用IntersectionObserver来做图片懒

用IntersectionObserver来做图片懒加载

端开发中经常会遇到图片懒加载的需求，也是性能优化的一部分。具体说来就是某个位置的图片快要滑动到可视区域（或指定容器）的时候，再去加载图片的内容。

过?去，要检测一个元素是否可见或者两个元素是否相交并不容易，很多解决办法不可靠或性能很差。如果用js去实现的话，不仅复杂，而且性能得不到保证，因为滚动事件触发的回调太多了。今天我们介绍Intersection Observer API，能够十分快速简单的实现“元素可视检测”。废话不多说，先看个效果：

Intersection Observer

从上图中可以看到，元素即将进入外部container的可视区时，会触发一个事件。再结合我们前面提到的图片懒加载功能，是不是感觉一拍即合？图中的颜色块可以替换为图片，未进入可视区时，先用一个div占位，等到即将进入可视区时，再在回调函数里将img写入。

这样的功能需要多少代码量呢？

intersection observer

你没看错，就这么简单。下面我们来解析一下Intersection Observer：

首先，我们要创建一个观察者对象：IntersectionObserver，用来观察子元素的位置，这个对象接收两个参数：被观察者可视状态变化的回调函数，观察者的配置选项。

Observer的配置选项有：

root：根元素，被观察者的参考目标，必须是被观察者的父级元素。如果没有指定具体的dom，则默认为浏览器视窗。
rootMargin：根元素的外边距。类似于 CSS 中的margin属性。该属性值是用作root元素和target发生交集时候的计算交集的区域范围，使用该属性可以控制root元素每一边的收缩或者扩张。默认值为0。
threshold：被观察者和观察者的相交程度，达到这个程度的时候触发回调函数。取值范围是0-1，为1时代表完全出现在被观察者中，才触发回调函数。

定义好observer后，使用observer.observe来绑定被观察者。

回调函数有两个参数，可视状态发生变化的元素集合（IntersectionObserverEntry）和观察者，重点了解一下IntersectionObserverEntry对象的属性isIntersecting。代表可视状态，值为true或false。出现在IntersectionObserverEntry中就代表元素的状态将发生变化，如果isIntersecting是false，则表示元素即将从可视区中消失；如果时true，则代表元素即将出现在可视区域。

总的来说，IntersectionObserver是一个十分有用的API，图片懒加载只是它的一个场景，只要是需要判断一个元素是否被用户看见、是否出现在某个位置，都可以用它来实现。唯一的问题是，浏览器的支持还不是100%。

IntersectionObserver浏览器支持

机页面的滑动加载功能

下载地址：http://www.xwcms.net/js/bddm/39369.html

演示地址：http://www.xwcms.net/webAnnexImages/fileAnnex/201607/48731/index.html

下载

css -样式文件

images -图片

lib -所需js在里面

index.html -demo页面

我们主要需要它的样式文件dropload.css 和js 文件 dropload.js

页面引入

dropload.css

dropload.js

jquery-2.2.3.min.js

参考khData.js 编写代码即可

主要是这里

var dropload=$('.khfxWarp').dropload({
scrollArea: window,
domDown: {
domClass: 'dropload-down',
domRefresh: '<div class="dropload-refresh">上拉加载更多</div>',
domLoad: '<div class="dropload-load"><span class="loading"></span>加载中...</div>',
domNoData: '<div class="dropload-noData">已无数据</div>'
},
loadDownFn: function (me) {
setTimeout(function () {
//ajax 加载数据编写你的代码
me.resetload();
}, 500);
}
});
没有数据调用
me.resetload();
me.lock();
me.noData();
me.resetload();
return;

--end

果图

各位老铁，大家好！

今天俺给大兄弟、大妹子们带来的是 jQuery全屏焦点图滑动切换源码

全屏滑动，属于暗黑炫酷系列！(适合高科技产品的展示)

若是想要文件源码，请看评论区！

废话不多说，上源码！

CSS源码：

@charset "utf-8";

*{margin:0;padding:0;list-style-type:none;}

a,img{border:0;}

body{font:12px/180% "Arial","微软雅黑";color:#666666;background-color:#ffffff;}

a,a:hover{color:#666666;text-decoration:none;border:0px;}

/* focusBar */

#focusBar{width:100%;height:644px;clear:both;position:relative;background:#242424;overflow:hidden;}

#focusBar li{width:100%;clear:both;height:644px;display:none;position:absolute;/* position:relative;*/}

#focusBar .focusL{height:644px;position:absolute;z-index:9;/*left:-1000px;*/ top:0px;left:-500px;margin-left:50%;}

#focusBar .focusR{height:644px;position:absolute;z-index:19;/* right:1000px;*/top:0px;left:-500px;margin-left:50%;}

a.arrL,a.arrR{display:block;width:52px;height:134px;position:absolute;top:250px;z-index:99;cursor:pointer;background:url(../images/bt_arr.png) no-repeat;}

a.arrL{left:-52px;background-position:0px 0px;}

a.arrR{right:-52px;background-position:-68px 0px;}

a.arrL:hover{background-position:0px -144px;}

a.arrR:hover{background-position:-68px -144px;}

/*HTML5特效库 jQuery全屏焦点图滑动切换源码 */

JS源码：

* jQuery Easing v1.3 - http://gsgd.co.uk/sandbox/jquery/easing/

* Uses the built in easing capabilities added In jQuery 1.1

* to offer multiple easing options

* TERMS OF USE - jQuery Easing

* Open source under the BSD License.

