expose firstIndex in virtual scroller

[lygaret]

I'd like to be able to use a virtual list with an <ol> element, since that's semantically correct for my application.

Currently, in order to correctly set the start attribute, I'm currently dividing viewTop by rowHeight, which is just how it's defined, and lets me get at the value correctly, but it's a very ugly hack to get the value which is just hidden under the lambda.

In use, this looks like:

  const [virtual, virtualOnScroll] = createVirtualList({
    items: entries,
    overscanCount: 5,
    rootHeight,
    rowHeight
  })

  return (
    <div class={styles['list-wrapper']} onScroll={virtualOnScroll}>
      <ol start={virtual().firstIndex} style={{top: `${virtual().viewerTop}px`}}>
        <For
          each={virtual().visibleItems}
          fallback={<li class={styles['list-empty-marker']} />}
        >
        /* whatever ... */
      </ol>
    </div>
  )

[changeset-bot]

🦋 Changeset detected

Latest commit: 16727c1

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

This PR includes changesets to release 1 package

Name	Type
@solid-primitives/virtual	Patch

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[atk]

while you're at it, you can also expose lastIndex.

[lygaret]

Sure, I can. I did originally, but removed it because its a little weird semantically: when a list is empty, should it return -1, or undefined?

firstIndex being zero makes sense, kinda, but I didn't know what to do with last's typing, so I skipped it.

[lygaret]

Right now, it's more like nextIndex than last because it's an exclusive range.

[atk]

That's fine, too.

[lygaret]

@atk I ended up going with lastIndex: number | undefined -- semantically, it's undefined for an empty list, though I'm not allowing firstIndex to be undefined, and returning 0 in that case, maybe that's wrong?

nextIndex also has a weird semantic, but imo nextIndex: undefined means more like "there's no more of the list", which might be a better thing to expose?

let me know your thoughts and I'll move that direction, thank you!

[atk]

Shouldn't it be items.length if it is -1?

[lygaret]

I'm sorry, I'm not sure what you're asking.

The -1 is an explanation of the reason we're not just passing it in straight, and it's already based on items.length, hence the whole thing ?

[atk]

Ah, I read the whole thing in a different order. My bad, I'm currently down with a flu.

[atk]

Now all that is left to do is add a changeset using npx changeset.