From 55ba2551b5f1060047e99d2a3b585924aa01875d Mon Sep 17 00:00:00 2001
From: mertsincan <sincan.mert@gmail.com>
Date: Sun, 5 Dec 2021 20:19:49 +0300
Subject: [PATCH] Fixed #1844 - Improve VirtualScroller implementation for Data
 components

---
 .../virtualscroller/VirtualScroller.vue       | 555 ++++++++++--------
 1 file changed, 298 insertions(+), 257 deletions(-)

diff --git a/src/components/virtualscroller/VirtualScroller.vue b/src/components/virtualscroller/VirtualScroller.vue
index 6cfa0f690..dadd8ea01 100644
--- a/src/components/virtualscroller/VirtualScroller.vue
+++ b/src/components/virtualscroller/VirtualScroller.vue
@@ -1,41 +1,50 @@
 <template>
     <template v-if="!disabled">
-        <div :ref="elementRef" :class="containerClass" :style="style" @scroll="onScroll">
-            <slot name="content" styleClass="p-virtualscroller-content" :contentRef="contentRef" :items="loadedItems" :getItemOptions="getOptions">
-                <div :ref="contentRef" class="p-virtualscroller-content">
+        <div :ref="elementRef" :class="containerClass" :tabindex="0" :style="style" @scroll="onScroll">
+            <slot name="content" :styleClass="contentClass" :items="loadedItems" :getItemOptions="getOptions" :loading="d_loading" :getLoaderOptions="getLoaderOptions" :itemSize="itemSize"
+                :rows="loadedRows" :columns="loadedColumns" :contentRef="contentRef" :spacerStyle="spacerStyle" :contentStyle="contentStyle"
+                :vertical="isVertical()" :horizontal="isHorizontal()" :both="isBoth()">
+                <div :ref="contentRef" :class="contentClass" :style="contentStyle">
                     <template v-for="(item, index) of loadedItems" :key="index">
                         <slot name="item" :item="item" :options="getOptions(index)"></slot>
                     </template>
                 </div>
             </slot>
-            <div :ref="spacerRef" class="p-virtualscroller-spacer"></div>
-            <div :class="loaderClass" v-if="d_loading">
-                <template v-for="(loadItem, index) of loaderArr" :key="index">
-                    <slot name="loader" :options="getLoaderOptions(index)">
-                        <i class="p-virtualscroller-loading-icon pi pi-spinner pi-spin"></i>
-                    </slot>
+            <div class="p-virtualscroller-spacer" :style="spacerStyle" v-if="showSpacer"></div>
+            <div :class="loaderClass" v-if="!loaderDisabled && showLoader && d_loading">
+                <template v-if="$slots && $slots.loader">
+                    <template v-for="(_, index) of loaderArr" :key="index">
+                        <slot name="loader" :options="getLoaderOptions(index, isBoth() && { numCols: d_numItemsInViewport.cols })"></slot>
+                    </template>
                 </template>
+                <i v-else class="p-virtualscroller-loading-icon pi pi-spinner pi-spin"></i>
             </div>
         </div>
     </template>
     <template v-else>
         <slot></slot>
-        <slot name="content" :items="items"></slot>
+        <slot name="content" :items="items" :rows="items" :columns="loadedColumns"></slot>
     </template>
 </template>
 
