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Concurrent Rendering in React 18

Maurice de Beijer

Maurice de Beijer

Independent Software Consultant and Trainer

Introduction to Concurrent Rendering in React 18

React 18 introduces a significant advancement in rendering capabilities with concurrent rendering. This feature aims to enhance the performance and responsiveness of applications by allowing React to work more efficiently in handling asynchronous operations and rendering tasks. The primary goal of concurrent rendering is to make applications feel more fluid and responsive, especially when dealing with complex UI updates and data fetching scenarios.

In the traditional rendering model, React processes updates synchronously, which can lead to performance bottlenecks, especially in applications with heavy computational tasks or frequent data fetching. Concurrent rendering changes this by enabling React to interrupt ongoing rendering tasks to handle more urgent updates, ensuring a smoother user experience.

Understanding Suspense and Its Role in React

Suspense is a powerful feature in React that helps manage asynchronous operations, such as data fetching, by allowing components to 'suspend' their rendering until certain conditions are met. This mechanism is crucial for creating seamless user experiences, as it enables developers to display loading indicators or fallback UI while waiting for data to load.

With the introduction of concurrent rendering, Suspense becomes even more effective. It provides a way to coordinate multiple asynchronous operations, ensuring that components render only when their data is ready. This reduces the risk of displaying incomplete or inconsistent UI states, which can occur when rendering tasks are not properly managed.

Exploring the Use of Suspense in React 17 and 18

Suspense was first introduced in React 17, offering a way to manage asynchronous operations within components. In React 18, enhancements to Suspense allow for more flexible and efficient handling of data fetching and rendering tasks. Developers can now take advantage of improved capabilities such as Suspense List and concurrent rendering modes.

One of the key improvements in React 18 is the ability to use Suspense with server-side rendering (SSR). This was not possible in React 17, but with React 18's CreateRoute API, developers can now implement SSR with Suspense, providing a more robust solution for rendering complex UIs.

The Importance of Error Boundaries

Error boundaries are a critical component in React applications, providing a way to catch and handle errors in the component tree. They work similarly to Suspense, allowing developers to specify fallback UI when errors occur. This ensures that applications remain stable and informative, even when unexpected errors arise.

By nesting error boundaries within the component tree, developers can isolate errors to specific parts of the UI. This prevents the entire application from crashing and allows users to continue interacting with unaffected areas of the interface. React 18's concurrent rendering capabilities further enhance error handling by allowing more granular control over how errors are managed during the rendering process.

Nesting Suspense Boundaries for Better UI Control

In React 18, developers can nest Suspense boundaries to gain finer control over the rendering process. This allows for more targeted management of asynchronous operations, ensuring that specific parts of the UI update independently based on their data dependencies.

Nesting Suspense boundaries is particularly useful in complex applications where different components rely on separate data sources. By isolating these components within their own Suspense boundaries, developers can optimize the user experience, minimizing unnecessary re-renders and improving overall application performance.

Coordinating Suspense with Suspense List

React 18 introduces the Suspense List component, which allows developers to coordinate multiple Suspense boundaries within an application. This is particularly beneficial when managing components that rely on different data sources but need to render in a synchronized manner.

With Suspense List, developers can specify the order in which Suspense boundaries resolve, ensuring a consistent and predictable UI. Options such as 'together', 'forwards', and 'backwards' provide flexibility in managing how components display their loading states and final content, enhancing the overall user experience.

Leveraging Concurrent Rendering for Performance

Concurrent rendering is a game-changer for React developers, offering a way to handle complex rendering tasks more efficiently. By breaking down rendering processes into smaller, manageable chunks, React can prioritize updates and improve application responsiveness, especially in scenarios with heavy computational demands.

This new rendering model allows React to pause and resume rendering tasks as needed, ensuring that high-priority updates, such as user interactions or data fetches, are handled promptly. This leads to a more fluid and responsive application, even under heavy load.

Utilizing Start Transition for Responsive UIs

The Start Transition API in React 18 provides a way to differentiate between high-priority and low-priority rendering tasks. By marking certain updates as low-priority, developers can ensure that crucial interactions, such as user inputs, remain responsive while less critical updates are delayed until resources are available.

This approach is particularly useful in scenarios where UI elements need to remain interactive while background computations are performed. Start Transition allows developers to optimize the user experience by prioritizing important updates, resulting in a more responsive and efficient application.

Enhancing User Feedback with Use Transition

Use Transition is another powerful tool in React 18 that complements Start Transition by providing additional feedback during rendering transitions. It enables developers to track the pending state of transitions, allowing for more informed UI updates and visual cues.

