semaphore/routes/_components/pseudoVirtualList/PseudoVirtualList.html

<div class="pseudo-virtual-list {{shown ? '' : 'hidden'}}" on:initializedVisibleItems ref:node>
  {{#if wrappedItems}}
    {{#each wrappedItems as wrappedItem, i @item}}
      <PseudoVirtualListLazyItem
        component="{{component}}"
        index="{{i}}"
        length="{{wrappedItems.length}}"
        makeProps="{{makeProps}}"
        key="{{wrappedItem.item}}"
        intersectionObserver="{{intersectionObserver}}"
        isIntersecting="{{isIntersecting(wrappedItem.item, $intersectionStates)}}"
        isCached="{{isCached(wrappedItem.item, $intersectionStates)}}"
        height="{{getHeight(wrappedItem.item, $intersectionStates)}}"
      />
    {{/each}}
  {{/if}}
</div>
<style>
  .pseudo-virtual-list {
    position: relative;
    transition: opacity 0.25s linear;
  }
</style>
<script>

  import PseudoVirtualListLazyItem from './PseudoVirtualListLazyItem.html'
  import { getRectFromEntry } from '../../_utils/getRectFromEntry'
  import { mark, stop } from '../../_utils/marks'
  import { pseudoVirtualListStore } from './pseudoVirtualListStore'

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

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

      this.set({intersectionObserver: new IntersectionObserver(this.onIntersection.bind(this), {
        root: document.getElementsByClassName('container')[0], // TODO: fix this
        rootMargin: '300% 0px'
      })})
      this.observe('allItemsHaveHeight', allItemsHaveHeight => {
        console.log('allItemsHaveHeight', allItemsHaveHeight)
        if (allItemsHaveHeight && !this.get('initialized')) {
          this.set({initialized: true})
          console.log('initializedVisibleItems')
          this.fire('initializedVisibleItems')
        }
      })
      stop('PseudoVirtualList oncreate()')
    },
    ondestroy() {
      let intersectionObserver = this.get('intersectionObserver')
      if (intersectionObserver) {
        intersectionObserver.disconnect()
      }
    },
    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: {
      scrollToPosition(rect) {
        if (this.get('scrolledToPosition')) {
          return
        }
        this.set({scrolledToPosition: true})
        console.log('scrollToPosition', rect.top)
        // TODO: there should be some cleaner way to grab the container element
        let container = document.getElementsByClassName('container')[0]
        if (!container) {
          return
        }
        let containerRect = container.getBoundingClientRect()
        let scrollTop = rect.top - containerRect.top
        if (scrollTop !== 0) {
          requestAnimationFrame(() => {
            console.log('setting scrollTop to ', scrollTop)
            container.scrollTop = scrollTop
          })
        }
      },
      onIntersection(entries) {
        mark('onIntersection')
        let newIntersectionStates = {}
        let scrollToItem = this.get('scrollToItem')
        let intersectionStates = this.store.get('intersectionStates')
        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(rect)
          }
        }
        intersectionStates = Object.assign(intersectionStates, newIntersectionStates)
        this.store.setForRealm({intersectionStates: intersectionStates})
        stop('onIntersection')
      }
    },
    computed: {
      wrappedItems: (items) => items && items.map(item => ({item: item})),
      allItemsHaveHeight: (items, $intersectionStates) => {
        if (!items) {
          return false
        }
        for (let item of items) {
          if (!$intersectionStates[item]) {
            return false
          }
        }
        return true
      }
    },
    components: {
      PseudoVirtualListLazyItem
    },
    store: () => pseudoVirtualListStore
  }
</script>