 <script>
 export default {
     name: 'VirtualScroller',
-    emits: ['update:numToleratedItems', 'scroll-index-change', 'lazy-load'],
+    emits: ['update:numToleratedItems', 'scroll', 'scroll-index-change', 'lazy-load'],
     props: {
+        id: {
+            type: String,
+            default: null
+        },
+        style: null,
+        class: null,
         items: {
             type: Array,
             default: null
         },
         itemSize: {
             type: [Number,Array],
-            default: null
+            default: 0
         },
         scrollHeight: null,
         scrollWidth: null,
@@ -55,17 +64,27 @@ export default {
             type: Boolean,
             default: false
         },
-        showLoader: {
+        disabled: {
             type: Boolean,
             default: false
         },
+        loaderDisabled: {
+            type: Boolean,
+            default: false
+        },
+        columns: {
+            type: Array,
+            default: null
+        },
         loading: {
             type: Boolean,
             default: false
         },
-        style: null,
-        class: null,
-        disabled: {
+        showSpacer: {
+            type: Boolean,
+            default: true
+        },
+        showLoader: {
             type: Boolean,
             default: false
         }
@@ -78,15 +97,19 @@ export default {
             lastScrollPos: this.isBoth() ? { top: 0, left: 0 } : 0,
             d_numToleratedItems: this.numToleratedItems,
             d_loading: this.loading,
-            loaderArr: null
+            loaderArr: [],
+            spacerStyle: {},
+            contentStyle: {}
         }
     },
     element: null,
     content: null,
-    spacer: null,
+    lastScrollPos: null,
     scrollTimeout: null,
     mounted() {
         this.init();
+
+        this.lastScrollPos = this.isBoth() ? { top: 0, left: 0 } : 0;
     },
     watch: {
         numToleratedItems(newValue) {
@@ -95,67 +118,185 @@ export default {
         loading(newValue) {
             this.d_loading = newValue;
         },
-        items(newValue, oldVal) {
-            if (!oldVal || oldVal.length !== (newValue || []).length) {
+        items(newValue, oldValue) {
+            if (!oldValue || oldValue.length !== (newValue || []).length) {
                 this.init();
             }
+        },
+        orientation() {
+            this.lastScrollPos = this.isBoth() ? { top: 0, left: 0 } : 0;
         }
     },
     methods: {
         init() {
-            if (!this.disabled) {
-                this.setSize();
-                this.calculateOptions();
-                this.setSpacerSize();
+            this.setSize();
+            this.calculateOptions();
+            this.setSpacerSize();
+        },
+        isVertical() {
+            return this.orientation === 'vertical';
+        },
+        isHorizontal() {
+            return this.orientation === 'horizontal';
+        },
+        isBoth() {
+            return this.orientation === 'both';
+        },
+        scrollTo(options) {
+            this.element && this.element.scrollTo(options);
+        },
+        scrollToIndex(index, behavior = 'auto') {
+            const both = this.isBoth();
+            const horizontal = this.isHorizontal();
+            const contentPos = this.getContentPosition();
+            const calculateFirst = (_index = 0, _numT) => (_index <= _numT ? 0 : _index);
+            const calculateCoord = (_first, _size, _cpos) => (_first * _size) + _cpos;
+            const scrollTo = (left = 0, top = 0) => this.scrollTo({ left, top, behavior });
+
+            if (both) {
+                const newFirst = { rows: calculateFirst(index[0], this.d_numToleratedItems[0]), cols: calculateFirst(index[1], this.d_numToleratedItems[1]) };
+                if (newFirst.rows !== this.first.rows || newFirst.cols !== this.first.cols) {
+                    scrollTo(calculateCoord(newFirst.cols, this.itemSize[1], contentPos.left), calculateCoord(newFirst.rows, this.itemSize[0], contentPos.top))
+                    this.first = newFirst;
+                }
+            }
+            else {
+                const newFirst = calculateFirst(index, this.d_numToleratedItems);
+
+                if (newFirst !== this.first) {
+                    horizontal ? scrollTo(calculateCoord(newFirst, this.itemSize, contentPos.left), 0) : scrollTo(0, calculateCoord(newFirst, this.itemSize, contentPos.top));
+                    this.first = newFirst;
+                }
             }
         },
-        getLast(last, isCols) {
-            return this.items ? Math.min((isCols ? this.items[0].length : this.items.length), last) : 0;
+        scrollInView(index, to, behavior = 'auto') {
+            if (to) {
+                const both = this.isBoth();
+                const horizontal = this.isHorizontal();
+                const { first, viewport } = this.getRenderedRange();
+                const scrollTo = (left = 0, top = 0) => this.scrollTo({ left, top, behavior });
+                const isToStart = to === 'to-start';
+                const isToEnd = to === 'to-end';
+
+                if (isToStart) {
+                    if (both) {
+                        if (viewport.first.rows - first.rows > index[0]) {
+                            scrollTo(viewport.first.cols * this.itemSize[1], (viewport.first.rows - 1) * this.itemSize[0]);
+                        }
+                        else if (viewport.first.cols - first.cols > index[1]) {
+                            scrollTo((viewport.first.cols - 1) * this.itemSize[1], viewport.first.rows * this.itemSize[0]);
+                        }
+                    }
+                    else {
+                        if (viewport.first - first > index) {
+                            const pos = (viewport.first - 1) * this.itemSize;
+                            horizontal ? scrollTo(pos, 0) : scrollTo(0, pos);
+                        }
+                    }
+                }
+                else if (isToEnd) {
+                    if (both) {
+                        if (viewport.last.rows - first.rows <= index[0] + 1) {
+                            scrollTo(viewport.first.cols * this.itemSize[1], (viewport.first.rows + 1) * this.itemSize[0]);
+                        }
+                        else if (viewport.last.cols - first.cols <= index[1] + 1) {
+                            scrollTo((viewport.first.cols + 1) * this.itemSize[1], viewport.first.rows * this.itemSize[0]);
+                        }
+                    }
+                    else {
+                        if (viewport.last - first <= index + 1) {
+                            const pos = (viewport.first + 1) * this.itemSize;
+                            horizontal ? scrollTo(pos, 0) : scrollTo(0, pos);
+                        }
+                    }
+                }
+            }
+            else {
+                this.scrollToIndex(index, behavior);
+            }
         },
-        calculateOptions() {
-            const isBoth = this.isBoth();
-            const isHorizontal = this.isHorizontal();
-            const first = this.first;
-            const itemSize = this.itemSize;
-            const contentPadding = this.getContentPadding();
-            const contentWidth = this.element ? this.element.offsetWidth - contentPadding.left : 0;
-            const contentHeight = this.element ? this.element.offsetHeight - contentPadding.top : 0;
-            const calculateNumItemsInViewport = (_contentSize, _itemSize) => Math.ceil(_contentSize / (_itemSize || _contentSize));
-            const numItemsInViewport = isBoth ?
-                { rows: calculateNumItemsInViewport(contentHeight, itemSize[0]), cols: calculateNumItemsInViewport(contentWidth, itemSize[1]) } :
-                calculateNumItemsInViewport((isHorizontal ? contentWidth : contentHeight), itemSize);
+        getRenderedRange() {
+            const calculateFirstInViewport = (_pos, _size) => Math.floor(_pos / (_size || _pos));
 