By leveraging Use Transition, developers can create more interactive and informative user interfaces. This API provides insight into the rendering process, enabling more precise control over how components update and display loading states, further enhancing the user experience.

Conclusion

React 18's concurrent rendering and Suspense features mark a significant step forward in managing complex UI updates and asynchronous operations. By enabling more efficient rendering processes and providing greater control over component states, developers can create more responsive and user-friendly applications.

The combination of concurrent rendering, Suspense, and new APIs like Start Transition and Use Transition offers developers the tools needed to build highly performant and interactive applications. As React continues to evolve, these features will play a crucial role in shaping the future of web development, ensuring that applications remain efficient, responsive, and capable of handling the demands of modern user experiences.

Watch full talk with demos and examples:

From Author:

With the release of React 18 we finally get the long awaited concurrent rendering. But how is that going to affect your application? What are the benefits of concurrent rendering in React? What do you need to do to switch to concurrent rendering when you upgrade to React 18? And what if you don’t want or can’t use concurrent rendering yet?

There are some behavior changes you need to be aware of! In this workshop we will cover all of those subjects and more.

Join me with your laptop in this interactive workshop. You will see how easy it is to switch to concurrent rendering in your React application. You will learn all about concurrent rendering, SuspenseList, the startTransition API and more.

This workshop has been presented at React Advanced 2021, check out the latest edition of this React Conference.

FAQ

React 18 focuses on introducing concurrent rendering capabilities that allow for more efficient handling of updates by breaking them into smaller chunks. This helps in managing priorities of rendering tasks and can interrupt lower-priority rendering to handle more urgent updates, enhancing the application's responsiveness.

The 'useTransition' hook in React 18 allows developers to mark updates as non-urgent or low priority. This enables the application to stay responsive by delaying these updates until higher-priority tasks are completed, thus preventing less important updates from blocking the user interface.

The 'suspense list' component in React 18 allows developers to control the order in which multiple suspense boundaries reveal their content. It supports different configurations such as 'forwards', 'backwards', and 'together' to manage how grouped suspense boundaries handle loading states, improving the coordination of loading sequences in complex applications.

The 'startTransition' API in React allows developers to specify non-urgent updates, which can be interrupted if more important updates arrive. This helps in keeping the application responsive by not blocking user interactions with less critical updates, ultimately enhancing user experience by prioritizing more critical tasks.

Yes, 'useTransition' can be used along with 'suspense' in React applications. While 'useTransition' allows for deferring less urgent updates, 'suspense' handles the loading states of components. Using them together can significantly enhance the handling of asynchronous operations and the responsiveness of applications.

While the exact release date for React 18 is not specified, it is expected to be released after an alpha phase, followed by beta and release candidate phases. The release is hoped to be around late 2021 or early 2022, depending on the completion of testing and feedback incorporation.

React 18 introduces new capabilities for server-side rendering, particularly around the 'suspense' feature, which was previously not supported. This allows developers to use suspense for data fetching on the server, improving the performance and user experience by rendering placeholder content until the data is fully loaded.

react 18 react concurrent rendering react concurrent mode react suspense

Maurice de Beijer

Maurice de Beijer

132 min

27 Oct, 2021

Comments

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Narges Haeri
Aspiring Professional
Thank you so much for such great content. Your workshop was incredibly helpful!
22:38 Dec 25, 2023

Video Summary and Transcription

The workshop covers the new features of React 17 and React 18, including suspense, error handling, and concurrent mode. It emphasizes the importance of hands-on learning and typing code instead of just copying. The workshop explores using suspense and error boundaries to handle errors and improve the user experience. It also introduces new features in React 18, such as start transition and useTransition, to enhance responsiveness. The interaction between suspense and start transition is discussed, highlighting their collaboration in handling fallback UI and communication.

Available in Español: Aventuras de Renderizado Concurrente en React 18

1. Introduction to Concurrent Rendering

Welcome to the workshop on concurrent rendering adventures in React18. I'm Maurice de Beer, a freelance developer instructor and Microsoft MVP. Follow me on Twitter and check out my website for more information. Subscribe to my weekly React newsletter for updates. You'll receive a copy of the slides as well.

Read also

React Suspense: Building Better User Experiences Through Modern Loading Patterns

2. Exploring React 17 and React 18 Features

We'll explore the new features of React 17 and React 18, including suspense, handling errors, rendering existing applications, suspense list, transitions, and concurrent mode. Please note that we won't cover all the features of React 18, such as server-side rendering with suspense.

