[group] control scroll position

[gitbreaker222]

This is a summary of issues around similar use-cases:

• Updating items only updates on scroll #15 lazy loading: [bug, resolved?] adding more items keeps scroll position and increases scroll length below
• Feature request: Initial "start" value (auto-scroll to certain element) #28 scroll to song: bring an item (by index) into view
• Viewport goes empty if scrolled down and items array changes. #30 filtering list: [bug] removing more items than current scroll position assumes causes "empty" viewport
• autoscroll (to the bottom) feature #32 messenger: autoscroll to bottom if new items loades

I hope the use-case-namings are helpful - all these issues have in common, that we expect certain behaviours from the scroll position. To prevent solving one issue while re-creating another, I suggest to work towards one PR that solves them together 🚄 🤝

REPL: https://svelte.dev/repl/1c36db7c1e7e4ef2bfb04874321412e5?version=3.20.1

---

comparable vanilla implementations:

• https://github.com/sergi/virtual-list
• https://github.com/voltraco/infinitely

[gitbreaker222]

attempting to create a "scrollTo" method, that scrolls an element by index at start, if list borders not hit. Problems so far:

• programmatically scrolling until item is in view works gets blocked in repl after about 10 calls to viewport.scrollTo as of "infinite loop" detection (even if its a for loop with a limit of 99 iterations)
  • This approach is not elegant and just does, what user would do manually with the mouse wheel - but it still works for rows with variable height
• calculating exact scroll disatance only reliable, if items have a fixed height by CSS. The property itemHeight does not control the actual row style, so the height has to be manually synced by user with correct calculation of box-height / border-width / padding / margin / box-sizing => to px value
  • as a result I find it more reliable to avoid itemHeight, instead just mind for a fixed height in CSS and let average_height take care of getting the rows computed pixel height
• refresh is missing a check, if new list length is shorter than current scroll start
  • solution: call scrollTo(list.length - 1)
• handle_scroll has this part, that should stabilize scrolling up: // prevent jumping if we scrolled up into unknown territory but this block somehow causes the position to be 0
  • removing it works better
• if rows have dynamic height, list seems not to scroll to the very bottom (jumps up and down). This especially is problematic with the "chat" use case, because messages vary greatly in content size

[gitbreaker222]

solved problems to this point:

• "lazy loading" ✔️ : Works as expected without further changes
• "jump to song" use case ✔️ : As soon rows have a fixed height, the "jumpToIndex" method works very well
• "filter" use case ✔️ ❌ The new check fires, but items only rendered sometimes
  • problem seems to appear if list.length = 1000, start is at about 400, then filter reduces length to about 250. It works though, if scrolled to bottom or on second filter call
  • edit: fixed ✔️

tested in firefox

[gitbreaker222]

update: after almost giving up fixing the remaining issue (adjust scroll position when filter list) I finally solved it! The demo has been updated and next I'd like to do a PR with clean code

So @Rich-Harris maybe now is a good time to get your attention :) My proposal:

• add a new method "scrollToIndex"
  • <VirtualList {items} bind:scrollToIndex}></VirtualList>
• update README documentation
• close 4 related issues

Any thoughts before I start coding?

[gitbreaker222]

Here is a webm of the demo 👼
https://windtanz.windcloud.de/index.php/s/d43AbLtXw9mimj9

[rsousacode]

Very nice. Thanks! Apparently your _scrollTo3 solves my problem!! 👍
#50

[Metis77]

great work!
REPL: https://svelte.dev/repl/1c36db7c1e7e4ef2bfb04874321412e5?version=3.20.1
should be merged.

[gitbreaker222]

I am preparing a pull request. Fork, cleanup, testing is done. One issue remains in logs:

"<VirtualList> was created with unknown prop 'scrollToIndex'"

How to properly expose a function / "method" so it can be called from outside? I played with bind: let: use: but don't get it :/

update:
bam got it - the consumer of binds to the exposed function scrollToIndex. Outside I've initialized a same named variable to bind to ... with a noop function

<script>
	export let scrollToIndex = () => {} // this should be "undefined"
</script>

<div class="List">
	<VirtualList {items}
		bind:start
		bind:end
		bind:scrollToIndex
		let:item
		>
		<ListItem {item}/>
	</VirtualList>
</div>

initializing with explicit undefined is correct https://stackoverflow.com/a/58574086/3313410 - thx @Rich-Harris

updating demo and opening PR shortly 👀

[gitbreaker222]

ℹ️ published fork
https://www.npmjs.com/package/svelte-virtual-list-ce