前端加载百万条数据列表，如果采用真实的DOM插入100万个div（或li）标签，肯定是非常卡顿的。这就不得不使用虚拟列表技术方案，但是虚拟列表技术方案网上有很详细的实现方法，今天我就来谈谈根据网上的方案，把虚拟列表改造成虚拟表格的过程。
我就直接列出代码：
index.html,里面需要引入hyperHTML-min.js，我也提供了。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>virtual table</title>
<style>
.list-container {
overflow: auto;
border: 1px solid black;
height: 500px;
width: 700px;
box-sizing: border-box;
}
.list-container-inner{
}
.list-container-inner .box{
}
.list-container-inner .box .layer1{
display: table;
width: 100%;
height:100%;
font-size:0;line-height: 0;margin: 0;padding: 0;
}
.list-container-inner .box .layer1 .row-box{
overflow: hidden;
}
.list-container-inner .box .layer1 .row{
box-sizing: border-box;
height:calc(100% + 2px);
position: relative;
}
.list-container-inner .box .layer1 .row .cell-box{
box-sizing: border-box;
position: absolute;
height: 100%;
overflow: hidden;
}
.list-container-inner .box .layer1 .cell{
font-size:12px;
line-height: 12px;
}
/* .list-container-inner .box>div>div:first-child{
border-left: 0px solid red;
}
.list-container-inner .box>div>div:last-child{
border-right: 0px solid red;
} */
.list-container-inner .box .layer1>div>div:active{
border: 0px solid red;
}
.wh100{
width: calc(100% + 1px);
padding-right: 1px;
box-sizing: border-box;
height: 100%;
}
</style>
<script src="./hyperHTML-min.js"></script>
</head>
<body>
<div id="root"></div>
<!-- 外部容器用来固定列表容器的高度，同时生成滚动条 -->
<div class="list-container">
<!-- 内部容器用来装元素，高度是所有元素高度的和 -->
<div class="list-container-inner">
<div class="box">
<div class="layer1"></div>
<div class="layer2"></div>
</div>
</div>
</div>
<script>
/** --------- 一些基本变量 -------- */
const itemHeight = 60
const columnWidth = 100
const width = 700
const height = 500
const random = (min, max) => {
return Math.floor(Math.random() * (max - min)) + min;
}
const getRandomColor = () => {
let colors = [
'red',
'blue',
'yellow',
'orange',
'brown',
'purple',
'pink',
'green',
'gray',
'mauve'
]
return colors[random(0,10)]
}
/** --------- 生成数据 -------- */
const getRandomHeight = () => {
// 返回 [60, 150] 之间的随机数
return Math.floor(Math.random() * (80 - itemHeight + 1) + itemHeight)
}
const getRandomWidth = () => {
// 返回 [60, 150] 之间的随机数
return Math.floor(Math.random() * (220 - columnWidth + 1) + columnWidth)
}
const columns = () => {
const data = []
for (let i = 0; i < 50; i++) {
data.push({
content:i,
width:getRandomWidth(),
color: getRandomColor()
})
}
return data
}
const _columns = columns()
const initData = () => {
const data = []
for (let i = 0; i < 500000; i++) {
data.push({
content: _columns,
rowIndex:i,
height: getRandomHeight(),
color: i % 2 ? 'blue' : 'red'
})
}
return data
}
const data = initData()
const cacheWidthMap = {}
const cacheHeightMap = {}
const outerContainer = document.querySelector('.list-container')
const scrollCallback = () => {
//
let contentWidth = 0
let paddingLeft = 0
let upperWidth = 0
let startColIndex
let endColIndex
//
let contentHeight = 0
let paddingTop = 0
let upperHeight = 0
let startIndex
let endIndex
const innerContainer = document.querySelector('.list-container-inner')
const scrollTop = Math.max(outerContainer.scrollTop, 0)
const scrollLeft = Math.max(outerContainer.scrollLeft, 0)
for (let i = 0; i < (data[0] ? (data[0].content || []) : []).length; i++) {
const cacheWidth = cacheWidthMap[i]
const usedWidth = cacheWidth === undefined ? columnWidth : cacheWidth
contentWidth += usedWidth;
if (contentWidth >= scrollLeft && startColIndex === undefined) {
startColIndex = i
paddingLeft = contentWidth - usedWidth
}
if (contentWidth > scrollLeft + width && endColIndex === undefined) {
endColIndex = i
upperWidth = contentWidth
}
}
// 遍历所有的元素，获取当前元素的高度、列表总高度、startIndex、endIndex
for (let i = 0; i < data.length; i++) {
// 初始化的时候因为元素还没有渲染，无法获取元素的高度
// 所以用元素的最小高度itemHeight来进行计算，保证渲染的元素个数能占满列表
const cacheHeight = cacheHeightMap[i]
const usedHeight = cacheHeight === undefined ? itemHeight : cacheHeight
contentHeight += usedHeight
if (contentHeight >= scrollTop && startIndex === undefined) {
startIndex = i
paddingTop = contentHeight - usedHeight