3. Advice on Learning and Typing

Your memory works better if you do things. Try and do the exercises instead of just watching and copying code. Making typos and recognizing errors will help you learn. Copying and pasting isn't enough in real application development. Typing things out is beneficial for learning.

4. Prerequisites and Setup

To start, ensure you have Node and npm set up. Check the versions using 'node -v' and 'npm -v'. Clone the starter repository from GitHub and install the npm packages. Open Visual Studio Code and run 'npm start'. The application displays a list of users with details and a list of prime numbers. The rendering can be slow, especially with larger numbers. The workshop includes interactive slides with code examples and links to the GitHub repository.

5. Breakout Rooms and Task Execution

GitHub is playing up. We're going to use the Zoom breakout rooms. I'll open them whenever it's time to do something. Most tasks are relatively short, so I'll open them in like five minutes. If you're done with whatever needs to be done, feel free to leave the breakout room, come back to the main room.

6. Understanding React 17 and Suspense

We start with React 17 and use suspense to suspend rendering of components. Suspense is triggered by throwing a promise, typically an AJAX request, inside the render. When the promise resolves, the component is re-rendered. Errors are handled by error boundaries. The SWR library can be configured to use suspense. Adding a suspense boundary and fallback mechanism fixes broken applications. TypeScript is used in this workshop.

7. Using Suspense and Error Boundaries

In this section, we learned about using suspense in React 17. We used TypeScript to handle potential undefined values and eliminated unnecessary code. We also explored the SWR library for data fetching and added suspense boundaries to our application. The loading behavior can be improved, which we'll address later.

8. Exploring Error Handling and Suspense Boundaries

Let's open the breakout rooms again for a longer duration. I'll provide the repository link to Ravi. Then, we'll explore error handling, orchestrating suspense boundaries, and the resulting outcomes.

9. Error Boundaries and Handling Errors

In this section, we learned about error boundaries in React and how they handle errors in components. We saw that without an error boundary, React unmounts the whole application when an error occurs, resulting in a blank page for the end user. However, by adding error boundaries, we can catch errors in specific components and have the rest of the application continue rendering. We also explored nesting error boundaries and the benefits of doing so. I recommend using the standard React error boundary npm package, which allows for error retries. Please add error boundaries to your application. Next, we'll move on to nesting suspense components.

10. Nesting Error Boundaries and Suspense Boundaries

The suspense component catches promises that are thrown, while the error boundary catches errors. React starts at the component that triggers suspense and walks up the tree to find the first error boundary. The difference between the two is that the error boundary has a fallback property that can potentially suspend, while the suspense component is inside the error boundary. It's recommended to have an error boundary at the root of the application to handle all errors. Nesting error boundaries and suspense boundaries is useful. Putting a suspense boundary around the app and a suspense boundary around the specific component can improve the user experience. However, it's important to consider the impact on the application's performance. Keeping a list of users and showing a spinner for user details can be achieved by nesting suspense components.

11. Nesting Suspense Boundaries

We can nest suspense boundaries to achieve a decent UI. React 18 respects null as the fallback for suspense, unlike React 17. Nesting suspense components serves us well even with React 17. Let's move on to suspense in parallel, which is similar to error boundaries.

Parallel Suspense and Handling Questions

We can add suspense in parallel by duplicating suspense components for different details. However, the UI may not be perfect, and React 17 doesn't provide much control over independent suspensions. With React 18, we'll have suspense list components that coordinate multiple suspense boundaries and allow more control over rendering and resolution. After the break, a question was asked about making an existing network layer compatible with suspense, and a demonstration of throwing promises in custom code was shown.

Suspend and React 18 Installation

We explore how to suspend a component using a new promise. By throwing a new promise, we can suspend the component. Resolving the promise after a timeout resolves the suspension. This technique can be used in any function called from the render cycle. Moving the code outside the render cycle, such as in an event handler, will not cause suspension. We then switch to React 18 and discuss the installation process. The latest version of React 18 can be installed using npm, and there are different tags available, including 'latest' and 'next' for the alpha version.

Installing React 18 and Updating Code

To install React 18, use the 'next' branch with the tag next. Use 'npm install React-at-next' and 'React-dom-at-next' with the '--force' flag to install the dependencies. React Router DOM 6 is compatible with React 18 and React 17. The router API has changed, now based on hooks. Use the 'useRoutes' hook to define routes. React DOM.render is no longer supported in React 18, use React root instead. Update the code to use 'createRoot' and pass the DOM element as a parameter. TypeScript may show a compile error, but it can be fixed by using the exclamation mark to assert the type of 'getElementById'.

