APIs for Concurrent Rendering in React: Optimizing Asynchronous UI Updates

What is React Concurrent Mode:

React Concurrent Mode makes React applications more efficient by allowing them to handle complex updates in the background without affecting the user experience. It introduces interruptible rendering, time-slicing, and prioritization, which together improve responsiveness and performance, especially in applications dealing with large amounts of data or asynchronous tasks. Here’s an explanation and example for each of the key React Concurrent Mode APIs, including Suspense, useTransition, startTransition, and useDeferredValue:

1. Suspense

• Suspense is a React component that lets you display a fallback UI while waiting for asynchronous operations (like data fetching) to complete. It's often used with concurrent features like React.lazy for code-splitting or to manage data loading states.

In this example, the Suspense component shows the text "Loading Table..." while the Table component is being lazily loaded.

2. useTransition

• useTransition is a hook that allows you to mark a state update as less urgent (transitional). React will prioritize more urgent updates (like typing or clicking) over these transitions, making the UI feel more responsive.

In this example, useTransition delays the state update for the search results, allowing more urgent updates (like typing) to happen without lag.

3. startTransition

• startTransition is a function that works similarly to useTransition. It allows you to mark state updates as transitions explicitly without using hooks. It’s useful when you want to control when certain updates should be considered "less urgent."

In this example, the count updates immediately, but deferredCount is updated as part of a transition, meaning it may not update until after more urgent work is done.

4. useDeferredValue

• useDeferredValue is a hook that allows you to defer updating a value, making it less urgent. This is helpful when you want to ensure the most critical updates (like user interactions) happen before less important ones (like rendering large lists or complex components).

In this example, useDeferredValue delays the rendering of the large list (items) based on the input. As a result, the UI remains responsive while typing in the input field, even though the rendering of the list is deferred.

Summary of Each API:

• Suspense: Used to display a fallback UI until asynchronous content (like lazy-loaded components) is ready.
• useTransition: Marks updates as non-urgent, allowing React to prioritize more critical work.
• startTransition: A function to manually mark updates as transitions.
• useDeferredValue: Defers the rendering of a value to prioritize more urgent updates.

For an in-depth exploration of React Concurrent Mode, be sure to check out this blog:
React Concurrent Mode: Optimizing React Performance