implement AsyncLayout

routes/_components/VirtualListItem.html

@@ -20,28 +20,20 @@
 </style>
 <script>
   import { virtualListStore } from '../_utils/virtualListStore'
+  import { AsyncLayout } from '../_utils/AsyncLayout'
   import { mark, stop } from '../_utils/marks'
 
   let updateItemHeights = {}
   let promise = Promise.resolve()
-  let onIntersectionCallbacks = {}
 
-  let intersectionObserver = new IntersectionObserver(entries => {
-    entries.forEach(entry => {
-      let key = entry.target.getAttribute('virtual-list-key')
-      onIntersectionCallbacks[key](entry)
-    })
-  })
+  const asyncLayout = new AsyncLayout(node => node.getAttribute('virtual-list-key'))
 
   export default {
     oncreate() {
       let key = this.get('key')
-      // TODO: implement AsyncLayout
       // TODO: implement batchUpdate
       // TODO: fix resize on media
-      onIntersectionCallbacks[key] = entry => {
-        console.log('onIntersection', key, entry.boundingClientRect.height)
-        let rect = entry.boundingClientRect
+      asyncLayout.observe(key, this.refs.node, (rect) => {
         updateItemHeights[key] = rect.height
         promise = promise.then(() => {
           // update all item heights in one microtask batch for better perf
@@ -61,22 +53,13 @@
           updateItemHeights = {}
           stop('batch update VirtualListItem')
         })
-        this.set({ unobserved: true })
-        intersectionObserver.unobserve(this.refs.node)
-      }
-      intersectionObserver.observe(this.refs.node)
+      })
     },
     ondestroy() {
       let key = this.get('key')
-      if (!this.get('unobserved')) {
-        intersectionObserver.unobserve(this.refs.node)
-      }
-      delete onIntersectionCallbacks[key]
+      asyncLayout.unobserve(key, this.refs.node)
       delete updateItemHeights[key]
     },
-    data: () => ({
-      unobserved: false
-    }),
     store: () => virtualListStore,
     computed: {
       'shown': ($itemHeights, key) => $itemHeights[key] > 0

routes/_utils/AsyncLayout.js

@@ -0,0 +1,31 @@
+// Use intersection observer to calculate rects asynchronously
+class AsyncLayout {
+  constructor(generateKeyFromNode) {
+    this._onIntersectionCallbacks = {}
+
+    this._intersectionObserver = new IntersectionObserver(entries => {
+      entries.forEach(entry => {
+        let key = generateKeyFromNode(entry.target)
+        this._onIntersectionCallbacks[key](entry)
+      })
+    })
+  }
+
+  observe(key, node, callback) {
+    this._onIntersectionCallbacks[key] = (entry) => {
+      callback(entry.boundingClientRect)
+      this.unobserve(key, node)
+    }
+    this._intersectionObserver.observe(node)
+  }
+
+  unobserve(key, node) {
+    if (key in this._onIntersectionCallbacks) {
+      return
+    }
+    this._intersectionObserver.unobserve(node)
+    delete this._onIntersectionCallbacks[key]
+  }
+}
+
+export { AsyncLayout }