start on infinite scrolling

routes/_components/Timeline.html

@@ -1,6 +1,7 @@
 <div class="timeline">
-  <VirtualList component="{{StatusListItem}}" items="{{statuses}}" />
-  <button type="button" on:click="addMoreItems()">Add more items</button>
+  <VirtualList component="{{StatusListItem}}"
+               items="{{statuses}}"
+               on:scrollToBottom="addMoreItems()" />
 </div>
 <style>
   .timeline {
@@ -17,7 +18,7 @@
 
   let i = -1
 
-  const createData = () => fixture.slice(0, 20).map(_ => ({
+  const createData = () => fixture.slice(0, 5).map(_ => ({
     key: `${++i}`,
     props: _
   }))
@@ -35,6 +36,7 @@
     methods: {
       splice: splice,
       addMoreItems() {
+        console.log('addMoreItems')
         this.splice('statuses', this.get('statuses').length, 0, ...createData())
       }
     }

routes/_components/VirtualList.html

@@ -20,12 +20,15 @@
   import throttle from 'lodash/throttle'
 
   const THROTTLE_TIME = 500
+  const DISTANCE_FROM_BOTTOM_TO_FIRE = 400
 
   export default {
     oncreate () {
-      this.observe('innerHeight', throttle(innerHeight => {
+      let container = document.body.querySelector('.container')
+      this.observe('innerHeight', throttle(() => {
+        // respond to window resize events
         this.store.set({
-          innerHeight: innerHeight
+          offsetHeight: container.offsetHeight
         })
       }, THROTTLE_TIME))
       this.observe('items', (items) => {
@@ -33,14 +36,27 @@
           'items': items
         })
       })
-      document.body.querySelector('.container').addEventListener('scroll', throttle((e) => {
+      this.store.observe('distanceFromBottom', distanceFromBottom => {
+        console.log('distanceFromBottom', distanceFromBottom)
+        if (distanceFromBottom >= 0 &&
+            distanceFromBottom <= DISTANCE_FROM_BOTTOM_TO_FIRE) {
+          this.fire('scrollToBottom')
+        }
+      })
+      container.addEventListener('scroll', throttle((e) => {
         this.store.set({
-          scrollTop: e.target.scrollTop
+          scrollTop: e.target.scrollTop,
+          scrollHeight: e.target.scrollHeight
         }, {
           leading: false,
           trailing: true
         })
       }, THROTTLE_TIME))
+      this.store.set({
+        scrollTop: container.scrollTop,
+        scrollHeight: container.scrollHeight,
+        offsetHeight: container.offsetHeight
+      })
     },
     data: () => ({
       component: null

routes/_utils/virtualListStore.js

@@ -6,13 +6,12 @@ class VirtualListStore extends Store {
 const virtualListStore = new VirtualListStore({
   items: [],
   itemHeights: {},
-  scrollTop: 0
 })
 
 virtualListStore.compute('visibleItems',
-    ['items', 'scrollTop', 'height', 'itemHeights', 'innerHeight'],
-    (items, scrollTop, height, itemHeights, innerHeight) => {
-  let renderBuffer = 1.5 * innerHeight
+    ['items', 'scrollTop', 'itemHeights', 'offsetHeight'],
+    (items, scrollTop, itemHeights, offsetHeight) => {
+  let renderBuffer = 1.5 * offsetHeight
   let visibleItems = []
   let totalOffset = 0
   let len = items.length
@@ -28,7 +27,7 @@ virtualListStore.compute('visibleItems',
         continue // below the area we want to render
       }
     } else {
-      if (currentOffset > (scrollTop + innerHeight + renderBuffer)) {
+      if (currentOffset > (scrollTop + offsetHeight + renderBuffer)) {
         break // above the area we want to render
       }
     }
@@ -36,12 +35,24 @@ virtualListStore.compute('visibleItems',
       offset: currentOffset,
       props: props,
       key: key,
-      index: i
+      index: i,
+      height: height
     })
   }
   return visibleItems
 })
 
+virtualListStore.compute('distanceFromBottom',
+    ['scrollHeight', 'scrollTop', 'offsetHeight'],
+    (scrollHeight, scrollTop, offsetHeight) => {
+  if (typeof scrollHeight === 'undefined' ||
+      typeof scrollTop === 'undefined' ||
+      typeof offsetHeight === 'undefined') {
+    return -1
+  }
+  return scrollHeight - scrollTop - offsetHeight
+})
+
 virtualListStore.compute('height', ['items', 'itemHeights'], (items, itemHeights) => {
   let sum = 0
   let i = -1