js圖片延遲加載的實現方法及思路

2019-11-20 22:30:11

字體：大中小

來源：轉載

供稿：網友

大概的實現方式是：
在頁面的load沒有觸發之前，把所有的指定id的元素內的img放入到imgs中，將所有的圖片的src值放入到一個新建的_src屬性中，把src設置為指定的顯示圖片。
然后，在document.body的scroll事件觸發時，循環計算imgs中的img元素位置是否正好在瀏覽器顯示框范圍內，如果是，則將img元素的_src屬性的值賦給src，這樣圖片就能顯示出來。
這里比較麻煩地方是，如何計算img的位置，獲得元素的相對于頁面的絕對位置。通常是用offsetLeft和offsetTop，但這兩個屬性是元素的offsetParent指向的元素的相對位置，如果offsetParent所指的元素是設置了浮動的或使用絕對定位，那么offsetLeft來獲得絕對位置就不正確呢。
在這里我是將元素的所有父級元素的offsetTop之和來獲得文檔絕對位置的。

復制代碼代碼如下:

//取元素的頁面絕對 X位置
        var getLeft = function(El){
            var left = 0;
            do{
                left += El.offsetLeft;
            }while((El = El.offsetParent).nodeName != 'BODY');
            return left;
        };
        //取元素的頁面絕對 Y位置
        var getTop = function(El){
            var top = 0;
            do{
                top += El.offsetTop;
            }while((El = El.offsetParent).nodeName != 'BODY');
            return top;
        };

在設置窗口的scroll事件時，ie使用是document.documentElement，而其他的瀏覽器都使用document。
接下來是要獲得瀏覽器顯示窗口現對于文檔的位置，用了下面的代碼來計算

復制代碼代碼如下:

//讀取滾動條的位置和瀏覽器窗口的顯示大小
             var top = isGoo ? document.body.scrollTop : document.documentElement.scrollTop,
                 left = isGoo ? document.body.scrollLeft :document.documentElement.scrollLeft,
                 width = document.documentElement.clientWidth,
                 height = document.documentElement.clientHeight;

谷歌瀏覽器要通過body來獲得scrollTop，而其他瀏覽器通過documentElement。
最后迭代判斷img的位置，并顯示圖片

復制代碼代碼如下:

//對所有圖片進行批量判斷是否在瀏覽器顯示區域內
            for(var i=0 ; i < imgs.length; i++){
                var _top = getTop(imgs[i]),_left = getLeft(imgs[i]);
                //判斷圖片是否在顯示區域內
                if( _top >= top &&
                    _left >= left &&
                    _top <= top+height &&
                    _left <= left+width){
                    var _src = imgs[i].getAttribute('_src');
                    //如果圖片已經顯示，則取消賦值
                    if(imgs[i].src !== _src){
                        imgs[i].src = _src;
                    }
                }
            }

