Angular SSR: Respond to scroll events to set css class

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04-10-2024

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Angular SSR: Dynamically Adding CSS Classes on Scroll with Server-Side Rendering

Server-Side Rendering (SSR) in Angular is a powerful technique for improving SEO and initial page load times. However, it can introduce challenges when working with features that rely on user interactions, such as scroll events.

Imagine a scenario where you want to dynamically add a CSS class to an element when the user scrolls past a certain point on the page. In a traditional client-side rendered application, this would be a simple task using JavaScript event listeners. However, with SSR, the initial rendering happens on the server, before the browser can process any JavaScript. This creates a problem as the scroll event wouldn't be triggered, and the CSS class wouldn't be applied until the browser fully loads the page.

The Problem:

• SSR renders the page before the browser can process JavaScript.
• Scroll events that are triggered by the user won't fire on the server, making it impossible to apply CSS classes based on scroll position.

The Solution:

To tackle this, we need a way to simulate the scroll event and apply the CSS class on the server-side. This can be achieved by using a combination of Angular's AfterViewInit lifecycle hook and a library like @ngneat/scroll.

Implementation Example:

import { Component, AfterViewInit, ElementRef, ViewChild } from '@angular/core';
import { ScrollService } from '@ngneat/scroll';

@Component({
  selector: 'app-scroll-class',
  template: `
    <div class="container" #scrollContainer>
      <div class="content" [ngClass]="{'scrolled': isScrolled}">
        <!-- Your content goes here -->
      </div>
    </div>
  `,
  styles: [`
    .scrolled {
      background-color: #f0f0f0; /* Example style change */
    }
  `]
})
export class ScrollClassComponent implements AfterViewInit {
  @ViewChild('scrollContainer') scrollContainer!: ElementRef;
  isScrolled = false;

  constructor(private scrollService: ScrollService) {}

  ngAfterViewInit(): void {
    this.scrollService.on('scroll')
      .pipe(
        // Add the scroll position logic based on your requirements
        // Example: Trigger the change when the scroll position exceeds 100px
        map(() => window.pageYOffset > 100),
      )
      .subscribe((isScrolled) => {
        this.isScrolled = isScrolled;
      });
  }
}

Explanation:

1. Import @ngneat/scroll: This library provides helpful utilities for listening to scroll events.
2. Create a component: The ScrollClassComponent contains the scrollContainer element and a content element that will have the CSS class scrolled applied when the user scrolls.
3. Use @ViewChild: Retrieve a reference to the scrollContainer element.
4. Implement AfterViewInit: Inside ngAfterViewInit, we use the ScrollService from @ngneat/scroll to listen for scroll events.
5. Use map operator: Transform the scroll event into a boolean value indicating whether the scroll position has exceeded a certain threshold.
6. Update isScrolled: Update the isScrolled property based on the transformed scroll event.
7. Conditional CSS class: Use [ngClass] directive to apply the scrolled class to the content element based on the value of isScrolled.

Key Points:

• This implementation leverages server-side rendering by simulating the scroll event and updating the isScrolled property.
• The CSS class is applied based on the value of isScrolled, ensuring the correct visual state is rendered on the initial page load.
• Angular's change detection mechanism will automatically update the DOM when isScrolled changes, ensuring the CSS class is applied correctly.

Benefits:

• Improved SEO: The page renders with the correct CSS classes applied, making it visible to search engines.
• Faster perceived load times: The user sees the styled elements immediately, even before JavaScript fully loads.
• Enhanced user experience: Provides a smooth transition between the initial server-rendered page and the fully interactive client-side version.

Further Optimization:

• Lazy loading: If your application is large, you can lazy load the @ngneat/scroll library to reduce the initial bundle size.
• Performance tuning: Optimize the scroll event logic and the map operator to avoid unnecessary calculations.
• Custom scroll events: You can customize the scroll event logic to trigger different CSS classes based on different scroll positions or other conditions.

Conclusion:

By using a library like @ngneat/scroll and Angular's lifecycle hooks, you can dynamically apply CSS classes based on scroll events in SSR applications. This approach improves the user experience, enhances SEO, and ensures a seamless transition from server-side to client-side rendering.