To learn more, check out this article about data fetching with Hooks. 6. By measuring our render differences with the React Developer Tools GUI, we’re able to more deeply see the varying results. Well you guessed it … React.memo to the rescue! You may also like my React starter kit ️ React relies on the order in which Hooks are called Some of the first custom Hooks to hit the shelves were several useFetch implementations that use Hooks to query a remote API. Update isScrollingDown. React will re-run the component with updated state immediately after exiting the first render so it wouldn’t be expensive. // Will be memoized even if `text` changes: 'Cannot call an event handler while rendering.'. We recommend trying Hooks in new code. Let’s click on the increment button six times and then hit the record button again to record our measurements. Things to note: Logic is now nicely encapsulated in separate methods, rather than in one giant switch statement. Feel free to code along and even follow the video demonstration at the bottom or to simply observe the differences in the documentation below. For additional resources, please check out the Performance Optimization section on React Dev 2020, which inspired this documentation I’ll link below. Importantly, custom Hooks give you the power to constrain React API if you’d like to type them more strictly in some way. By first building our application without preemptively applying optimizations and instead having an initial comparison value, we’re able to measure our performance with our own test implementations — as well as bring in helpful extensions such as the React Developer Tools. For example: Only do this if you couldn’t find a better alternative, as relying on mutation makes components less predictable. It accepts a new state value and enqueues a re-render of the component. On the Create a New Design page, we have several input forms for the user to fill out. // If we want to perform an action, we can get dispatch from context. For example, we could split our component state into position and size objects, and always replace the position with no need for merging: Separating independent state variables also has another benefit. However, for the purposes of this demonstration, let’s move onto the Profiler to measure the performance of our render times. In the future, React may choose to “forget” some previously memoized values and recalculate them on next render, e.g. Is it safe to omit functions from the list of dependencies? We plan to provide more ergonomic alternatives in the future, but the safest solution right now is to always invalidate the callback if some value it depends on changes. React.memo is a function that you can use to optimize the render performance of pure function components and hooks. Back in our weather application, we can wrap React.memo around our export of Weather, like such: And it’s as simple as that. How do I implement shouldComponentUpdate? For example, if an effect specifies [] as the second argument but reads someProp inside, it will keep “seeing” the initial value of someProp. In Computer Science, memoization is a concept used in general when we don’t need to recompute the function with a given argument for the next time as it returns the cached result. We provide the exhaustive-deps ESLint rule as a part of the eslint-plugin-react-hooks package. React expects that, no matter the props or state values, the component between re-rendering always invokes the hooks in the same order. If you want to be notified any time a component resizes, you may want to use ResizeObserver or a third-party Hook built on it. React will call that callback whenever the ref gets attached to a different node. ), Now, the setInterval callback executes once a second, but each time the inner call to setCount can use an up-to-date value for count (called c in the callback here.). Normally, you shouldn’t mutate local state in React. First, examine a JavaScript file containing our Weather child component with the following code: Our simple functional component takes the destructured weather property as its props and returns two p tags, displaying further dot notation accessing city and temperature. This hook has the potential to improve performance in your application. It makes it easy to later extract some related logic into a custom Hook, for example: Note how we were able to move the useState call for the position state variable and the related effect into a custom Hook without changing their code. React.memo is a function that you can use to optimize the render performance of pure function components and hooks. Don’t do anything there that you wouldn’t normally do while rendering. In our example below we have an expensive function called computeLetterCount (for demo purposes we make it slow … Whether a component is a class or a function that uses Hooks is an implementation detail of that component. It’s possible that in the future React will provide a usePrevious Hook out of the box since it’s a relatively common use case. The problem here becomes evident whenever we click on the Increment button. When a computational biologist starts creating a new biological design, there is a lot of information we must gather to get started. The controlled component You can wrap a function component with React.memo to shallowly compare its props: It’s not a Hook because it doesn’t compose like Hooks do. You can’t use Hooks inside a class component, but you can definitely mix classes and function components with Hooks in a single tree. The hook will return a new value only when one of the dependencies value changes (referential equality). Yes! useCallback(function, [dependency]) The function of the useCallback hook is that it caches all the functions used in the previous render and checks the value of the properties in its dependency each time it is rendered. We recognize this heuristic isn’t perfect and there may be some false positives, but without an ecosystem-wide convention there is just no way to make Hooks work well — and longer names will discourage people from either adopting Hooks or following the convention. Then you can test the component you wrote. (Note: if this seems undesirable to you, see the end of this post.) Tagged with react, hooks, statemanagement, usereducer. the recommended pattern for derived state, even if a child component displays the measured node later. One of the built-in Hooks that was introduced in 16.8 is useMemo. For example, consider code like this: If you first click “Show alert” and then increment the counter, the alert will show the count variable at the time you clicked the “Show alert” button. Most skirt around the issue of calling the remote API from an event handler because Hooks can only be called from the start of a functional component. // Spreading "...state" ensures we don't "lose" width and height, // Note: this implementation is a bit simplified. Things are blowing up in the React community lately! Say we have an application with two basic components: a button that increments consecutively on each click by one and a weather component that displays a city and the weather in that city. If you’re coming from classes, you might be tempted to always call useState() once and put all state into a single object. It means that the result of the function wrapped in React.memo is saved in memory and returns the cached result if it's being called with the same input again. As of November 26, 2018, react native does not correspond to hooks… It helps you find components that don’t handle updates consistently. Even though it is more explicit, it can feel like a lot of “plumbing”. This is how multiple useState() calls each get independent local state. No. However, it’s important to note that by default, React Memo will only shallowly compare complex objects in the props object. A weekly newsletter sent every Friday with the best articles we published that week. It is only safe to omit a function from the dependency list if nothing in it (or the functions called by it) references props, state, or values derived from them. This page answers some of the frequently asked questions about Hooks. Hooks avoid a lot of the overhead that classes require, like the cost of creating class instances and binding event handlers in the constructor. If you miss automatic merging, you could write a custom useLegacyState Hook that merges object state updates. We’ll now see six new renders of our Weather component per each application render, averaging each at about one extra millisecond per render. How much of my React knowledge stays relevant? The useMemo hook is used to memoize the function return value, so that function only recomputes the value when one of its dependencies are changed. To use in functional components we just need to wrap our components with React.memo. In this example, the callback ref will be called only when the component mounts and unmounts, since the rendered

component stays present throughout any rerenders. But you can make children pure too, or even optimize individual children with useMemo. Conveniently, useMemo also lets you skip an expensive re-render of a child: Note that this approach won’t work in a loop because Hook calls can’t be placed inside loops. If you intentionally want to read the latest state from some asynchronous callback, you could keep it in a ref, mutate it, and read from it. In addition, consider that the design of Hooks is more efficient in a couple ways: Traditionally, performance concerns around inline functions in React have been related to how passing new callbacks on each render breaks shouldComponentUpdate optimizations in child components. The following CodeSandbox wraps-up our discussion about re-rendering. Hooks embrace functions, but without sacrificing the practical spirit of React. 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