Named layouts thingy

[rChaoz]

Describe the problem

This has been discussed a lot in the past (for example #627), but now with Svelte 5 released, I think we can take a look at this from a different angle, considering all the new Svelte 5 toys.

Describe the proposed solution

Let's consider a rather simple usecase - header that changes based on page. Most pages would want to use the same (default) header, while some can customize it. This could be achieved with snippets or with additional +page-<name>.svelte files, for example +page-appbar.svelte.

With snippets (demo)

Allow pages to export snippets:

<script>
  export const snippets = { appbar }
</script>

{#snippet appbar()} ... {/snippet}

that can then be used in layouts:

<script>
  const { children, appbar } = $props()
</script>

<aside> {@render appbar?.()} </aside>
<main> {@render children()} </main>

Pros: Code sharing between app bar and page, easy to use
Cons: Not SSR-friendly. The page needs to first render before the snippet is created, which then needs to be sent to the layout for re-rendering.

With additional files (demo)

Allow placing additional +page files that can be used in layouts:

• +page.svelte
• +page-appbar.svelte

<script>
	const { children, appbar } = $props()
</script>

<aside> {@render appbar()} </aside>
<main> {@render children()} </main>

Pros: SSR-friendly, intuitive (+page goes to children, +page-x goes to x)
Cons: Need a communication mechanism to synchronize the page and named slots, as they don't share code.

Both of these method can sort-of be achieved in user land, but neither of them is great.

With magic (not demo)

Syntactically I think this is great, but it would need a bit (more) help from the Svelte compiler to work.

<!-- +page.svelte -->
<script>
	const { data, Header, Footer } = $props()
</script>

<Header> This is a header </Header>
Main content
<Footer> This is a footer </Footer>


<!-- +layout.svelte -->
<script>
	const { children, header, footer } = $props()
</script>

<header> {@render header()} </header>
<main> {@render children()} </main>
<footer> {@render footer()} </footer>

For this to work, a div with display: contents would need to be created that would hold the header/footer content, which is rendered inside the snippet, and a virtual component with this div as its target would be passed to the page:

<script>
  import Layout from "./+layout.svelte"
  import Page from "./+page.svelte"

  const headerDiv = createDivDisplayContents()
  const footerDiv = createDivDisplayContents()
  const Header = createComponent({ target: headerDiv })
  const Footer = createComponent({ target: footerDiv })
</script>

<Layout>
  {#snippet header()}
    {@insert headerDiv}
  {/snipper}
  {#snippet footer()}
    {@insert footerDiv}
  {/snipper}
  <Page {Header} {Footer} />
</Layout>

Conclusions

This is pretty unhinged, but would it be doable?

[brunnerh]

This would be something you can do already:

// $lib/layout-slots.svelte.js
import { getContext, setContext } from 'svelte';

const key = Symbol('layout-slots');

export function initSlots() {
	const slots = $state({});
	return setContext(key, slots);
}

export function setSlots(slots) {
	const context = getContext(key);
	Object.assign(context, slots);
}

<!-- +layout.svelte -->
<script>
	import { initSlots } from '$lib/layout-slots.svelte.js';

	let { children } = $props();

	const slots = initSlots();
</script>

<!--
	run page logic first to get slot snippets,
	the page should have no output.
-->
{@render children()}

<h1>{@render slots.heading()}</h1>

{@render slots.children()}

<!-- +page.svelte -->
<script>
	import { setSlots } from '$lib/layout-slots.svelte.js';

	setSlots({ heading, children });
</script>

{#snippet heading()}
	Hello there
{/snippet}

{#snippet children()}
	Main slot content
{/snippet}

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