How to implement lazy loading of images?

Method to implement lazy loading: first customize attributes such as [data-imgurl] to store the path of the image; then use js to determine the scrolling position of the interface or whether the image has been loaded; finally load and obtain the attribute [ data-imgurl] value can be assigned to src.

Method to implement lazy loading of images:

First customize attributes such as [data-imgurl], which stores The path of the image; then use js to determine the scrolling position of the interface or whether the image has been loaded; finally load it and get the value of the attribute [data-imgurl] and assign it to src.

To implement image loading, the specific method is as follows:

$('img').each(function () {//在未触发滚动事件时先判断图片是否已经出现在视窗中，打开页面时先遍历一次
if (checkShow($(this)) && !isLoaded($(this)) ){ loadImg($(this));//加载当前img  } })
$(window).on('scroll',function () {//滚动的触发事件
        $('img').each(function () {//遍历img标签
            if (checkShow($(this)) && !isLoaded($(this)) ){
                loadImg($(this));//加载当前img
            }
        })
    })
function checkShow($img) {}；// 定义checkShow函数判断当前img是否已经出现在了视野中，传入img对象
function isLoaded ($img) {}；//定义isLoaded函数判断当前img是否已经被加载过了
function loadImg ($img) {}；//定义loadImg函数加载图片

1. Determine whether the target label appears in the field of view:

function checkShow($img) { // 传入img对象
        var sTop = $(window).scrollTop();  //即页面向上滚动的距离
        var wHeight = $(window).height(); // 浏览器自身的高度
        var offsetTop = $img.offset().top;  //目标标签img相对于document顶部的位置
        if (offsetTop < (scrollTop + windowHeight) && offsetTop > scrollTop) { //在2个临界状态之间的就为出现在视野中的
            return true;
        }
        return false;
    }

2 . Determine whether the target tag has been loaded:

 function isLoaded ($img) {
        return $img.attr('data-imgurl') === $img.attr('src'); //如果data-imgurl和src相等那么就是已经加载过了
    }

3. Load the target tag:

  function loadImg ($img) {
        $img.attr('src',$img.attr('data-imgurl')); // 把自定义属性中存放的真实的src地址赋给src属性
    }

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