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三十五、懒加载（原生封装）

var Lazy = {
        $:function(arg,context){
            var tagAll,n,eles=[],i,sub = arg.substring(1);
            context = context|| document;
            if(typeof arg =='string'){
                switch(arg.charAt(0)){
                    case '#':
                        return document.getElementById(sub);
                        break;
                    case '.':
                        if(context.getElementsByClassName) return context.getElementsByClassName(sub);
                        tagAll = Lazy.$('*');
                        n = tagAll.length;
                        for(i = 0;i<n;i++){
                            if(tagAll[i].className.indexOf(sub) > -1) eles.push(tagAll[i]);
                        }
                        return eles;
                        break;
                    default:
                        return context.getElementsByTagName(arg);
                        break;
                }
            }
        },
        getPos:function (node) {
            var scrollx = document.documentElement.scrollLeft || document.body.scrollLeft,
                    scrollt = document.documentElement.scrollTop || document.body.scrollTop;
            var pos = node.getBoundingClientRect();
            return {top:pos.top + scrollt, right:pos.right + scrollx, bottom:pos.bottom + scrollt, left:pos.left + scrollx }
        },
        bind:function(node,type,handler){
            node.addEventListener?node.addEventListener(type, handler, false):node.attachEvent('on'+ type, handler);
        },
        unbind:function(node,type,handler){
            node.removeEventListener?node.removeEventListener(type, handler, false):node.detachEvent('on'+ type, handler);
        },
        toArray:function(eles){
            var arr = [];
            for(var i=0,n=eles.length;i<n;i++){
                arr.push(eles[i]);
            }
            return arr;
        }
    };
    function imgLazyLoad(){
        var timer,screenHeight = document.documentElement.clientHeight;
        // 选择所有图片
        var allimg = Lazy.$('img');
        // 筛选CLASS为lazyload的图片
        var elems = Lazy.$('.lazyload',allimg);
        // 转换为真正的数组
        elems = Lazy.toArray(elems);
        if(!elems.length) return;
        // 没有发生滚动事件时如果图片在可视范围之内，也显示
        for(var i = 0;i < elems.length;i++){
            // 获取图像的父元素即包裹图像的元素，判断图像是否在可视区域即直接判断父元素是否可视
            var parent = elems[i].parentNode;
            var pos = Lazy.getPos(parent);
            var posT = pos.top;
            var posB = pos.bottom;
            // 没有滚动条情况如果距离顶部的距离小于屏幕的高度则赋值SRC
            if(posT < screenHeight){
                elems[i].src = elems[i].getAttribute('data-img');
                // 移除后，数组的长度减一，下一个下标需减一
                elems.splice(i--,1);
            }
        }
        // 绑定scroll事件
        Lazy.bind(window,'scroll',loading);
        Lazy.bind(window,'resize',loading);
        function loading(){
            timer && clearTimeout(timer);
            timer = setTimeout(function(){
                var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
                screenHeight = document.documentElement.clientHeight;
                for(var i = 0;i < elems.length;i++){
                    var parent = elems[i].parentNode;
                    var pos = Lazy.getPos(parent);
                    var posT = pos.top;
                    var posB = pos.bottom;
                    var screenTop = screenHeight+scrollTop;
                    // 元素顶部出现在可视区  或者  元素底部出现在可视区
                    if((posT > scrollTop && posT <  screenTop) || (posB > scrollTop && posB < screenTop)){
                        elems[i].src = elems[i].getAttribute('data-img');
                        elems.splice(i--,1);
                    }else{
                        // 去掉以下注释开启图片预加载
//                         new Image().src = elems[i].getAttribute('data-img');
                    }
                }
                if(!elems.length){
                    Lazy.unbind(window,'scroll',loading);
                    Lazy.unbind(window,'resize',loading);
                }
            },10);
        }
    }
    imgLazyLoad();