Timeline virtual scrolling

src/components/timeline/timeline.js

@@ -33,7 +33,8 @@ const Timeline = {
     return {
       paused: false,
       unfocused: false,
-      bottomedOut: false
+      bottomedOut: false,
+      virtualScrollIndex: 0
     }
   },
   components: {
@@ -78,6 +79,16 @@ const Timeline = {
     },
     pinnedStatusIdsObject () {
       return keyBy(this.pinnedStatusIds)
+    },
+    statusesToDisplay () {
+      const amount = this.timeline.visibleStatuses.length
+      const statusesPerSide = Math.ceil(Math.max(3, window.innerHeight / 80))
+      const min = Math.max(0, this.virtualScrollIndex - statusesPerSide)
+      const max = Math.min(amount, this.virtualScrollIndex + statusesPerSide)
+      return this.timeline.visibleStatuses.slice(min, max).map(_ => _.id)
+    },
+    virtualScrollingEnabled () {
+      return this.$store.getters.mergedConfig.virtualScrolling
     }
   },
   created () {
@@ -85,7 +96,7 @@ const Timeline = {
     const credentials = store.state.users.currentUser.credentials
     const showImmediately = this.timeline.visibleStatuses.length === 0
 
-    window.addEventListener('scroll', this.scrollLoad)
+    window.addEventListener('scroll', this.handleScroll)
 
     if (store.state.api.fetchers[this.timelineName]) { return false }
 
@@ -104,9 +115,10 @@ const Timeline = {
       this.unfocused = document.hidden
     }
     window.addEventListener('keydown', this.handleShortKey)
+    setTimeout(this.determineVisibleStatuses, 250)
   },
   destroyed () {
-    window.removeEventListener('scroll', this.scrollLoad)
+    window.removeEventListener('scroll', this.handleScroll)
     window.removeEventListener('keydown', this.handleShortKey)
     if (typeof document.hidden !== 'undefined') document.removeEventListener('visibilitychange', this.handleVisibilityChange, false)
     this.$store.commit('setLoading', { timeline: this.timelineName, value: false })
@@ -146,6 +158,48 @@ const Timeline = {
         }
       })
     }, 1000, this),
+    determineVisibleStatuses () {
+      if (!this.$refs.timeline) return
+      if (!this.virtualScrollingEnabled) return
+
+      const statuses = this.$refs.timeline.children
+      const cappedScrollIndex = Math.max(0, Math.min(this.virtualScrollIndex, statuses.length - 1))
+
+      if (statuses.length === 0) return
+
+      const height = Math.max(document.body.offsetHeight, window.pageYOffset)
+
+      const centerOfScreen = window.pageYOffset + (window.innerHeight * 0.5)
+
+      // Start from approximating the index of some visible status by using the
+      // the center of the screen on the timeline.
+      let approxIndex = Math.floor(statuses.length * (centerOfScreen / height))
+      let err = statuses[approxIndex].getBoundingClientRect().y
+
+      // if we have a previous scroll index that can be used, test if it's
+      // closer than the previous approximation, use it if so
+
+      const virtualScrollIndexY = statuses[cappedScrollIndex].getBoundingClientRect().y
+      if (Math.abs(err) > virtualScrollIndexY) {
+        approxIndex = cappedScrollIndex
+        err = virtualScrollIndexY
+      }
+
+      // if the status is too far from viewport, check the next/previous ones if
+      // they happen to be better
+      while (err < -20 && approxIndex < statuses.length - 1) {
+        err += statuses[approxIndex].offsetHeight
+        approxIndex++
+      }
+      while (err > window.innerHeight + 100 && approxIndex > 0) {
+        approxIndex--
+        err -= statuses[approxIndex].offsetHeight
+      }
+
+      // this status is now the center point for virtual scrolling and visible
+      // statuses will be nearby statuses before and after it
+      this.virtualScrollIndex = approxIndex
+    },
     scrollLoad (e) {
       const bodyBRect = document.body.getBoundingClientRect()
       const height = Math.max(bodyBRect.height, -(bodyBRect.y))
@@ -155,6 +209,10 @@ const Timeline = {
         this.fetchOlderStatuses()
       }
     },
+    handleScroll: throttle(function (e) {
+      this.determineVisibleStatuses()
+      this.scrollLoad(e)
+    }, 200),
     handleVisibilityChange () {
       this.unfocused = document.hidden
     }