React 18 Rendering and New Hooks

We're using React 18 rendering, but the application is still sluggish. React 18 introduces new hooks, including useDivertValue, useTransitionHook, useMutableSource, and useOpaqueIdentifier (or useID). useOpaqueIdentifier is useful for automatically focusing input fields associated with labels. It can be challenging to achieve this behavior when rendering on both the server and client sides, but useOpaqueIdentifier simplifies the process. Note that useOpaqueIdentifier may be renamed to useID in the current version of React 18.

Using the useOpaqueIdentifier Hook

The useOpaqueIdentifier hook in React 18 provides an opaque ID, which is a string without any meaning. It is useful for server-side rendering and recommended to use in such cases.

Using the Suspense List Component

We can use the suspense list component to control multiple suspense boundaries and customize their display, resolution, and fallback components. By specifying the reveal order as 'together', the suspense components inside the suspense list will resolve simultaneously, resulting in a better user experience. The 'forwards' option allows us to decide how to handle remaining components, with the 'hidden' value hiding them. By adding a key to the component, we ensure that it is always remounted, providing consistent behavior. The suspense list is a valuable addition to managing individual suspense boundaries and controlling their interaction.

Concurrent Rendering and Mode

React 18 introduces concurrent rendering, which allows rendering in chunks and provides small time slices for JavaScript runtime to execute important tasks. It's not multi-threading, but it coordinates concurrent mode where React lets someone else execute between rendering chunks.

React State and Rendering

React detects state changes in the component tree and determines whether to continue rendering or start over. If a state change occurs, React checks if any APIs like useState or setStates were called, which may require a redraw. To prevent invalid UI states, React provides APIs for state management libraries to notify it of state updates and decide whether to re-render the complete or partial component tree.

Improving Responsiveness with Start Transition

When we installed React 18 and started using create route, we began using concurrent rendering. However, the prime numbers list remained sluggish. By default, applications behave the same way as before. To make a difference, we can use the start transition API to differentiate between high and low priority work. By assigning the prime number part as a lower priority than the range input value, we can improve responsiveness. This change, achieved by using start transition, allows the list of prime numbers to update only when the value stabilizes, resulting in a more responsive and faster application. The difference between start transition and debounce or throttle is that start transition is not time-based, but rather based on available CPU time. This simple change has a significant impact on the UI and responsiveness of the application.

Introduction to Start Transitions

Start transitions should be used judiciously. In some cases, like a responsive slider, it makes sense to use them. However, there is a drawback to start transition - it doesn't provide feedback when the list of prime numbers becomes stale. We'll explore another API that addresses this issue.

Using the useTransition API in React 18

In this section, we learned how to use the useTransition API in React 18 to start transitions and get the current state of pending transitions. By using useTransition, we can show a specific UI when in a transition state. We made a simple change in the PrimeNumbers component to use useTransition instead of startTransition, and passed the isPending flag to the CheckNumber component to show a sandglass icon during transitions. This change improved the responsiveness of the UI, and we can now see the pending state during transitions. Please make this change in your code. After that, we'll discuss the reasons for having both startTransition and useTransition APIs. Note that useTransition is unrelated to suspense and does not involve throwing promises.

Using StartTransition and UseTransition

You don't need to use suspense. In fact, there is no suspense components, or actually there is at the root, but not specifically used for this. StartTransition is faster, but no additional feedback. So, yeah, they both have their advantages and disadvantages and both are useful. With Use Transition, I can show the fact that we're loading a new user here inside of this list and have some more control over how everything appears. The click and the transition here are actually in different suspense boundaries now, so they don't cooperate. But if I go into user details, and let's get rid of these suspense boundaries, now they live in the same suspense boundary. We see it's been grayed out, and we don't see any spinners anymore, but we're inside of a transition, and we control exactly what we want to do.

Improving User Experience with useTransition

To improve the user experience, we can update the active style of the user immediately upon clicking, rather than waiting for the transition to complete. This can be achieved by duplicating the state and using the useTransition hook. By using useTransition, we can have full control over how the user details are updated and prevent spinners from appearing. We can even render additional indicators, such as an hourglass, conditionally to show that the user is loading. It's important to duplicate the state and set the state inside the transaction for the data we don't care about immediately, and set the state outside the transaction for the data we do care about immediately. This change can significantly improve the responsiveness of the application. Please make this change and I'll see you all back here in five minutes.

Interaction between Suspense and Start Transition

Suspense and start transition work together to handle the fallback UI and communicate between each other. The React source code is complex and developed by talented individuals.

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