Prefetch timeout

Describe the problem

SvelteKit include the ability to prefetch pages while the user is hovering over a link. However, a the prefetched data might not be relevant after a period of time, so when the user will click on the link, say after a minute, she will get old data.

Describe the proposed solution

I think the simple solution is to allow to specify timeout. Either allow to specify timeout in a syntax similar to <a sveltekit:prefetch={timeout: (time in seconds)} ...>, or get the timeout from an optional “timeout” property in the return value of load() function, or even support both solutions.

A better/additional aprouch to the simple “timeout” property in my opinion, is to allow the developer to give a predicate: function dataValidity(lastPrefetchedTime): boolean

Alternatives considered

The manual alternative would be to use some custom svelte action in the <a> tag which calls prefetch() manually and call it again if the user hover/click on the link after a certain amount of time.

Sadly, as far as I know, there is not even a way to invalidate the prefetched data manually. (the invalidate() works only for the currently active page acoording to the docs)

Importance

would make my life easier

Additional Information

No response

[dummdidumm]

@Christian-E once #6924 is released invalidate will also invalidate prefetched data.

[Rich-Harris]

We recently made a change such that instead of returning maxage from load, you return a cache object. One idea that occurs to me is adding a prefetchTimeout property (name subject to bikeshedding, obviously) that, if maxage isn’t specified, defaults to the global config.kit.prefetchTimeout (which defaults to some number of milliseconds — say 5000 for the sake of argument).

I think that makes more sense than putting it in the attribute, since there’s no reason you’d ever have different values for two links that pointed to the same page.

<script context="module">
  export const load = ({ props }) => ({
    props,
    cache: {
      // for this page, treat data as stale if users hovers on link for >1s
      prefetchTimeout: 1000
    }
  });
</script>