-            let numToleratedItems = this.d_numToleratedItems || Math.ceil((isBoth ? numItemsInViewport.rows : numItemsInViewport) / 2);
-            const calculateLast = (_first, _num, _isCols) => this.getLast(_first + _num + ((_first < numToleratedItems ? 2 : 3) * numToleratedItems), _isCols);
-            const last = isBoth ?
-                { rows: calculateLast(first.rows, numItemsInViewport.rows), cols: calculateLast(first.cols, numItemsInViewport.cols, true) } :
-                calculateLast(first, numItemsInViewport);
+            let firstInViewport = this.first;
+            let lastInViewport = 0;
 
-            this.d_numToleratedItems = numToleratedItems;
-            this.$emit('update:numToleratedItems', this.d_numToleratedItems);
-            this.last = last;
-            this.numItemsInViewport = numItemsInViewport;
+            if (this.element) {
+                const both = this.isBoth();
+                const horizontal = this.isHorizontal();
+                const scrollTop = this.element.scrollTop;
+                const scrollLeft = this.element.scrollLeft;
 
-            if (this.showLoader) {
-                if (this.$slots && this.$slots.loader) {
-                    this.loaderArr = Array.from({ length: (isBoth ? numItemsInViewport.rows : numItemsInViewport) });
+                if (both) {
+                    firstInViewport = { rows: calculateFirstInViewport(scrollTop, this.itemSize[0]), cols: calculateFirstInViewport(scrollLeft, this.itemSize[1]) };
+                    lastInViewport = { rows: firstInViewport.rows + this.numItemsInViewport.rows, cols: firstInViewport.cols + this.numItemsInViewport.cols };
                 }
                 else {
-                    this.loaderArr = Array.from({ length: 1});
+                    const scrollPos = horizontal ? scrollLeft : scrollTop;
+                    firstInViewport = calculateFirstInViewport(scrollPos, this.itemSize);
+                    lastInViewport = firstInViewport + this.numItemsInViewport;
                 }
             }
 
