pinafore/routes/_components/pseudoVirtualList/PseudoVirtualList.html

<div class="pseudo-virtual-list" on:initialized ref:node>
  {#each safeItems as item, i (item)}
    <PseudoVirtualListLazyItem
      {component}
      index={i}
      length={safeItems.length}
      {makeProps}
      key={item}
      {intersectionObserver}
      isIntersecting={isIntersecting(item, $intersectionStates)}
      isCached={isCached(item, $intersectionStates)}
      height={getHeight(item, $intersectionStates)}
    />
  {/each}
</div>
<style>
  .pseudo-virtual-list {
    position: relative;
  }
</style>
<script>
  import PseudoVirtualListLazyItem from './PseudoVirtualListLazyItem.html'
  import { getRectFromEntry } from '../../_utils/getRectFromEntry'
  import { mark, stop } from '../../_utils/marks'
  import { pseudoVirtualListStore } from './pseudoVirtualListStore'
  import { observe } from 'svelte-extras'

  export default {
    oncreate () {
      mark('PseudoVirtualList oncreate()')
      let { realm } = this.get()
      this.store.setCurrentRealm(realm)

      // When re-rendering, assume all items are non-intersecting until told otherwise.
      // We already have the heights cached.
      let { intersectionStates } = this.store.get()
      let keys = Object.keys(intersectionStates)
      for (let key of keys) {
        intersectionStates[key].isCached = true
      }
      this.store.setForRealm({intersectionStates: intersectionStates})

      let { containerQuery } = this.get()
      let intersectionObserver = new IntersectionObserver(this.onIntersection.bind(this), {
        root: document.querySelector(containerQuery),
        rootMargin: '300% 0px'
      })
      this.set({intersectionObserver})
      this.observe('allItemsHaveHeight', allItemsHaveHeight => {
        console.log('allItemsHaveHeight', allItemsHaveHeight)
        let { initialized } = this.get()
        if (allItemsHaveHeight && !initialized) {
          this.set({initialized: true})
          console.log('initialized PseudoVirtualList')
          this.fire('initialized')
        }
      })
      stop('PseudoVirtualList oncreate()')
    },
    ondestroy () {
      let { intersectionObserver } = this.get()
      if (intersectionObserver) {
        intersectionObserver.disconnect()
      }
      this.store.setCurrentRealm(null)
    },
    helpers: {
      isIntersecting (key, $intersectionStates) {
        return !!($intersectionStates[key] && $intersectionStates[key].isIntersecting)
      },
      isCached (key, $intersectionStates) {
        return !!($intersectionStates[key] && $intersectionStates[key].isCached)
      },
      getHeight (key, $intersectionStates) {
        return $intersectionStates[key] && $intersectionStates[key].height
      }
    },
    methods: {
      observe,
      scrollToPosition (element) {
        let { scrolledToPosition } = this.get()
        if (scrolledToPosition) {
          return
        }
        this.set({scrolledToPosition: true})
        requestAnimationFrame(() => {
          console.log('scrolling element into view')
          element.scrollIntoView(true)
        })
      },
      onIntersection (entries) {
        mark('onIntersection')
        let newIntersectionStates = {}
        let { scrollToItem } = this.get()
        let { intersectionStates } = this.store.get()
        for (let entry of entries) {
          let key = entry.target.getAttribute('pseudo-virtual-list-key')
          let rect = getRectFromEntry(entry)
          newIntersectionStates[key] = {
            isIntersecting: entry.isIntersecting,
            height: rect.height
          }
          if (scrollToItem === key) {
            this.scrollToPosition(entry.target)
          }
        }
        intersectionStates = Object.assign(intersectionStates, newIntersectionStates)
        this.store.setForRealm({intersectionStates: intersectionStates})
        stop('onIntersection')
      }
    },
    computed: {
      safeItems: ({ items }) => items || [],
      allItemsHaveHeight: ({ items, $intersectionStates }) => {
        if (!items) {
          return false
        }
        for (let item of items) {
          if (!$intersectionStates[item]) {
            return false
          }
        }
        return true
      }
    },
    components: {
      PseudoVirtualListLazyItem
    },
    data: () => ({
      intersectionObserver: void 0
    }),
    store: () => pseudoVirtualListStore
  }
</script>