spiffytech

Recently, while building a simple Reddit clone, I wanted to lazy-load images and comments. That is, rather that loading all of the images and comments the instant I added a component to the DOM, I wanted to wait until the component was actually visible. This spreads out the impact of loading a page, both for the client and the server.

To do this, I used the IntersectionObserver API to create a higher-order Svelte component:

VisibilityGuard.svelte:

<script>
  import { onMount } from "svelte";

  let el = null;

  let visible = false;
  let hasBeenVisible = false;

  onMount(() => {
    const observer = new IntersectionObserver(entries => {
      console.log("entry", entries[0]);
      visible = entries[0].isIntersecting;
      hasBeenVisible = hasBeenVisible || visible;
    });
    observer.observe(el);

    return () => observer.unobserve(el);
  });
</script>

<div bind:this={el}>
  <slot {visible} {hasBeenVisible} />
</div>

This renders a div and stores the rendered HTML element in the el variable. Once the component has been rendered (onMount), we use IntersectionObserver to watch the component, calling a callback whenever the element intersects with our viewport. We track whether the element is currently visible, and whether it has ever been visible, and we pass those to our child component (slot).

So how do we use this component?

App.html:

<script>
	import VisibilityGuard from './VisibilityGuard.svelte';
	
	const images = new Array(100).fill(null).map((n, i) => [
		Math.floor(Math.random() * 500),
		Math.floor(Math.random() * 750)
	]);
</script>

{#each images as [x, y]}
	<VisibilityGuard let:hasBeenVisible>
		<div style="border: 1px solid black; min-height: 40px; min-width: 40px; padding: 5px; border-radius: 5px; margin-bottom: 5px;">
			<header>I am {hasBeenVisible ? 'visible' : 'invisible'}</header>
			<img src={hasBeenVisible ? `https://placekitten.com/${x}/${y}` : null} alt="a kitten" />
		</div>
	</VisibilityGuard>
{/each}

We import the VisibilityGuard component and use it to wrap our image. The let:hasBeenVisible directive declares a new variable that comes from our higher-order component, and we use that to determine whether to display our image (by setting or not setting a src attribute).

And that's that! Here's a REPL of this in action.