+            return {
+                first: this.first,
+                last: this.last,
+                viewport: {
+                    first: firstInViewport,
+                    last: lastInViewport
+                }
+            };
+        },
+        calculateOptions() {
+            const both = this.isBoth();
+            const horizontal = this.isHorizontal();
+            const contentPos = this.getContentPosition();
+            const contentWidth = this.element ? this.element.offsetWidth - contentPos.left : 0;
+            const contentHeight = this.element ? this.element.offsetHeight - contentPos.top : 0;
+            const calculateNumItemsInViewport = (_contentSize, _itemSize) => Math.ceil(_contentSize / (_itemSize || _contentSize));
+            const calculateNumToleratedItems = (_numItems) => Math.ceil(_numItems / 2);
+            const numItemsInViewport = both ?
+                { rows: calculateNumItemsInViewport(contentHeight, this.itemSize[0]), cols: calculateNumItemsInViewport(contentWidth, this.itemSize[1]) } :
+                calculateNumItemsInViewport((horizontal ? contentWidth : contentHeight), this.itemSize);
+
+            let numToleratedItems = this.d_numToleratedItems || (both ?
+                [calculateNumToleratedItems(numItemsInViewport.rows), calculateNumToleratedItems(numItemsInViewport.cols)] :
+                calculateNumToleratedItems(numItemsInViewport));
+
+            const calculateLast = (_first, _num, _numT, _isCols) => this.getLast(_first + _num + ((_first < _numT ? 2 : 3) * _numT), _isCols);
+            const last = both ?
+                { rows: calculateLast(this.first.rows, numItemsInViewport.rows, numToleratedItems[0]), cols: calculateLast(this.first.cols, numItemsInViewport.cols, numToleratedItems[1], true) } :
+                calculateLast(this.first, numItemsInViewport, numToleratedItems);
+
+            this.last = last;
+            this.numItemsInViewport = numItemsInViewport;
+            this.d_numToleratedItems = numToleratedItems;
+            this.$emit('update:numToleratedItems', this.d_numToleratedItems);
+
+            if (this.showLoader) {
+                this.loaderArr = both ?
+                    Array.from({ length: numItemsInViewport.rows }).map(() => Array.from({ length: numItemsInViewport.cols })) :
+                    Array.from({ length: numItemsInViewport });
+            }
+
             if (this.lazy) {
-                this.$emit('lazy-load', { first, last });
+                this.$emit('lazy-load', { first: this.first, last });
             }
         },
-        getContentPadding() {
+        getLast(last = 0, isCols) {
+             if (this.items) {
+                return Math.min((isCols ? (this.columns || this.items[0]).length : this.items.length), last);
+            }
+
+            return 0;
+        },
+        getContentPosition() {
             if (this.content) {
                 const style = getComputedStyle(this.content);
-                const left = parseInt(parseFloat(style.paddingLeft.slice(0, -2)), 10);
-                const right = parseInt(parseFloat(style.paddingRight.slice(0, -2)), 10);
-                const top = parseInt(parseFloat(style.paddingTop.slice(0, -2)), 10);
-                const bottom = parseInt(parseFloat(style.paddingBottom.slice(0, -2)), 10);
+                const left = parseInt(style.paddingLeft, 10) + Math.max(parseInt(style.left, 10), 0);
+                const right = parseInt(style.paddingRight, 10) + Math.max(parseInt(style.right, 10), 0);
+                const top = parseInt(style.paddingTop, 10) + Math.max(parseInt(style.top, 10), 0);
+                const bottom = parseInt(style.paddingBottom, 10) + Math.max(parseInt(style.bottom, 10), 0);
 
                 return { left, right, top, bottom, x: left + right, y: top + bottom };
             }
@@ -164,126 +305,125 @@ export default {
         },
         setSize() {
             if (this.element) {
-                const isBoth = this.isBoth();
-                const isHorizontal = this.isHorizontal();
+                const both = this.isBoth();
+                const horizontal = this.isHorizontal();
                 const parentElement = this.element.parentElement;
                 const width = this.scrollWidth || `${(this.element.offsetWidth || parentElement.offsetWidth)}px`;
                 const height = this.scrollHeight || `${(this.element.offsetHeight || parentElement.offsetHeight)}px`;
                 const setProp = (_name, _value) => this.element.style[_name] = _value;
 
-                if (isBoth) {
+                if (both || horizontal) {
                     setProp('height', height);
                     setProp('width', width);
                 }
                 else {
-                    isHorizontal ? setProp('width', width) : setProp('height', height);
+                    setProp('height', height);
                 }
             }
         },
         setSpacerSize() {
             const items = this.items;
 
-            if (this.spacer && items) {
-                const isBoth = this.isBoth();
-                const isHorizontal = this.isHorizontal();
-                const itemSize = this.itemSize;
-                const contentPadding = this.getContentPadding();
-                const setProp = (_name, _value, _size, _padding = 0) => this.spacer.style[_name] = (((_value || []).length * _size) + _padding) + 'px';
+            if (items) {
+                const both = this.isBoth();
+                const horizontal = this.isHorizontal();
+                const contentPos = this.getContentPosition();
+                const setProp = (_name, _value, _size, _cpos = 0) => this.spacerStyle = { ...this.spacerStyle, ...{ [`${_name}`]: (((_value || []).length * _size) + _cpos) + 'px' } };
 
