How to implement delayed loading of images using dataset
下面小编就为大家分享一篇基于dataset的使用和图片延时加载的实现方法，具有很好的参考价值，希望对大家有所帮助。一起跟随小编过来看看吧
首先，先介绍一下关于javascript中dataset属性。。
html5中可以使用data-前缀设置我们需要的自定义属性,来进行一些数据的存放。
下面是元素应用data属性的一个例子：
<p id="day-meal-expense" data-drink="tea" data-food="noodle" data-meal="lunch">$18.3</p>
要想获取某个属性的值,可以像下面这样使用dataset对象:
var expenseday=document.getElementById('day-meal-expense'); var typeOfDrink=expenseday.dataset.drink; console.log(typeOfDrink);//tea console.log(expenseday.dataset.food);//noodle console.log(expenseday.dataset.meal);//lunch
如果浏览器支持dataset,则会弹出注释内容,如果浏览器不支持dataset则会报错,无法获取属性drink/food/meal的值:对象为null或未定义(如IE9版本).
data属性基本上所有的浏览器都是支持的,但是dataset对象支持的就比较特殊了,目前仅在Opera 11.1+,Chrome 9+下可以通过javascript,使用dataset访问你自定义的data属性.至于其他浏览器,FireFox 6+(未出)以及Safari 6+(未出)会支持dataset对象,至于IE浏览器,目前看来还是遥遥无期的趋势.
问：不是有getAttribute来获取自定义属性么。要这个干嘛？？
答：如果使用传统的方法获取属性值应该会类似下面：
var typeOfDrink=document.getElementById('day-meal-expense').getAttribute('data-drink');
现在,如果我们要获得多个自定义的属性值,就要用下面N行代码来实现了:
var attrs=expenseday.attributes, expense={},i,j;
for (i=0,j=attrs.length;i<j;i++){
if(attrs[i].name.substring(0,5)=='data-'){
expense[attrs[i].name.substring(5)]=attrs[i].value;
}
}而使用dataset属性,我们根本不需要任何循环去获取你想要的那个值,直接秒杀：
expense=document.getElementById('day-meal-expense').dataset;
问：怎么操作这玩意~
答：可以像下面这样操作名-值对:
charInput=[];
for(var item in expenseday){
charInput.push(expenseday[item]);
}让所有的自定义属性塞到一个数组中.
如果你想删除一个data属性,可以这么做:
delete expenseday.dataset.meal; console.log(expenseday.dataset.meal)//undefined
如果你想给元素添加一个属性,可以这么做:
expenseday.dataset.dessert='icecream'; console.log(expenseday.dataset.dessert);//icecream
dataset并不是典型意义上的JavaScript对象,而是个DOMStringMap对象,DOMStringMap是HTML5一种新的含有多个名-值对的交互变量.
下面来个实际的应用吧~
（当然图片地址肯定是空的。想要看效果的自己去加个图片地址，这个效果附上图片也是看不出什么所以就不上截图了）
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>图片延迟加载</title>
<style>
#box{
width: 100%;
height: 500px;
}
#box img{
width: 100%;
height: 500px;
transition: 1s;
opacity: 0;
}
</style>
</head>
<body>
<p id="box"></p>
<script type="text/javascript">
var data=["img/1.jpg","img/2.jpg","img/3.jpg","img/4.jpg"]
var box=document.getElementById('box');
// var img=document.createElement(img);
for (var i = 0; i < data.length; i++) {
var img=document.createElement('img');
img.dataset.src=data[i];
// img.style.opacity=1;
box.appendChild(img);
// console.log(box);
}
var imgs=document.querySelectorAll('img');
window.addEventListener('scroll',loadFn);
window.addEventListener('load',loadFn);
function loadFn(){
for (var i = 0; i < imgs.length; i++) {
if(imgs[i].getBoundingClientRect().top<window.innerHeight){
if(imgs[i].dataset.src){
imgs[i].src=imgs[i].dataset.src;
imgs[i].style.opacity=1;
imgs[i].removeAttribute('src');
}
}
}
}
</script>
</body>
</html>上面是我整理给大家的，希望今后会对大家有帮助。
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