Thanks! // One of the values has changed! It's a hook initialised with useRef. One thing it’s not good for is making DOM changes that are visible to the user. // want to run this effect NOW, we only want to queue it up. This fun fact is also true for the dispatch function returned by useReducer. We'll learn about cleaning up our effects, and how to pass an empty array into the second argument of our effect to create a componentDidMount like effect. 值得注意的是，这里的回调函数会在每次渲染后调用，因此不仅可以访问 componentDidUpdate，还可以访问componentDidMount，如果只想模拟 componentDidUpdate，我们可以这样来实现。 Just as we were able to set up useEffect to run when a state variable changed, the same can be done with props.
So, if the state is updated synchronously within an effect method users will see a flicker as the first frame is replaced with the second frame. Ignore all of them :) In this case: Bottom, then Middle, then Top.
Methods like componentDidMount() revolve around lifecycles and render time whilst hooks are designed around state and synchronization with the DOM. React does not do this – it will only re-render in response to state changes.
Effects are made to be used atomically, to maintain good readability : If the effect concernX have to be trigged only on updates right after any user action or async stuff, IMO you better should rely on your component scope (props, states, sometimes ref or context, ...). (Notice that if you click on Bottom, you won’t see “rendered” messages from Top or Middle). React's useEffect hook combines componentDidMount, componentDidUpdate and componentWillUnmount lifecycle methods. // <-- we didn't pass a value. How to mimic componentDidUpdate() with React Hooks, // I also made it to support running when specific values update in deps, // The default value for deps will be undefined if you did not pass it, // and will have the same effect as not passing the parameter to useEffect, // so it watch for general updates by default, // concern 4 (on each re-render because we can :) ), Best practices to authenticate with Passport.js, Mutability and reassignability in JavaScript. We're a place where coders share, stay up-to-date and grow their careers. The useEffect hook gives you a better way. componentDidMount; componentDidUpdate; componentWillUnmount; useEffect(() => {}, [...values]) componentDidUpdate being one of them.
Let’s look at another common use case: fetching data and displaying it. That seems weird…. 使用 hooks 模拟 componentDidUpdate. With useEffect, you can handle lifecycle events directly inside function components. Let’s expand on the example to cover another common problem: how to re-fetch data when something changes, like a user ID, or in this case, the name of the subreddit. Network requests, manual DOM mutations, and logging are common examples of effects that don’t require a cleanup. Re-run this effect when `subreddit` changes: // 2 pieces of state: one to hold the input value. Sometimes, we'd like only … Before hooks were introduced we were taught to use functions like componentDidMount(). Rather than thinking of useEffect as one function doing the job of 3 separate lifecycles, it might be more helpful to think of it simply as a way to run side effects after render – including the potential cleanup you’d want to do before each one, and before unmounting. Thanks for sharing your notes . got me around my issue, but I can't see how I could use @devdufutur Have a look at the docs for useEffect and particularly the section on the timing of effects if you have a need to make visible DOM changes. Note! Remember they’re all regular variables! In future versions, it may be handled by an argument. You’ll see 3 messages printed to the console.