An equivalent of solid's isPending utility ($state.pending)

[Thiagolino8]

Describe the problem

Svelte has a way to tell if there are pending asynchronous derivations within the scope of a boundary with $effect.pending()
And it has a way to tell if a state is causing an asynchronous derivation with $state.eager(state) !== state
But it doesn't have a way to tell if a specific derived state is currently in an asynchronous derivation
Version 2 of Solid is bringing a utility to fulfil this function and it would be very useful to have an equivalent

Describe the proposed solution

A rune like $state.pending(state) which is an equivalent of $effect.pending() but instead of indicating asynchronous derivations within the nearest boundary indicates whether the state passed as a parameter is a derived state with a pending asynchronous derivation

Importance

would make my life easier

[dummdidumm]

What would you use it for?

[Thiagolino8]

The UI in the solid example is a commonly needed one, so I ported it to Svelte

The only thing missing is disabling selects during derivations

If $effect.pending were used, all selects would be disabled in any derivation, which is not the goal

I could wait for the promise only at the point of use and put a boundary on each select, but besides making the code a mess, this would break the composition since it wouldn't be possible to have a global pending state on initial load

It would also be possible to have the select snippet receive a pending state and use a comparison of the derivation's dependencies with their eager versions to create this state, but besides making the code a lot more complex, this doesn't scale well to more complex versions, since it is necessary to do this comparison with all the dependencies of each derivation

The only way to complete this UI without introducing too many complications would be with a utility that lets you know if a derived state has new pending derivations

[Rich-Harris]

Not sure I understand how this would differ from $state.eager(state) !== state?

[Thiagolino8]

In addition to needing to add a pending prop to the select snippet, which will have its value computed with a state that doesn't actually directly concern the snippet, it's also necessary to perform this comparison with all the dependencies of each derivative

In this example, each derivation has only one direct dependency, but if it were the case of having 10 dependencies, it would be necessary to do:

pending: dep1 !== $state.eager(dep1) ||
	dep2 !== $state.eager(dep2) ||
	// n more diffs
	dep10 !== $state.eager(dep10)