Loading UI and Streaming are important features in Next.js that help enhance the user experience by optimizing how content is loaded and displayed. These features allow for a smoother, more responsive application by effectively handling content loading states and progressively serving data as it becomes available.

1. Loading UI in Next.js

Loading UI in Next.js refers to the implementation of components that visually indicate to users that content is still loading. This ensures users are not left with a blank screen while waiting for data to arrive.

a. Loading States in Server-side Rendering

Next.js provides different data-fetching methods such as getStaticProps, getServerSideProps, and API routes that can benefit from custom loading states.

• You can manage loading states using a combination of client-side state management tools like useState and useEffect.

Example:

import { useState, useEffect } from 'react';

const Page = () => {
  const [loading, setLoading] = useState(true);
  const [data, setData] = useState(null);

  useEffect(() => {
    async function fetchData() {
      try {
        const response = await fetch('/api/data');
        const result = await response.json();
        setData(result);
      } catch (error) {
        console.error('Error fetching data:', error);
      } finally {
        setLoading(false);
      }
    }

    fetchData();
  }, []);

  if (loading) {
    return <div>Loading...</div>;
  }

  return (
    <div>
      <h1>Data Loaded</h1>
      <pre>{JSON.stringify(data, null, 2)}</pre>
    </div>
  );
};

export default Page;

In this example:

• While data is being fetched, the <div>Loading...</div> is shown to the user.
• Once the data is loaded, it displays the content.

b. Loading UI in App Router (Next.js 13 and Later)

In Next.js 13, the App Router was introduced, which comes with a new, simplified approach to managing loading states for pages, called Loading UI segments.

You can define a loading.js file within a folder that represents a route. This file is automatically used by Next.js to render a loading state while the content is being loaded.

• Directory Structure:

  app/
    dashboard/
      page.js
      loading.js
  

• loading.js Example:

  // app/dashboard/loading.js
  export default function Loading() {
    return <div>Loading Dashboard...</div>;
  }
  

When a user navigates to /dashboard, the loading.js component is shown while page.js is still loading. This allows for a seamless experience where users see a loading indicator until the content is ready.

c. Suspense for Data Fetching

Next.js supports React's Suspense for data fetching, especially with the App Router in Next.js 13. Suspense allows components to "wait" for something, such as data, before rendering.

• Example:

  import { Suspense } from 'react';
  import UserData from './UserData';
  
  export default function Page() {
    return (
      <div>
        <h1>User Profile</h1>
        <Suspense fallback={<div>Loading user data...</div>}>
          <UserData />
        </Suspense>
      </div>
    );
  }
  

In this example, while UserData is loading, the fallback <div>Loading user data...</div> is displayed, providing a smoother experience.

2. Streaming in Next.js

Streaming in Next.js allows for progressively rendering parts of a page as they become available. This means the user can start interacting with some content while the rest is still being loaded. Streaming is especially powerful for enhancing perceived performance and minimizing the time users wait for an entire page to load.

a. React Server Components and Streaming

With the App Router introduced in Next.js 13, you can leverage React Server Components and streaming to improve server-side rendering. Instead of waiting for all components to finish rendering on the server, parts of the UI are streamed to the client as soon as they are ready.

Streaming works well for pages that have:

• Data from multiple sources.
• Slow-to-load components.
• Content that doesn’t need to be loaded all at once.

b. How Streaming Works in Next.js

• Streaming in Next.js works by leveraging React's concurrent features, where server-rendered HTML is progressively sent to the client.
• Pages are split into segments, and each segment is streamed as it becomes available, providing users with visible content sooner.

Consider a page where some components rely on API calls that may be slow. Instead of delaying the entire page until everything is ready, streaming sends segments immediately as they’re rendered on the server, giving users a faster initial view of the page.

c. Example of Streaming in Next.js

In Next.js 13, you can use a combination of server components and client components to enable streaming.

• Example:

  // app/profile/page.js
  import UserDetails from './UserDetails';
  import UserPosts from './UserPosts';
  
  export default function ProfilePage() {
    return (
      <div>
        <h1>User Profile</h1>
        {/* UserDetails is rendered immediately */}
        <UserDetails />
        {/* UserPosts might be streamed later as they become available */}
        <UserPosts />
      </div>
    );
  }
  

In this example:

• UserDetails can be rendered immediately while UserPosts might take longer (perhaps due to a data fetch). Streaming allows the page to be progressively enhanced as data arrives, ensuring a faster load experience.

d. Using Suspense for Streaming Components

Suspense can also be combined with streaming to indicate to users that some parts of the page are loading while others are available.

import { Suspense } from 'react';
import UserDetails from './UserDetails';
import UserPosts from './UserPosts';

export default function ProfilePage() {
  return (
    <div>
      <h1>User Profile</h1>
      <UserDetails />
      <Suspense fallback={<div>Loading user posts...</div>}>
        <UserPosts />
      </Suspense>
    </div>
  );
}

In this example:

• UserDetails will be rendered immediately.
• While UserPosts is loading, the fallback UI (<div>Loading user posts...</div>) is displayed.

3. Benefits of Loading UI and Streaming

• Improved User Experience: Loading states provide visual feedback, so users understand that the application is processing and hasn't crashed.
• Faster Time to Interactive: Streaming reduces the time until users see meaningful content, allowing them to interact with the page even if some parts are still loading.
• Perceived Performance: By rendering critical content quickly while progressively loading non-critical parts, users perceive the page as faster.

Summary

• Loading UI:

  • Helps manage loading states, providing feedback to users while content is being fetched.
  • Implemented with client-side tools like useState, useEffect, or using components like loading.js in Next.js 13.
  • Suspense can be used to handle asynchronous components with fallback loading UI.

• Streaming:

  • Allows progressively rendering parts of a page as data becomes available.
  • Leveraged in React Server Components in the App Router (Next.js 13).
  • Enhances user experience by reducing time to first contentful paint, giving users visible content faster.

Together, Loading UI and Streaming ensure a more seamless and responsive experience, minimizing waiting times for users and enhancing the performance of Next.js applications.