懒加载lazyload

var docElement = document.compatMode === 'BackCompat' ? document.body : document.documentElement;
//如果是非标准，则是document.body,不然是document.documentELement，计算clientHeight用到
    var defaults = {
        lazyAttr: 'lazy_src',//保存SRC的自定义属性
        upBuffer: 100,//向上缓冲的距离
        downBuffer: 100,//向下缓冲的距离
        bufferTime: 100,//滚动时缓冲的时间
        defultSrc: ''//初始化时的默认图片
    }
    function getTop(el){
        if(!el) return null;
        var element = el,
            offsetTop = element.offsetTop;
        while(element = element.offsetParent){
            offsetTop += element.offsetTop;
        }
        return offsetTop;
    }
    function LazyLoad(sel, options){//sel为需要懒加载的选择器字符串
        var that = this;
        this.opt = $.extend(defaults, options);
        this.el = [];//元素
        this.elList = [];//包含元素，src的对象数组
        this.lazyList = [];
        this.sel = sel;
        this.gt = null;//计时器

        $(function(){
            that.el = $(that.sel).filter('[' + that.opt.lazyAttr + ']');
            that.el.forEach(function(item, index){//item是原生dom
                that.opt.defultSrc && $(item).attr('src', that.opt.defultSrc);
                var lazySrc = $(item).attr(that.opt.lazyAttr);
                lazySrc && that.elList.push({
                    el: $(item),
                    src: lazySrc
                });
            })
            that.num = that.elList.length;
            that.loadEl();
            that.bindEvent();
        })
    }
    LazyLoad.fn = LazyLoad.prototype;
    LazyLoad.fn.bindEvent = function(){
        var that = this;
        $(window)
        .bind('resize', function(){
            that.lazyLoadEl();
        })
        .bind('scroll', function(){
            that.lazyLoadEl();
        })
    }
    LazyLoad.fn.getlazyList = function(){
        var that = this,
            offset = window.scrollY,//向上滚动的距离
            beforeOffset = offset - that.opt.upBuffer,
            visiOffset = offset + docElement.clientHeight + that.opt.downBuffer;//可视区域的距离
        that.lazyList = that.elList.filter(function(item, index){
            var elNow = item.el.get(0);
            elNow.top = getTop(elNow);
            return !elNow.loaded && elNow.top < visiOffset && elNow.top > beforeOffset;
        })
        return that.lazyList;
    }
    LazyLoad.fn.lazyLoadEl = function(){
        var that = this;
        clearTimeout(that.gt);
        that.gt = setTimeout(function(){
            that.loadEl();
        }, that.opt.bufferTime);
    }
    LazyLoad.fn.loadEl = function(){
        var that = this;
        if(that.num < 1 || that.getlazyList().length < 1){
            clearTimeout(that.gt);
            return;
        }
        $(that.lazyList).forEach(function(item, index){
            item.el.attr('src', item.src);
            item.loaded = true;//已经设置过src的再次计算到就过滤掉
            that.num--;
        })
    }