feat: Add handleLoad hooks on client and server

[Lms24]

This PR is a PoC implementation of our feature request from #9542. It adds support for a handleLoad hook in hooks.client.js as well as handleLoad and handleServerLoad hooks in hooks.server.js.

The usage of these hooks is very similar to the usage of the server-side handle hook, where users are provided with an event and a resolve function.

Please take a look at #9542 for further details.

At this stage, this PR definitely needs more tests and of course a proper review. I'm more than happy to continue working on this if you decide to go forward with the proposed handleLoad hooks. Please feel free to request changes or apply changes yourself - whatever you prefer :)

Also, happy to split this PR up into the individual hooks if you prefer that!

EDIT: I only ran pnpm changeset after opening this PR (sorry!). Should I re-open a new PR or does this still work?

---

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[changeset-bot]

🦋 Changeset detected

Latest commit: f7de955

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@sveltejs/kit	Minor

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[david-plugge]

Awesome stuff!

Would this also allow some sort of locals for shared load functions?

// hooks.client.js

export const handleLoad = ({event, resolve}) => {
  event.locals.myFetch = createFetchClient(event);
  //       ^ better choose a different name

  return resolve(event);
}

// routes/some/page/+page.js

export const load = async ({ locals }) => {
  const res = locals.myFetch(...)
  ...
}

This would make libraries so much easier to build while also resolving the waterfall problem (await parent()) for global state.

[Lms24]

I haven't touched the type of LoadEvent or ServerLoadEvent types which we'd probably need to do if we wanted to add properties, like locals there. But generally, yes, I guess adding properties to event in the hook could work. Depending on how #9530 is going, we might actually use the handleLoad hook to instrument event.fetch.

I'd argue though that event type expansion is a bit out of scope for this intial PR/feature but I'd leave it up to the Kit maintainers to decide on that :)

[dummdidumm]

Re changeset: Just amend the PR, changesets will pick it up, and we sqash everything anyway.

[david-plugge]

Seems like this pr got forgotten. I would absolutely love to see shared locals in the future and this pr would be a good first step.

[dummdidumm]

What do you mean by "shared locals"? Could you add your use case to the related issue?

[david-plugge]

What do you mean by "shared locals"? Could you add your use case to the related issue?

Basicly event.locals for shared load functions. Would make it easier to support custom fetch functions and sdks like supabase.

[devunt]

Would like to see this get merged since we are also leveraging dynamic load function generation on +page.ts file. For now we put empty +page.ts file besides of +page.svelte file everytime so vite transform plugin can pick it up.

[dummdidumm]

Closing in favor of #11313 - thank you!