-                if (isBoth) {
-                    setProp('height', items[0], itemSize[0], contentPadding.y);
-                    setProp('width', items[1], itemSize[1], contentPadding.x);
+                if (both) {
+                    setProp('height', items, this.itemSize[0], contentPos.y);
+                    setProp('width', (this.columns || items[1]), this.itemSize[1], contentPos.x);
                 }
                 else {
-                    isHorizontal ? setProp('width', items, itemSize, contentPadding.x) : setProp('height', items, itemSize, contentPadding.y);
+                    horizontal ? setProp('width', (this.columns || items), this.itemSize, contentPos.x) : setProp('height', items, this.itemSize, contentPos.y);
                 }
             }
         },
         setContentPosition(pos) {
             if (this.content) {
-                const isBoth = this.isBoth();
-                const isHorizontal = this.isHorizontal();
-                const content = this.content;
+                const both = this.isBoth();
+                const horizontal = this.isHorizontal();
                 const first = pos ? pos.first : this.first;
-                const itemSize = this.itemSize;
                 const calculateTranslateVal = (_first, _size) => (_first * _size);
-                const setTransform = (_x = 0, _y = 0) => content.style.transform = `translate3d(${_x}px, ${_y}px, 0)`;
+                const setTransform = (_x = 0, _y = 0) => {
+                    this.contentStyle = { ...this.contentStyle, ...{ transform: `translate3d(${_x}px, ${_y}px, 0)` } };
+                };
 
-                if (isBoth) {
-                    setTransform(calculateTranslateVal(first.cols, itemSize[1]), calculateTranslateVal(first.rows, itemSize[0]));
+                if (both) {
+                    setTransform(calculateTranslateVal(first.cols, this.itemSize[1]), calculateTranslateVal(first.rows, this.itemSize[0]));
                 }
                 else {
-                    const translateVal = calculateTranslateVal(first, itemSize);
-                    isHorizontal ? setTransform(translateVal, 0) : setTransform(0, translateVal);
+                    const translateVal = calculateTranslateVal(first, this.itemSize);
+                    horizontal ? setTransform(translateVal, 0) : setTransform(0, translateVal);
                 }
             }
         },
         onScrollPositionChange(event) {
             const target = event.target;
-            const isBoth = this.isBoth();
-            const isHorizontal = this.isHorizontal();
-            const itemSize = this.itemSize;
-            const contentPadding = this.getContentPadding();
-            const calculateScrollPos = (_pos, _padding) => _pos ? (_pos > _padding ? _pos - _padding : _pos) : 0;
+            const both = this.isBoth();
+            const horizontal = this.isHorizontal();
+            const contentPos = this.getContentPosition();
+            const calculateScrollPos = (_pos, _cpos) => _pos ? (_pos > _cpos ? _pos - _cpos : _pos) : 0;
             const calculateCurrentIndex = (_pos, _size) => Math.floor(_pos / (_size || _pos));
-            const calculateTriggerIndex = (_currentIndex, _first, _last, _num, _isScrollDownOrRight) => {
-                return (_currentIndex <= this.d_numToleratedItems ? this.d_numToleratedItems : (_isScrollDownOrRight ? (_last - _num - this.d_numToleratedItems) : (_first + this.d_numToleratedItems - 1)))
+            const calculateTriggerIndex = (_currentIndex, _first, _last, _num, _numT, _isScrollDownOrRight) => {
+                return (_currentIndex <= _numT ? _numT : (_isScrollDownOrRight ? (_last - _num - _numT) : (_first + _numT - 1)))
             };
-            const calculateFirst = (_currentIndex, _triggerIndex, _first, _last, _num, _isScrollDownOrRight) => {
-                if (_currentIndex <= this.d_numToleratedItems)
+            const calculateFirst = (_currentIndex, _triggerIndex, _first, _last, _num, _numT, _isScrollDownOrRight) => {
+                if (_currentIndex <= _numT)
                     return 0;
                 else
-                    return _isScrollDownOrRight ?
-                        (_currentIndex < _triggerIndex ? _first : _currentIndex - this.d_numToleratedItems) :
-                        (_currentIndex > _triggerIndex ? _first : _currentIndex - (2 * this.d_numToleratedItems));
+                    return Math.max(0, _isScrollDownOrRight ?
+                            (_currentIndex < _triggerIndex ? _first : _currentIndex - _numT) :
+                            (_currentIndex > _triggerIndex ? _first : _currentIndex - (2 * _numT)));
             };
-            const calculateLast = (_currentIndex, _first, _last, _num, _isCols) => {
-                let lastValue = _first + _num + (2 * this.d_numToleratedItems);
+            const calculateLast = (_currentIndex, _first, _last, _num, _numT, _isCols) => {
+                let lastValue = _first + _num + (2 * _numT);
 
-                if (_currentIndex >= this.d_numToleratedItems) {
-                    lastValue += (this.d_numToleratedItems + 1);
+                if (_currentIndex >= _numT) {
+                    lastValue += (_numT + 1);
                 }
 
                 return this.getLast(lastValue, _isCols);
             };
 