}
if (contentHeight > scrollTop + height && endIndex === undefined) {
endIndex = i
// console.log(endIndex,startIndex == endIndex, contentHeight, scrollTop + height)
upperHeight = contentHeight
}
}
if(endColIndex === undefined) {
endColIndex = (data[0] ? (data[0].content || []) : []).length - 1
upperWidth = contentWidth
}
// 应对列表所有元素没有占满整个容器的情况
if (endIndex === undefined) {
endIndex = data.length - 1
upperHeight = contentHeight
}
const paddingRight = contentWidth - upperWidth
// innerContainer.setAttribute('style', `padding-left: ${paddingLeft}px; padding-bottom: ${paddingRight}px`)
// 未渲染的元素的高度由padding-top和padding-bottom代替，保证滚动条位置正确
// 这里如果把设置pading的操作放在渲染元素之后，部分浏览器滚动到最后一个元素时会有问题
const paddingBottom = contentHeight - upperHeight
innerContainer.setAttribute('style', `padding-left: ${paddingLeft}px; padding-right: ${paddingRight}px;padding-top: ${paddingTop}px; padding-bottom: ${paddingBottom}px`)
// 从data取出要渲染的元素并渲染到容器中
const viewData = data.slice(startIndex, endIndex + 1).map((item,index) => {
let _item = {...item, content: item.content.slice(startColIndex, endColIndex + 1)}
_item.content = _item.content.map((item,index) => {
if(index==0) {
item.left = 0
} else {
item.left = _item.content[index - 1].left + _item.content[index - 1].width
}
return item
})
return _item
})
// ${itemData.content.map(column => `<div class="cell-box" style="width:${column.width}px;left:${column.left}px;"><div class="wh100 cell" style="background:${getRandomColor()}">第${itemData.rowIndex}行，第${column.content}列</div></div>`)}
innerContainer.querySelector('.box').style.width = (viewData[0] ? viewData[0].content : []).reduce(function(accumulator, column) {return accumulator + column.width;}, 0)+ 'px'
innerContainer.querySelector('.box').style.height = viewData.map(item => item.height).reduce(function(accumulator, currentValue) {return accumulator + currentValue;}, 0) + 'px'
function update(render, state) {
render`
${state.viewData.map(itemData => hyperHTML.wire(itemData)`
<section class="row-box" style="height: ${itemData.height}px;background: ${itemData.color}">
<div class="row">
${itemData.content.map(column => `<div class="cell-box" style="width:${column.width}px;left:${column.left}px;"><div class="wh100 cell" style="background:${getRandomColor()}">第${itemData.rowIndex}行，第${column.content}列</div></div>`)}
</div>
</section>
`)}
`;
}
const render = hyperHTML.bind(innerContainer.querySelector('.box .layer1'))
update(render, {
viewData:viewData
})
// 存储已经渲染出来的元素的高度，供后面使用
const children = innerContainer.querySelector('.box .layer1').children
const columnChildren = innerContainer.querySelector('.box .layer1 .row').children
let widthFlag = startColIndex
for (const child of columnChildren) {
cacheWidthMap[widthFlag] = child.clientHeight
widthFlag++
}
let flag = startIndex
for (const child of children) {
cacheHeightMap[flag] = child.clientHeight
flag++
}
}
// 首屏渲染
scrollCallback()
// 监听外部容器的滚动事件
outerContainer.addEventListener('scroll', scrollCallback)
</script>
</body>
</html>hyperHTML-min.js
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n=this.state,r="function"==typeof e?e.call(this,n):e;for(var i in r)n[i]=r[i];return!1!==t&&this.render(),this}}}),gt}(document);原理 我就不过多赘述，主要就是说以下优点和缺点：
1、在27万条数据以内，背景填充没有问题。超过27万的div的背景不能正确填充div绘制的范围，导致看上去上下间有问题，有一条白色的水平线条。曾做过如下方法解决：
a 、我已经把font-size改为0了，不能解决。
b、把padding-top padding-bottom 改为 真实的元素占位，不能解决。因为我以为这情况回事padding导致。
c、也把div从 position static 改为 absolute 定位来解决，发现也与这个无关。
d、设置过background相关的参数，不能解决。
通过审查解决，是背景不能填充，确定是背景的问题。如果是在firfox，低于27万就出现这种问题了。
上图是垂直方向的问题，水平方向也会这种问题，我就不截图了。
2、超过27万以后的数据在chrome浏览器中，当滚动时候，出现渲染不准确问题，比如说border都设置为1px，但是看上去并不一致，outline也一样。而且有时候应该隐藏outline的时候，还是出现一些颜色在边上，不知道是不是显卡问题，但是浏览器渲染div又不用显卡。
3、这种方案，从百万数据中找到 屏幕可视数据的算法是非常棒的！经过有以上问题，但是性能非常好，别说100万，200万也就秒渲染，只是渲染后有一些问题。
4、padding填充内部实现内部内容高度的增高大约只支持到 55.92w条左右，超过这个数，将无法显示，因为chrome浏览器似乎支持的最大padding-bottom的值是33551640px，所以如果有需要超过这个值的需求，padding撑起内部内容高度实现滚动条是不可取的，必须自定义滚动条支持超过55.9万条的数据。
总结，如果实际需求中碰不到超过10万条的数据，这种方式处理10万条的数据，还是很优秀的，但是我想要他支持百万，可超过27万就出现了不可解决的问题。
