How to use Reacts concurrent mode

This article introduces you to the idea behind Reacts concurrent mode as well as some of its usage and boons. Reacts concurrent mode is an innovative set of ingredients designed to better the handling of asynchronous giveing. These betterments make for a better end-user experience.

One of the perpetual issues that has obstinate web clients since time primitive is intercourse with giveing of asynchronous updates. The React team continues its tradition of introducing ambitious solutions into the framework by adding concurrent mode support to the React 16.x release line.

[ Also on InfoWorld: How to use React officeal ingredients ]

There are a difference of cases where naive giveing of changing state leads to less-than-desirable conduct including wearisome loading screens choppy input handling and unnamed spinners to name a few.

Addressing such issues piecemeal is error-prone and incongruous. Reacts concurrent mode represents a wholesale baked-into-the-framework solution. The core idea: React now draws updates concurrently in remembrance supports interruptible giveing and proposes ways for application code to interact with that support.

Enabling concurrent mode in React

The API for harnessing these capabilities is quiet in flux and you have to establish it explicitly like so:

npm establish [email protected] [email protected]

Concurrent mode is a global change to the way React works and requires that the root level node be passed through the concurrent engine. This is done by calling createRoot on the app root instead of just recoilDOM.give(). This is seen in Listing 1.

Listing 1. Using the concurrent giveer

).give(lt;App /gt;);

Note that createRoot is useful only if youve established the experimental package. And owing it is a primary change existing codebases and libraries are likely not congruous with it. Eparticularly the lifecycle methods that are now prepended with UNSAFE_ are not congruous.

Because of this fact React introduces a middle step between the old-school give engine that we use today and the concurrent mode. This step is named ’blocking mode’ and it is more backward congruous but with fewer concurrent ingredients.

In the long-term concurrent mode will befit the lapse. In the mid-term React will support the following three modes as described here:

  1. Legacy mode: ReactDOM.give(lt;App /gt; rootNode). The existing legacy mode.
  2. Blocking mode: ReactDOM.createBlockingRoot(rootNode).give(lt;App /gt;). Fewer breaking changes fewer ingredients.
  3. Concurrent mode: ReactDOM.createRoot(rootNode).give(lt;App /gt;). Full concurrent mode with many breaking changes.

A new giveing standard in React

Concurrent mode primaryly alters the way React gives the interface to allow the interface to be giveed while fetching of data is in progress. This resources that React must know something almost your ingredients. Specifically React must now know almost the data-fetching status of your ingredients.

Reacts new Suspense ingredient

The most jutting ingredient is the new Suspense ingredient. You use this ingredient to enlighten React that a given area of the UI is hanging on asynchronous data loading and give it the status of such loading.

This cleverness acts at the framework level and resources that your data-fetching library must active React to its status by implementing the Suspense API. Currently Relay does this for GraphQL and the recoil-suspense-fetch project is tackling REST data fetching.

To repeat you are now required to use a more intelligent data fetching library that is capable of effective React what its status is thereby allowing React to optimize the way your UI gives.

Look at this sample from the React samples. Listing 2 has the significant view template details.

Listing 2. Using Suspense in the view

lt;Suspense fallback={lt;h1gt;Loading;/h1gt;}gt;
      lt;ProfileDetails /gt;
      lt;Suspense fallback={lt;h1gt;Loading;/h1gt;}gt;
        lt;ProfileTimeline /gt;

Notice that Suspense allows for the determination of alternate loading full. This is analogous to how you might use different recur values within a ingredient based on its loading status in the old giveing engine to give a placeholder until the data is prompt.

Inside the ingredients used by this view template no particular code is required to deal with the loading state. This is all now feeld behind the scenes by the framework and data fetching library.

For sample the ProfileDetails ingredient can innocently load its data and recur its markup as in Listing 3. Again this is hanging on the data store (in Listing 3 the resource object) implementing the Suspense API.

Listing 3. ProfileDetails

office ProfileDetails() {
  const user =; 
  recur lt;h1gt;{}lt;/h1gt;;

Concurrency in data fetching

An significant boon of this setup that bears repeating is that all data fetching will befall concurrently. So your UI boons from both an betterd give lifecycle and a single and automatic way to accomplish correspondent data fetching for multiple ingredients.

Reacts useTransition hook

The next major tool in your new concurrent React kit is the useTransition hook. This is a more fine-grained tool that allows you to tune how UI transitions befall. Listing 4 has an sample of wrapping a transition with useTransition.

Listing 4. useTransition

office App() {
  const [resource setResource] = useState(initialResource);
  const [ startTransition isPending ] = useTransition({ timeoutMs: 3000 });
  recur (
         onClick={() =gt; {
            startTransition(() =gt; {
              const nextUserId = getNextId(resource.userId);
       }}gt; Next lt;/buttongt;
        {isPending ? " Loading..." : null} 
       lt;ProfilePage resource={resource} /gt;

What the code in Listing 4 says is ’Delay showing the new state up to three seconds.’ This code works owing the ProfilePage when used is wrapped by a Suspense ingredient. React begins fetching the data and instead of showing the placeholder shows the existing full for as a long as the defined timeoutMs. As soon as fetching is complete React will show the updated full. This is a single mechanism for improving the perceived accomplishment of transitions.

The startTransition office unprotected by useTransition allows you to wrap the fetching portion of code while the isPending office exposes a boolean flag you can use to feel conditional loading show.

All of this enchantment is practicable owing Reacts concurrent mode has implemented a kind of background giveing mechanism: React gives your updated state UI in remembrance in the background while fetching is happening. You can get a more detailed perception of how this works here.

Reacts useDeferredValue hook

Our last sample involves fixing the problem of choppy typing when typing causes data loading. This is a fairly canonical problem that is frequently solved with debounce/throttle on the input. Concurrent mode opens up a more congruous and smoother solution: the useDeferredValue hook.

An sample is here. The talent of this solution is that it gets you the best of both worlds. The input stays answering and the list updates just as soon as the data is useful.

Listing 5. useDeferredValue in action

const [text setText] = useState("hello");
  const deferredText = useDeferredValue(text {    timeoutMs: 5000  });
  // ....  
  lt;MySlowList text={deferredText} /gt; 

Similar to how we wrapped a transition with useTransition we are wrapping a resource value with useDeferredValue. This allows the value to stay as-is for as long as the timeoutMs value. All the complexity of managing this betterd give is feeld behind the curtain by React and the data store.

Solution to race conditions

Another boon by using Suspense and concurrent mode is that race conditions introduced by manually loading data in lifecycle hooks and methods are avoided. Data is guaranteed to arrive and be applied in the order it is requested. (This is correspondent to how Redux fixes race conditions.) Therefore the new mode obviates the need for manually checking data staleness due to the interleaving of request responses.

These are some of the highlights to the new concurrent mode. They propose compelling boons that will befit the norm going advanced.