feat: forking

[Rich-Harris]

Definitely WIP. Opening this so that we can discuss the API and start coming up with more test cases. I've no doubt that this is missing all sorts of details, though at the same time I'm very pleasantly surprised at how little code it took to get this far. It gives me faith in the primitives we've built.

The idea is that you need to be able to change some state 'offscreen' in such a way that you can discover any async work resulting from the state change without committing it to the screen. This allows you to, for example, load the data that will be needed on the next page before the user navigates there. Future SvelteKit will use this to preload remote function calls, much as it preloads load functions today.

You can see an example here — if you click on the <a> elements fast enough, there will be a delay before navigation occurs, since it's doing async work (though the nav bar updates immediately, courtesy of $state.eager(...). But if you hover for a beat before clicking — which users generally do — the fork will be ready by the time navigation occurs.

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🦋 Changeset detected

Latest commit: 3a27726

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@17004

[dummdidumm]

Great stuff! Added a few comments + we might have a short description of this in the await documentation

[Rich-Harris]

Added some docs and whatnot, though realised in the course of creating this example that there's a memory leak at the moment — if you discard a fork, the effects that were created inside it don't get destroyed. You can see that my hovering over the button, hovering out, waiting a second, then immediately clicking on it — that should redo the work (since we discard() on pointerleave), but since the effect already exists in the tree that doesn't happen. Looking into it

[elliott-with-the-longest-name-on-github]

love it, just a nit. it's amazing that this is as simple as it is