perf: small code changes to maximize performance

Author: Akryum

Diff for: packages/vue-virtual-scroller/src/components/DynamicScroller.vue

@@ -111,7 +111,8 @@ export default {
       const result = []
       const { items, keyField, simpleArray } = this
       const sizes = this.vscrollData.sizes
-      for (let i = 0; i < items.length; i++) {
+      const l = items.length
+      for (let i = 0; i < l; i++) {
         const item = items[i]
         const id = simpleArray ? i : item[keyField]
         let size = sizes[id]

Diff for: packages/vue-virtual-scroller/src/components/DynamicScrollerItem.vue

@@ -149,7 +149,7 @@ export default {
     },
 
     updateWatchData () {
-      if (this.watchData) {
+      if (this.watchData && !this.vscrollResizeObserver) {
         this.$_watchData = this.$watch('item', () => {
           this.onDataUpdate()
         }, {
@@ -188,7 +188,7 @@ export default {
     },
 
     applySize (width, height) {
-      const size = Math.round(this.vscrollParent.direction === 'vertical' ? height : width)
+      const size = ~~(this.vscrollParent.direction === 'vertical' ? height : width)
       if (size && this.size !== size) {
         if (this.vscrollParent.$_undefinedMap[this.id]) {
           this.vscrollParent.$_undefinedSizes--

Diff for: packages/vue-virtual-scroller/src/components/RecycleScroller.vue

@@ -500,9 +500,7 @@ export default {
           if (view.nr.used) {
             // Update view item index
             if (checkItem) {
-              view.nr.index = items.findIndex(
-                item => keyField ? item[keyField] === view.item[keyField] : item === view.item,
-              )
+              view.nr.index = items.indexOf(view.item)
             }
 
             // Check if index is still in visible range