some more work on virtual scroll which is hard

routes/_components/LazyTimeline.html

@@ -9,7 +9,7 @@
 {{then constructor}}
 <:Component {constructor} :target />
 {{catch error}}
-<div>Component failed to load. Please try refreshing!</div>
+<div>Component failed to load. Please try refreshing! {{error}}</div>
 {{/await}}
 <style>
   .loading-page {

routes/_components/Timeline.html

@@ -1,5 +1,6 @@
 <div class="timeline">
   <VirtualList component="{{StatusListItem}}" items="{{statuses}}" />
+  <button type="button" on:click="addMoreItems()">Add more items</button>
 </div>
 <style>
   .timeline {
@@ -12,16 +13,30 @@
   import fixture from '../_utils/fixture.json'
   import StatusListItem from './StatusListItem.html'
   import VirtualList from './VirtualList.html'
+  import { splice } from 'svelte-extras'
+
+  let i = -1
+
+  const createData = () => fixture.slice(0, 5).map(_ => ({
+    key: `${++i}`,
+    props: _
+  }))
 
   export default {
     data: () => ({
       target: 'home',
-      statuses: fixture,
+      statuses: createData(),
       StatusListItem: StatusListItem
     }),
     store: () => store,
     components: {
       VirtualList
+    },
+    methods: {
+      splice: splice,
+      addMoreItems() {
+        this.splice('statuses', this.get('statuses').length, 0, ...createData())
+      }
     }
   }
 </script>

routes/_components/VirtualList.html

@@ -1,10 +1,12 @@
-<div class="virtual-list" ref:node style="height: {{height}}px;">
-  {{#each visibleItems as visibleItem, index}}
+<:Window bind:scrollY='scrollY'/>
+<div class="virtual-list" ref:node style="height: {{$height}}px;">
+  <!-- <div class="virtual-list-viewport" ref:viewport></div> -->
+  {{#each $virtualItems as virtualItem, virtualIndex}}
     <VirtualListItem :component
-                     :intersectionObserver
-                     virtualOffset="{{visibleItem.offset}}"
-                     virtualProps="{{visibleItem.props}}"
-                     virtualIndex="{{index}}" />
+                     props="{{virtualItem.props}}"
+                     index="{{virtualItem.index}}"
+                     key="{{virtualItem.key}}"
+    />
   {{/each}}
 </div>
 <style>
@@ -14,55 +16,22 @@
 </style>
 <script>
   import VirtualListItem from './VirtualListItem'
-
-  function sum(arr) {
-    return arr.reduce((a, b) => a + b, 0)
-  }
+  import { virtualListStore } from '../_utils/virtualListStore'
 
   export default {
     oncreate() {
-      let intersectionObserver = new IntersectionObserver((entries) => {
-        let totalHeight = sum(entries.map(entry => entry.boundingClientRect.height))
-        let offset = 0
-        let offsets = []
-        entries.forEach(entry => {
-          offsets.push(offset)
-          offset += entry.boundingClientRect.height
-        })
-        this.set({
-          height: totalHeight,
-          offsets: offsets
-        })
-        console.log('entries', entries.map(entry => entry.target.getAttribute('data-virtual-index')))
-      }, {
-        root: this.refs.node
+      this.observe('items', (items) => {
+        this.store.set({'items': items})
       })
-      this.set({
-        intersectionObserver: intersectionObserver
+      this.observe('scrollY', (scrollY) => {
+        console.log('scrollY', scrollY)
+        this.store.set({scrollTop: scrollY})
       })
     },
-    ondestroy() {
-      let intersectionObserver = this.get('intersectionObserver')
-      if (intersectionObserver) {
-        intersectionObserver.disconnect()
-      }
-    },
-    computed: {
-      visibleItems: (items, offsets) => {
-        return items.map((item, idx) => ({
-          props: item,
-          offset: offsets[idx]
-        }))
-      }
-    },
     data: () => ({
-      scrollHeight: 0,
-      component: null,
-      intersectionObserver: null,
-      items: [],
-      offsets: [],
-      height: 400
+      component: null
     }),
+    store: () => virtualListStore,
     components: {
       VirtualListItem
     }

routes/_components/VirtualListItem.html

@@ -1,8 +1,10 @@
 <div class="virtual-list-item"
      ref:node
-     style="transform: translate3d(0, {{virtualOffset}}px, 0);"
-     data-virtual-index="{{virtualIndex}}">
-  <:Component {component} :virtualProps :virtualIndex :intersectionObserver />
+     style="transform: translate3d(0, {{itemOffset}}px, 0);"
+     data-virtual-index="{{index}}"
+     data-virtual-key="{{key}}"
+>
+  <:Component {component} virtualProps="{{props}}" />
 </div>
 <style>
   .virtual-list-item {
@@ -12,18 +14,19 @@
   }
 </style>
 <script>
+  import { virtualListStore } from '../_utils/virtualListStore'
+
   export default {
     oncreate() {
-      this.observe('intersectionObserver', (intersectionObserver) => {
-        if (intersectionObserver) {
-          intersectionObserver.observe(this.refs.node)
-        }
-      })
+      let itemHeights = this.store.get('itemHeights')
+      itemHeights[this.get('key')] = this.refs.node.offsetHeight
+      this.store.set({itemHeights: itemHeights})
     },
-    methods: {
-      showRefs () {
-        console.log(this.refs)
+    computed: {
+      itemOffset: ($itemOffsets, key) => {
+        return $itemOffsets[key] || 0
       }
-    }
+    },
+    store: () => virtualListStore
   }
 </script>

routes/_utils/virtualListStore.js

@@ -0,0 +1,48 @@
+import { Store } from 'svelte/store.js'
+import { splice } from 'svelte-extras'
+
+class VirtualListStore extends Store {
+}
+
+VirtualListStore.prototype.splice = splice
+
+const virtualListStore = new VirtualListStore({
+  items: [],
+  itemHeights: {},
+  scrollTop: 0
+})
+
+virtualListStore.compute('virtualItems', ['items'], (items) => {
+  return items.map((item, idx) => ({
+    props: item.props,
+    key: item.key,
+    index: idx
+  }))
+})
+
+virtualListStore.compute('itemOffsets', ['virtualItems', 'itemHeights'], (virtualItems, itemHeights) => {
+  let itemOffsets = {}
+  let totalHeight = 0
+  virtualItems.forEach(item => {
+    let height = itemHeights[item.key] || 0
+    itemOffsets[item.key] = totalHeight
+    totalHeight += height
+  })
+  return itemOffsets
+})
+
+virtualListStore.compute('height', ['virtualItems', 'itemHeights'], (virtualItems, itemHeights) => {
+  let sum = 0
+  virtualItems.forEach(item => {
+    sum += itemHeights[item.key] || 0
+  })
+  return sum
+})
+
+if (process.browser && process.env.NODE_ENV !== 'production') {
+  window.virtualListStore = virtualListStore
+}
+
+export {
+  virtualListStore
+}