-            const scrollTop = calculateScrollPos(target.scrollTop, contentPadding.top);
-            const scrollLeft = calculateScrollPos(target.scrollLeft, contentPadding.left);
+            const scrollTop = calculateScrollPos(target.scrollTop, contentPos.top);
+            const scrollLeft = calculateScrollPos(target.scrollLeft, contentPos.left);
 
             let newFirst = 0;
             let newLast = this.last;
             let isRangeChanged = false;
 
-            if (isBoth) {
+            if (both) {
                 const isScrollDown = this.lastScrollPos.top <= scrollTop;
                 const isScrollRight = this.lastScrollPos.left <= scrollLeft;
-                const currentIndex = { rows: calculateCurrentIndex(scrollTop, itemSize[0]), cols: calculateCurrentIndex(scrollLeft, itemSize[1]) };
+                const currentIndex = { rows: calculateCurrentIndex(scrollTop, this.itemSize[0]), cols: calculateCurrentIndex(scrollLeft, this.itemSize[1]) };
                 const triggerIndex = {
-                    rows: calculateTriggerIndex(currentIndex.rows, this.first.rows, this.last.rows, this.numItemsInViewport.rows, isScrollDown),
-                    cols: calculateTriggerIndex(currentIndex.cols, this.first.cols, this.last.cols, this.numItemsInViewport.cols, isScrollRight)
+                    rows: calculateTriggerIndex(currentIndex.rows, this.first.rows, this.last.rows, this.numItemsInViewport.rows, this.d_numToleratedItems[0], isScrollDown),
+                    cols: calculateTriggerIndex(currentIndex.cols, this.first.cols, this.last.cols, this.numItemsInViewport.cols, this.d_numToleratedItems[1], isScrollRight)
                 };
 
                 newFirst = {
-                    rows: calculateFirst(currentIndex.rows, triggerIndex.rows, this.first.rows, this.last.rows, this.numItemsInViewport.rows, isScrollDown),
-                    cols: calculateFirst(currentIndex.cols, triggerIndex.cols, this.first.cols, this.last.cols, this.numItemsInViewport.cols, isScrollRight)
+                    rows: calculateFirst(currentIndex.rows, triggerIndex.rows, this.first.rows, this.last.rows, this.numItemsInViewport.rows, this.d_numToleratedItems[0], isScrollDown),
+                    cols: calculateFirst(currentIndex.cols, triggerIndex.cols, this.first.cols, this.last.cols, this.numItemsInViewport.cols, this.d_numToleratedItems[1], isScrollRight)
                 };
                 newLast = {
-                    rows: calculateLast(currentIndex.rows, newFirst.rows, this.last.rows, this.numItemsInViewport.rows),
-                    cols: calculateLast(currentIndex.cols, newFirst.cols, this.last.cols, this.numItemsInViewport.cols, true)
+                    rows: calculateLast(currentIndex.rows, newFirst.rows, this.last.rows, this.numItemsInViewport.rows, this.d_numToleratedItems[0]),
+                    cols: calculateLast(currentIndex.cols, newFirst.cols, this.last.cols, this.numItemsInViewport.cols, this.d_numToleratedItems[1], true)
                 };
-                isRangeChanged = (newFirst.rows !== this.first.rows || newFirst.cols !== this.first.cols) || (newLast.rows !== this.last.rows || newLast.cols !== this.last.cols);
+
+                isRangeChanged = (newFirst.rows !== this.first.rows && newLast.rows !== this.last.rows) || (newFirst.cols !== this.first.cols && newLast.cols !== this.last.cols);
 
                 this.lastScrollPos = { top: scrollTop, left: scrollLeft };
             }
             else {
-                const scrollPos = isHorizontal ? scrollLeft : scrollTop;
+                const scrollPos = horizontal ? scrollLeft : scrollTop;
                 const isScrollDownOrRight = this.lastScrollPos <= scrollPos;
-                const currentIndex = calculateCurrentIndex(scrollPos, itemSize);
-                const triggerIndex = calculateTriggerIndex(currentIndex, this.first, this.last, this.numItemsInViewport, isScrollDownOrRight);
+                const currentIndex = calculateCurrentIndex(scrollPos, this.itemSize);
+                const triggerIndex = calculateTriggerIndex(currentIndex, this.first, this.last, this.numItemsInViewport, this.d_numToleratedItems, isScrollDownOrRight);
 
-                newFirst = calculateFirst(currentIndex, triggerIndex, this.first, this.last, this.numItemsInViewport, isScrollDownOrRight);
-                newLast = calculateLast(currentIndex, newFirst, this.last, this.numItemsInViewport);
-                isRangeChanged = newFirst !== this.first || newLast !== this.last;
+                newFirst = calculateFirst(currentIndex, triggerIndex, this.first, this.last, this.numItemsInViewport, this.d_numToleratedItems, isScrollDownOrRight);
+                newLast = calculateLast(currentIndex, newFirst, this.last, this.numItemsInViewport, this.d_numToleratedItems);
+                isRangeChanged = newFirst !== this.first && newLast !== this.last;
 
                 this.lastScrollPos = scrollPos;
             }
@@ -302,27 +442,27 @@ export default {
 
                 this.setContentPosition(newState);
 
