • Trigger component re-rendering by changing the component's state or by re-rendering the component's parent, causing the child node to be re-rendered.
  • Execute the component function.
  • Render the component to the screen.
  • Execute the useEffect hook.

useLayoutEffect

useLayoutEffect is very similar to useEffect, the function signature is also the same, the only difference is that useEffect is asynchronous while useLayoutEffect is synchronous. When a function component refreshes and renders, the entire process including the component containing useLayoutEffect is as follows:

  • Trigger component re-rendering by changing the component's state or by re-rendering the component's parent, causing the child component to be re-rendered.
  • Execute the component function.
  • Execute the useLayoutEffect hook, and React waits for the useLayoutEffect function to finish execution.
  • Render the component to the screen.

Daily Challenge

https://github.com/WindrunnerMax/EveryDay

References

https://zhuanlan.zhihu.com/p/348701319
https://zhuanlan.zhihu.com/p/259766064
https://segmentfault.com/a/1190000039087645
http://www.ptbird.cn/react-hoot-useEffect.html
https://react.docschina.org/docs/hooks-effect.html
https://pengfeixc.com/blog/605af93600f1525af762a725
https://react.docschina.org/docs/hooks-faq.html#is-it-safe-to-omit-functions-from-the-list-of-dependencies