案例十二、JavaScript实现无穷滚动加载数据

首先实现滚动要借助onscroll事件处理程序。
无穷滚动就是滑动滚动条，实现数据块的无穷加载。
我们以瀑布流的布局来实现无穷加载。瀑布流就是一些等宽不等高的数据块布局。在滚动条下拉时，它会进行加载。那么问题就是在判断什么时候应该加载。

我们首先应该明白瀑布流布局的特点。它将下一个图片总是放在当前列数最低的那一列。所以当加载最后一个蓝色的图片时，也就无疑是最后一个图片了。所以要判断该图片加载到什么程度来触发滚动事件。
图上灰色的表示页面的大小，后面蓝色边框表示窗口的大小。当拖动滚动条时，灰色部分上移。我们希望页面最后一个图片（蓝色图片）加载一半时触发滚动事件。那么就要形成参照。

下来写代码：

//检测是否具备滚动条加载数据块的条件

function checkScrollSlide(){

    var oparent = document.getElementById('main');

    var oBoxs = getByClass(oparent,'box');

    var scrollH = document.body.scrollTop || document.documentElement.scrollTop + document.body.clientHeight || document.documentElement.clientHeight;

   var lastBoxH = oBoxs[oBoxs.length - 1].offsetTop + Math.floor(oBoxs[oBoxs.length - 1].offsetHeight/2);

    return (lastBoxH < scrollH )? true : false;
}

getClass是根据类名查找元素集合的自己写的函数，待会儿会附上源码

我们要进行加载的数据块的格式是这样的.每个图片被一个class= pic的div包裹。最后整体属于父元素main

<div id = "main">
    <div class = "box">
        <div class = "pic">
            "images/0.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/1.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/2.jpg">
        div>
    div>
<div>

首先我们要加载的数据块应该是从后台传过来的，在这里我们用json模拟一下就可以：

var DataIn = {"data":[{"src":'0.jpg'},{"src":'1.jpg'},{"src":'2.jpg'},{"src":'3.jpg'},{"src":'4.jpg'},{"src":'5.jpg'}]};

比如这个就是后台传过来的数据。

上面的函数返回一个布尔值，当为true时，触发滚动事件。

  window.onscroll = function(){
    var oparent = document.getElementById('main');
    //当满足加载条件时，就要向页面中加载数据块
    if(checkScrollSlide){
        for(var i = 0; i< DataIn.data.length; i++){
                var oBox = document.createElement('div');
                oBox.className = 'box';
                oparent.appendChild(oBox);

                var opic = document.createElement('div');
                opic.className = 'pic';
                oBox.appendChild(opic);

                var oImg = document.createElement('img');
                oImg.src = './images/'+DataIn.data[i].src;
                opic.appendChild(oImg);

            }
            waterFull('main','box');
        }
    }
}

附上源码：
css和html

"utf-8" />
<script src = "./jswaterfll.js">script>
    
    


<div id = 'main'>
    <div class = "box">
        <div class = "pic">
            "images/3.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/4.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/20.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/21.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/2.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/3.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/4.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/5.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/6.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/7.jpg">
        div>
    div>

    <div class = "box">
        <div class = "pic">
            "images/8.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/9.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/10.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/11.jpg">
        div>
    div>

    <div class = "box">
        <div class = "pic">
            "images/17.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/18.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/19.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/4.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/5.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/6.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/7.jpg">
        div>
    div>

    <div class = "box">
        <div class = "pic">
            "images/8.jpg">
        div>
    div>

    <div class = "box">
        <div class = "pic">
            "images/11.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/12.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/13.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/14.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/15.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/16.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/17.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/18.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/19.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/20.jpg">
        div>
    div>
    <div class = "box">
        <div class = "pic">
            "images/21.jpg">
        div>
    div>
div>

js代码：

window.onload = function(){
    waterFull('main','box');

    var DataIn = {"data":[{"src":'0.jpg'},{"src":'1.jpg'},{"src":'2.jpg'},{"src":'3.jpg'},{"src":'4.jpg'},{"src":'5.jpg'}]};
    window.onscroll = function(){
        var oparent = document.getElementById('main');
        if(checkScrollSlide){
            for(var i = 0; i< DataIn.data.length; i++){
                var oBox = document.createElement('div');
                oBox.className = 'box';
                oparent.appendChild(oBox);

                var opic = document.createElement('div');
                opic.className = 'pic';
                oBox.appendChild(opic);

                var oImg = document.createElement('img');
                oImg.src = './images/'+DataIn.data[i].src;
                opic.appendChild(oImg);

            }
            waterFull('main','box');
        }
    }
}

function waterFull(parent,children){
    var oParent = document.getElementById(parent);
    //var oBoxs = parent.querySelectorAll(".box");

     var oBoxs = getByClass(oParent,children);

    //计算整个页面显示的列数

    var oBoxW = oBoxs[0].offsetWidth;

    var cols = Math.floor(document.documentElement.clientWidth/oBoxW);

    //设置main的宽度，并且居中

    oParent.style.cssText = 'width:'+oBoxW * cols +'px; margin: 0 auto';

    //找出高度最小的图片，将下一个图片放在下面

    //定义一个数组，存放每一列的高度，初始化存的是第一行的所有列的高度

    var arrH = [];

    for(var i = 0; i< oBoxs.length ; i++){
        if(i < cols){
            arrH.push(oBoxs[i].offsetHeight);
        }
        else{
            var minH = Math.min.apply(null,arrH);

            var minIndex = getMinhIndex(arrH,minH);

            oBoxs[i].style.position = 'absolute';
            oBoxs[i].style.top= minH + 'px';
            //oBoxs[i].style.left = minIndex * oBoxW + 'px'; 
            oBoxs[i].style.left = oBoxs[minIndex].offsetLeft+'px';

            arrH[minIndex] += oBoxs[i].offsetHeight; 
        }
    }


}
function getByClass(parent,className){

    var boxArr = new Array();//用来获取所有class为box的元素

    oElement = parent.getElementsByTagName('*');

    for (var i = 0; i if(oElement[i].className == className){

            boxArr.push(oElement[i]);

        }
    };
    return boxArr;
}


//获取当前最小值得下标
function getMinhIndex(array,min){

    for(var i in array){

        if(array[i] == min)

            return i;
    }
}

//检测是否具备滚动条加载数据块的条件
function checkScrollSlide(){
    var oparent = document.getElementById('main');
    var oBoxs = getByClass(oparent,'box');
    var scrollH = document.body.scrollTop || document.documentElement.scrollTop + document.body.clientHeight || document.documentElement.clientHeight;
    var lastBoxH = oBoxs[oBoxs.length - 1].offsetTop + Math.floor(oBoxs[oBoxs.length - 1].offsetHeight/2);

    return (lastBoxH < scrollH )? true : false;
}