可以運行的代碼

復制代碼代碼如下:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<title>js圖片延遲加載</title>
</head>
<body>
<table id="out1" style="height:2450px;">
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_22/photo163115955184917.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_13/photo163175537548397.jpg" alt="" /></td>
</tr>
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_16/photo16314557927776.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_10/photo1631932180569094.jpg" alt="" /></td>
</tr>
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_1/photo1630540642223.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_2/photo1638657445827.jpg" alt="" /></td>
</tr>
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_29/photo16384188604.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_29/photo16384188604.jpg" alt="" /></td>
</tr>
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_13/photo16311444254618471.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_15/photo16318118126415.jpg" alt="" /></td>
</tr>
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo16310513957332.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo16310342238265.jpg" alt="" /></td>
</tr>
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo163103452142757.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo16310310447460.jpg" alt="" /></td>
</tr>
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo163103347259334.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo16310342238265.jpg" alt="" /></td>
</tr>
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo163102333593532.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo16323618120479.jpg" alt="" /></td>
</tr>
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo16323942409450.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_15/photo1630653584606.jpg" alt="" /></td>
</tr>
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_12/photo163233846816799.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_13/photo16313216736705.jpg" alt="" /></td>
</tr>
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_1/photo16313391752186220.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_1/photo163765186092776.jpg" alt="" /></td>
</tr>
</table>
<table id="out2" style="width:4883px">
<tr>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_22/photo163115955184917.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_13/photo163175537548397.jpg" alt="" /></td>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_16/photo16314557927776.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_10/photo1631932180569094.jpg" alt="" /></td>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_1/photo1630540642223.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_2/photo1638657445827.jpg" alt="" /></td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_29/photo16384188604.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_29/photo16384188604.jpg" alt="" /></td>
<td><img src="http://www.survivalescaperooms.com/small/2011_4_13/photo16311444254618471.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_15/photo16318118126415.jpg" alt="" /></td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo16310513957332.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo16310342238265.jpg" alt="" /></td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo163103452142757.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo16310310447460.jpg" alt="" /></td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo163103347259334.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo16310342238265.jpg" alt="" /></td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo163102333593532.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo16323618120479.jpg" alt="" /></td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_14/photo16323942409450.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_15/photo1630653584606.jpg" alt="" /></td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_12/photo163233846816799.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_13/photo16313216736705.jpg" alt="" /></td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_1/photo16313391752186220.jpg" alt="" /> </td>
<td><img src="http://www.survivalescaperooms.com/small/2011_3_1/photo163765186092776.jpg" alt="" /></td>
</tr>
</table>
</body>
</html>
  <script type="text/javascript">
        function delayload(option){
            //讀取參數
            //圖片未加載時顯示的圖片
            var src = option.src ? option.src : '',
            //指定那些id下的img元素使用延遲顯示
                id = option.id ? option.id : [];
            //圖片列表
            var imgs = [];
            //獲得所有的圖片元素
            for(var i=0 ; i < id.length ; i++){
                var idbox = document.getElementById(id[i]),_imgs;
                if(idbox && (_imgs = idbox.getElementsByTagName('img'))){
                    for(var t=0 ; t < _imgs.length ; t++){
                        imgs.push(_imgs[t]);
                    }
                }
            }
            //將所有的圖片設置為指定的loading圖片
            for(var i=0 ; i < imgs.length ; i++){
                //圖片本來的圖片路徑放入_src中
                imgs[i].setAttribute('_src',imgs[i].src);
                imgs[i].src = src;
            }
            //取元素的頁面絕對 X位置
            var getLeft = function(El){
                var left = 0;
                do{
                    left += El.offsetLeft;
                }while((El = El.offsetParent).nodeName != 'BODY');
                return left;
            };
            //取元素的頁面絕對 Y位置
            var getTop = function(El){
                var top = 0;
                do{
                    top += El.offsetTop;
                }while((El = El.offsetParent).nodeName != 'BODY');
                return top;
            };
            //是否為ie，并讀出ie版本
            var isIE = !!navigator.userAgent.match(/MSIE/b/s*([0-9]/.[0-9]);/img);
            isIE && (isIE = RegExp.$1);
            //是否為chrome
            var isGoo = !!navigator.userAgent.match(/AppleWebKit/b/img);
            //獲得可以觸發scroll事件的對象
            var box = isIE ? document.documentElement : document;
            //body元素的scroll事件
            var onscroll = box.onscroll = function(){
                //讀取滾動條的位置和瀏覽器窗口的顯示大小
                var top = isGoo ? document.body.scrollTop : document.documentElement.scrollTop,
                    left = isGoo ? document.body.scrollLeft :document.documentElement.scrollLeft,
                    width = document.documentElement.clientWidth,
                    height = document.documentElement.clientHeight;
                //對所有圖片進行批量判斷是否在瀏覽器顯示區域內
                for(var i=0 ; i < imgs.length; i++){
                    var _top = getTop(imgs[i]),_left = getLeft(imgs[i]);
                    //判斷圖片是否在顯示區域內
                    if( _top >= top &&
                        _left >= left &&
                        _top <= top+height &&
                        _left <= left+width){
                        var _src = imgs[i].getAttribute('_src');
                        //如果圖片已經顯示，則取消賦值
                        if(imgs[i].src !== _src){
                            imgs[i].src = _src;
                        }
                    }
                }
            };
            var load = new Image();
            load.src = src;
            load.onload = function(){
                onscroll();
            };
        }
        delayload({id:['out1','out2'],src:'//files.VeVB.COm/file_images/article/201307/2013072210300234.jpg'});
      </script>