fine-tune infinite scrolling list

package-lock.json

@@ -5848,6 +5848,11 @@
         }
       }
     },
+    "requestidlecallback": {
+      "version": "0.3.0",
+      "resolved": "https://registry.npmjs.org/requestidlecallback/-/requestidlecallback-0.3.0.tgz",
+      "integrity": "sha1-b7dOBzP5DfP6pIOPn2oqX5t0KsU="
+    },
     "require-directory": {
       "version": "2.1.1",
       "resolved": "https://registry.npmjs.org/require-directory/-/require-directory-2.1.1.tgz",

package.json

@@ -32,6 +32,7 @@
     "npm-run-all": "^4.1.2",
     "performance-now": "^2.1.0",
     "pify": "^3.0.0",
+    "requestidlecallback": "^0.3.0",
     "sapper": "^0.3.2",
     "serve-static": "^1.13.1",
     "style-loader": "^0.19.1",

routes/_components/Layout.html

@@ -10,17 +10,20 @@
 	import Nav from './Nav.html';
 	import { virtualListStore } from '../_utils/virtualListStore'
 
+  import debounce from 'lodash/debounce'
   import throttle from 'lodash/throttle'
-  const THROTTLE_DELAY = 500
+
+  const SCROLL_EVENT_DELAY = 300
+  const RESIZE_EVENT_DELAY = 700
 
 	export default {
 	  oncreate() {
-      this.observe('innerHeight', throttle(() => {
+      this.observe('innerHeight', debounce(() => {
         // respond to window resize events
         this.store.set({
           offsetHeight: this.refs.node.offsetHeight
         })
-      }, THROTTLE_DELAY))
+      }, RESIZE_EVENT_DELAY))
 	    this.store.set({
         scrollTop: this.refs.node.scrollTop,
         scrollHeight: this.refs.node.scrollHeight,
@@ -33,7 +36,10 @@
     store: () => virtualListStore,
     events: {
       scroll(node, callback) {
-        const onScroll = throttle(callback, THROTTLE_DELAY)
+        const onScroll = throttle(callback, SCROLL_EVENT_DELAY, {
+          leading: true,
+          trailing: true
+        })
         node.addEventListener('scroll', onScroll);
 
         return {

routes/_components/Timeline.html

@@ -14,7 +14,7 @@
   import fixture from '../_utils/fixture.json'
   import StatusListItem from './StatusListItem.html'
   import VirtualList from './VirtualList.html'
-  import { splice } from 'svelte-extras'
+  import { splice, push } from 'svelte-extras'
 
   let i = -1
 
@@ -35,12 +35,33 @@
     },
     methods: {
       splice: splice,
+      push: push,
       addMoreItems() {
         console.log('addMoreItems')
         let statuses = this.get('statuses')
         if (statuses) {
-          this.splice('statuses', statuses.length, 0, ...createData())
+          let itemsToAdd = createData()
+          if (itemsToAdd.length) {
+
+          }
+
+          let importantFirstItem = itemsToAdd
+          this.splice('statuses', statuses.length, 0, ...itemsToAdd)
         }
+      },
+      addTheseItems(items) {
+        if (!items.length) {
+          return
+        }
+        this.push(items.pop())
+        while (items.length) {
+          this.addItemLazily(items.pop())
+        }
+      },
+      addItemLazily(item) {
+        requestIdleCallback(() => {
+          this.push(item)
+        })
       }
     }
   }

routes/_components/VirtualList.html

@@ -1,5 +1,4 @@
-<div class="virtual-list">
+<div class="virtual-list" style="height: {{$height}}px;">
-  <!-- <div class="virtual-list-viewport" ref:viewport></div> -->
   {{#each $visibleItems as item @key}}
   <VirtualListItem :component
                    offset="{{item.offset}}"
@@ -27,9 +26,14 @@
         })
       })
 
+      let observedOnce = false
+
       this.observe('distanceFromBottom', (distanceFromBottom) => {
-        //console.log('distanceFromBottom', distanceFromBottom)
+        if (!observedOnce) {
-        if (distanceFromBottom > 0 && // hack: the first it's reported, it's always 0
+          observedOnce = true // TODO: the first time is always 0... need better way to handle this
+          return
+        }
+        if (distanceFromBottom >= 0 &&
             distanceFromBottom <= DISTANCE_FROM_BOTTOM_TO_FIRE) {
           this.fire('scrollToBottom')
         }

routes/_components/VirtualListItem.html

@@ -11,6 +11,7 @@
     top: 0;
     opacity: 0;
     pointer-events: none;
+    /* will-change: transform; */ /* causes jank in mobile Firefox */
   }
   .shown {
     opacity: 1;

routes/_utils/asyncModules.js

@@ -17,8 +17,13 @@ const importIntersectionObserver = () => import(
   /* webpackChunkname: 'intersection-observer' */ 'intersection-observer'
   )
 
+const importRequestIdleCallback = () => import(
+  /* webpackChunkName: 'requestidlecallback' */ 'requestidlecallback'
+  )
+
 export {
   importURLSearchParams,
   importTimeline,
-  importIntersectionObserver
+  importIntersectionObserver,
+  importRequestIdleCallback
 }

routes/_utils/virtualListStore.js

@@ -11,7 +11,7 @@ const virtualListStore = new VirtualListStore({
 virtualListStore.compute('visibleItems',
     ['items', 'scrollTop', 'itemHeights', 'offsetHeight'],
     (items, scrollTop, itemHeights, offsetHeight) => {
-  let renderBuffer = 1.5 * offsetHeight
+  let renderBuffer = 3 * offsetHeight
   let visibleItems = []
   let totalOffset = 0
   let len = items.length

templates/main.js

@@ -1,11 +1,15 @@
 import { init } from 'sapper/runtime.js'
-import { importURLSearchParams } from '../routes/_utils/asyncModules'
+import {
-import { importIntersectionObserver } from '../routes/_utils/asyncModules'
+  importURLSearchParams,
+  importIntersectionObserver,
+  importRequestIdleCallback
+} from '../routes/_utils/asyncModules'
 
 // polyfills
 Promise.all([
   typeof URLSearchParams === 'undefined' && importURLSearchParams(),
-  typeof IntersectionObserver === 'undefined' && importIntersectionObserver()
+  typeof IntersectionObserver === 'undefined' && importIntersectionObserver(),
+  typeof requestIdleCallback === 'undefined' && importRequestIdleCallback()
 ]).then(() => {
   // `routes` is an array of route objects injected by Sapper
   init(document.querySelector('#sapper'), __routes__)