-                if (this.lazy) {
-                    this.$emit('lazy-load', { first, last });
-                }
                 this.first = first;
                 this.last = last;
 
-                this.$emit('scroll-index-change', { first, last });
+                this.$emit('scroll-index-change', newState);
+
+                if (this.lazy) {
+                    this.$emit('lazy-load', newState);
+                }
             }
         },
         onScroll(event) {
-            if (this.delay && !this.lazy) {
+            this.$emit('scroll', event);
+
+            if (this.delay) {
                 if (this.scrollTimeout) {
                     clearTimeout(this.scrollTimeout);
                 }
 
                 if (!this.d_loading && this.showLoader) {
                     const { isRangeChanged: changed } = this.onScrollPositionChange(event);
-
-                    if (changed) {
-                        this.d_loading = true;
-                    }
+                    changed && (this.d_loading = true);
                 }
 
                 this.scrollTimeout = setTimeout(() => {
@@ -338,8 +478,8 @@ export default {
             }
         },
         getOptions(renderedIndex) {
-            let count = this.items.length;
-            let index = this.isBoth() ? this.first.rows + renderedIndex : this.first + renderedIndex;
+            const count = (this.items || []).length;
+            const index = this.isBoth() ? this.first.rows + renderedIndex : this.first + renderedIndex;
             return {
                 index,
                 count,
@@ -349,139 +489,23 @@ export default {
                 odd: index % 2 !== 0
             };
         },
-        getLoaderOptions(index) {
+        getLoaderOptions(index, extOptions) {
             let count = this.loaderArr.length;
             return {
-                loading: this.d_loading,
+                index,
+                count,
                 first: index === 0,
                 last: index === (count - 1),
                 even: index % 2 === 0,
-                odd: index % 2 !== 0
+                odd: index % 2 !== 0,
+                ...extOptions
             }
         },
-        isHorizontal() {
-            return this.orientation === 'horizontal';
-        },
-        isBoth() {
-            return this.orientation === 'both';
-        },
-        scrollTo(options) {
-            if (this.element) {
-                this.element.scrollTo(options);
-            }
-        },
-        scrollToIndex(index, behavior = 'auto') {
-            const isBoth = this.isBoth();
-            const isHorizontal = this.isHorizontal();
-            const itemSize = this.itemSize;
-            const contentPadding = this.getContentPadding();
-            const calculateFirst = (_index = 0) => (_index <= this.d_numToleratedItems ? 0 : _index);
-            const calculateCoord = (_first, _size, _padding) => (_first * _size) + _padding;
-            const scrollTo = (left = 0, top = 0) => this.scrollTo({ left, top, behavior });
-
-            if (isBoth) {
-                const newFirst = { rows: calculateFirst(index[0]), cols: calculateFirst(index[1]) };
-                if (newFirst.rows !== this.first.rows || newFirst.cols !== this.first.cols)
-                    scrollTo(calculateCoord(newFirst.cols, itemSize[1], contentPadding.left),calculateCoord(newFirst.rows, itemSize[0], contentPadding.top));
-            }
-            else {
-                const newFirst = calculateFirst(index);
-
-                if (newFirst !== this.first) {
-                    isHorizontal ? scrollTo(calculateCoord(newFirst, itemSize, contentPadding.left), 0) : scrollTo(0, calculateCoord(newFirst, itemSize, contentPadding.top));
-                }
-
-                this.first = newFirst;
-            }
-        },
-        scrollInView(index, to, behavior = 'auto') {
-            if (to) {
-                const isBoth = this.isBoth();
-                const isHorizontal = this.isHorizontal();
-                const { first, viewport } = this.getRenderedRange();
-                const itemSize = this.itemSize;
-                const scrollTo = (left = 0, top = 0) => this.scrollTo({ left, top, behavior });
-                const isToStart = to === 'to-start';
-                const isToEnd = to === 'to-end';
-
-                if (isToStart) {
-                    if (isBoth) {
-                        if (viewport.first.rows - first.rows > index[0]) {
-                            scrollTo(viewport.first.cols * itemSize[1], (viewport.first.rows - 1) * itemSize[0]);
-                        }
-                        else if (viewport.first.cols - first.cols > index[1]) {
-                            scrollTo((viewport.first.cols - 1) * itemSize[1], viewport.first.rows * itemSize[0]);
-                        }
-                    }
-                    else {
-                        if (viewport.first - first > index) {
-                            const pos = (viewport.first - 1) * itemSize;
-                            isHorizontal ? scrollTo(pos, 0) : scrollTo(0, pos);
