semaphore/routes/_components/VirtualListContainer.html

<div class="container"
     on:scroll="onScroll(event)"
     on:fullscreen="onFullscreenChange()"
     ref:node>
  <slot></slot>
</div>
<script>
  import { virtualListStore } from '../_utils/virtualListStore'

  import throttle from 'lodash/throttle'
  import { isFullscreen, attachFullscreenListener, detachFullscreenListener } from '../_utils/fullscreen'
  import { mark, stop } from '../_utils/marks'

  const SCROLL_EVENT_DELAY = 300

  const cachedVirtualStores = {}

  if (process.browser && process.env.NODE_ENV !== 'production') {
    window.cachedVirtualStores = cachedVirtualStores
  }

  export default {
    oncreate() {
      mark('onCreate VirtualListContainer')
      let node = this.refs.node
      let storeKey = this.get('storeKey')
      let cachedStore
      if (storeKey && (cachedStore = cachedVirtualStores[storeKey])) {
        this.store.set({
          scrollTop: cachedStore.state.scrollTop,
          scrollHeight: cachedStore.state.scrollHeight,
          offsetHeight: cachedStore.state.offsetHeight
        })
        this.store.set(cachedStore.state)

        // rehydrate scroll top
        let cachedScrollTop = cachedStore.state.scrollTop || 0
        if (cachedScrollTop === 0) {
          if (process.env.NODE_ENV !== 'production') {
            console.log('no need to force scroll top')
          }
          return
        }
        let initializedScrollTop = false
        let node = this.refs.node
        this.observe('allVisibleItemsHaveHeight', allVisibleItemsHaveHeight => {
          if (process.env.NODE_ENV !== 'production') {
            console.log('allVisibleItemsHaveHeight', allVisibleItemsHaveHeight)
          }
          if (!initializedScrollTop && allVisibleItemsHaveHeight && node) {
            initializedScrollTop = true
            requestAnimationFrame(() => {
              mark('set scrollTop')
              if (process.env.NODE_ENV !== 'production') {
                console.log('forcing scroll top to ', cachedScrollTop)
              }
              node.scrollTop = cachedScrollTop
              stop('set scrollTop')
            })
          }
        })
      } else {
        this.store.set({
          scrollTop: 0,
          scrollHeight: node.scrollHeight,
          offsetHeight: node.offsetHeight
        })
      }
      stop('onCreate VirtualListContainer')
    },
    ondestroy() {
      let storeKey = this.get('storeKey')
      if (storeKey) {
        let clonedState = this.store.cloneState()
        if (process.env.NODE_ENV !== 'production') {
          console.log('caching scroll top', clonedState.scrollTop)
        }
        cachedVirtualStores[storeKey] = {
          state: clonedState
        }
      }
    },
    store: () => virtualListStore,
    events: {
      scroll(node, callback) {
        const onScroll = throttle(event => {
          mark('onScroll')
          if (this.get('fullscreen')) {
            return
          }
          callback(event)
          stop('onScroll')
        }, SCROLL_EVENT_DELAY, {
          leading: true,
          trailing: true
        })
        node.addEventListener('scroll', onScroll)

        return {
          teardown() {
            node.removeEventListener('scroll', onScroll)
          }
        }
      },
      fullscreen(node, callback) {
        const onFullscreen = (() => {
          callback()
        })
        attachFullscreenListener(onFullscreen)
        return {
          teardown() {
            detachFullscreenListener(onFullscreen)
          }
        }
      }
    },
    methods: {
      onScroll(event) {
        this.store.set({
          scrollTop: event.target.scrollTop,
          scrollHeight: event.target.scrollHeight
        })
      },
      onFullscreenChange() {
        mark('onFullscreenChange')
        if (process.env.NODE_ENV !== 'production') {
          console.log('is fullscreen? ', isFullscreen())
        }
        this.set({ fullscreen: isFullscreen() })
        stop('onFullscreenChange')
      }
    },
    computed: {
      // TODO: bug in svelte/store – the observer in oncreate() never get removed without this hack
      allVisibleItemsHaveHeight: ($allVisibleItemsHaveHeight) => $allVisibleItemsHaveHeight
    }
  };
</script>