-                        }
-                    }
-                }
-                else if (isToEnd) {
-                    if (isBoth) {
-                        if (viewport.last.rows - first.rows <= index[0] + 1) {
-                            scrollTo(viewport.first.cols * itemSize[1], (viewport.first.rows + 1) * itemSize[0]);
-                        }
-                        else if (viewport.last.cols - first.cols <= index[1] + 1) {
-                            scrollTo((viewport.first.cols + 1) * itemSize[1], viewport.first.rows * itemSize[0]);
-                        }
-                    }
-                    else {
-                        if (viewport.last - first <= index + 1) {
-                            const pos = (viewport.first + 1) * itemSize;
-                            isHorizontal ? scrollTo(pos, 0) : scrollTo(0, pos);
-                        }
-                    }
-                }
-            }
-            else {
-                this.scrollToIndex(index, behavior);
-            }
-        },
-        getRenderedRange() {
-            const isBoth = this.isBoth();
-            const isHorizontal = this.isHorizontal();
-            const itemSize = this.itemSize;
-            const calculateFirstInViewport = (_pos, _size) => Math.floor(_pos / (_size || _pos));
-
-            let firstInViewport = this.first;
-            let lastInViewport = 0;
-
-            if (this.element) {
-                const scrollTop = this.element.scrollTop;
-                const scrollLeft = this.element.scrollLeft;
-
-                if (isBoth) {
-                    firstInViewport = { rows: calculateFirstInViewport(scrollTop, itemSize[0]), cols: calculateFirstInViewport(scrollLeft, itemSize[1]) };
-                    lastInViewport = { rows: firstInViewport.rows + this.numItemsInViewport.rows, cols: firstInViewport.cols + this.numItemsInViewport.cols };
-                }
-                else {
-                    const scrollPos = isHorizontal ? scrollLeft : scrollTop;
-                    firstInViewport = calculateFirstInViewport(scrollPos, itemSize);
-                    lastInViewport = firstInViewport + this.numItemsInViewport;
-                }
-            }
-
-            return {
-                first: this.first,
-                last: this.last,
-                viewport: {
-                    first: firstInViewport,
-                    last: lastInViewport
-                }
-            };
-        },
         elementRef(el) {
             this.element = el;
         },
         contentRef(el) {
             this.content = el;
-        },
-        spacerRef(el) {
-            this.spacer = el;
         }
     },
     computed: {
@@ -491,6 +515,11 @@ export default {
                 'p-horizontal-scroll': this.isHorizontal()
             }, this.class];
         },
+        contentClass() {
+            return ['p-virtualscroller-content', {
+                'p-virtualscroller-loading': this.d_loading
+            }];
+        },
         loaderClass() {
             return ['p-virtualscroller-loader', {
                 'p-component-overlay': !this.$slots.loader
@@ -499,22 +528,33 @@ export default {
         loadedItems() {
             const items = this.items;
             if (items && !this.d_loading) {
-                const isBoth = this.isBoth();
-
-                if (isBoth) {
-                    return items.slice(this.first.rows, this.last.rows).map((item) => {
-                        const items = item.slice(this.first.cols, this.last.cols);
-                        return items;
-                    });
-                }
-                else {
-                    return items.slice(this.first, this.last).map((item) => {
-                        return item;
-                    });
+                if (this.isBoth()) {
+                    return items.slice(this.first.rows, this.last.rows).map(item => this.columns ? item : item.slice(this.first.cols, this.last.cols));
                 }
+                else if (this.isHorizontal() && this.columns)
+                    return items;
+                else
+                    return items.slice(this.first, this.last);
             }
 
             return [];
+        },
+        loadedRows() {
+            return this.d_loading ? (this.loaderDisabled ? this.loaderArr : []) : this.loadedItems;
+        },
+        loadedColumns() {
+            if (this.columns) {
+                const both = this.isBoth();
+                const horizontal = this.isHorizontal();
+
+                if (both || horizontal) {
+                    return this.d_loading && this.loaderDisabled ?
+                    (both ? this.loaderArr[0] : this.loaderArr):
+                    this.columns.slice((both ? this.first.cols : this.first), (both ? this.last.cols : this.last));
+                }
+            }
+
+            return this.columns;
         }
     }
 }
@@ -527,6 +567,7 @@ export default {
     contain: strict;
     transform: translateZ(0);
     will-change: scroll-position;
+    outline: 0 none;
 }
 
 .p-virtualscroller-content {