* Copyright ? 2008 George McGinley Smith

* Redistribution and use in source and binary forms, with or without modification,

* are permitted provided that the following conditions are met:

* Redistributions of source code must retain the above copyright notice, this list of

* conditions and the following disclaimer.

* Redistributions in binary form must reproduce the above copyright notice, this list

* of conditions and the following disclaimer in the documentation and/or other materials

* provided with the distribution.

* Neither the name of the author nor the names of contributors may be used to endorse

* or promote products derived from this software without specific prior written permission.

* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY

* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF

* MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE

* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,

* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE

* GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED

* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING

* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED

* OF THE POSSIBILITY OF SUCH DAMAGE.

// t: current time, b: begInnIng value, c: change In value, d: duration

jQuery.easing['jswing']=jQuery.easing['swing'];

jQuery.extend( jQuery.easing,

{

def: 'easeOutQuad',

swing: function (x, t, b, c, d) {

//alert(jQuery.easing.default);

return jQuery.easing[jQuery.easing.def](x, t, b, c, d);

easeInQuad: function (x, t, b, c, d) {

return c*(t/=d)*t + b;

easeOutQuad: function (x, t, b, c, d) {

return -c *(t/=d)*(t-2) + b;

easeInOutQuad: function (x, t, b, c, d) {

if ((t/=d/2) < 1) return c/2*t*t + b;

return -c/2 * ((--t)*(t-2) - 1) + b;

easeInCubic: function (x, t, b, c, d) {

return c*(t/=d)*t*t + b;

easeOutCubic: function (x, t, b, c, d) {

return c*((t=t/d-1)*t*t + 1) + b;

easeInOutCubic: function (x, t, b, c, d) {

if ((t/=d/2) < 1) return c/2*t*t*t + b;

return c/2*((t-=2)*t*t + 2) + b;

easeInQuart: function (x, t, b, c, d) {

return c*(t/=d)*t*t*t + b;

easeOutQuart: function (x, t, b, c, d) {

return -c * ((t=t/d-1)*t*t*t - 1) + b;

easeInOutQuart: function (x, t, b, c, d) {

if ((t/=d/2) < 1) return c/2*t*t*t*t + b;

return -c/2 * ((t-=2)*t*t*t - 2) + b;

easeInQuint: function (x, t, b, c, d) {

return c*(t/=d)*t*t*t*t + b;

easeOutQuint: function (x, t, b, c, d) {

return c*((t=t/d-1)*t*t*t*t + 1) + b;

easeInOutQuint: function (x, t, b, c, d) {

if ((t/=d/2) < 1) return c/2*t*t*t*t*t + b;

return c/2*((t-=2)*t*t*t*t + 2) + b;

easeInSine: function (x, t, b, c, d) {

return -c * Math.cos(t/d * (Math.PI/2)) + c + b;

easeOutSine: function (x, t, b, c, d) {

return c * Math.sin(t/d * (Math.PI/2)) + b;

easeInOutSine: function (x, t, b, c, d) {

return -c/2 * (Math.cos(Math.PI*t/d) - 1) + b;

easeInExpo: function (x, t, b, c, d) {

return (t==0) ? b : c * Math.pow(2, 10 * (t/d - 1)) + b;

easeOutExpo: function (x, t, b, c, d) {

return (t==d) ? b+c : c * (-Math.pow(2, -10 * t/d) + 1) + b;

easeInOutExpo: function (x, t, b, c, d) {

if (t==0) return b;

if (t==d) return b+c;

if ((t/=d/2) < 1) return c/2 * Math.pow(2, 10 * (t - 1)) + b;

return c/2 * (-Math.pow(2, -10 * --t) + 2) + b;

easeInCirc: function (x, t, b, c, d) {

return -c * (Math.sqrt(1 - (t/=d)*t) - 1) + b;

easeOutCirc: function (x, t, b, c, d) {

return c * Math.sqrt(1 - (t=t/d-1)*t) + b;

easeInOutCirc: function (x, t, b, c, d) {

if ((t/=d/2) < 1) return -c/2 * (Math.sqrt(1 - t*t) - 1) + b;

return c/2 * (Math.sqrt(1 - (t-=2)*t) + 1) + b;

easeInElastic: function (x, t, b, c, d) {

var s=1.70158;var p=0;var a=c;

if (t==0) return b; if ((t/=d)==1) return b+c; if (!p) p=d*.3;

if (a < Math.abs(c)) { a=c; var s=p/4; }

else var s=p/(2*Math.PI) * Math.asin (c/a);

return -(a*Math.pow(2,10*(t-=1)) * Math.sin( (t*d-s)*(2*Math.PI)/p )) + b;

easeOutElastic: function (x, t, b, c, d) {

var s=1.70158;var p=0;var a=c;

if (t==0) return b; if ((t/=d)==1) return b+c; if (!p) p=d*.3;

if (a < Math.abs(c)) { a=c; var s=p/4; }

else var s=p/(2*Math.PI) * Math.asin (c/a);

return a*Math.pow(2,-10*t) * Math.sin( (t*d-s)*(2*Math.PI)/p ) + c + b;

easeInOutElastic: function (x, t, b, c, d) {

var s=1.70158;var p=0;var a=c;

if (t==0) return b; if ((t/=d/2)==2) return b+c; if (!p) p=d*(.3*1.5);

if (a < Math.abs(c)) { a=c; var s=p/4; }

else var s=p/(2*Math.PI) * Math.asin (c/a);

if (t < 1) return -.5*(a*Math.pow(2,10*(t-=1)) * Math.sin( (t*d-s)*(2*Math.PI)/p )) + b;

return a*Math.pow(2,-10*(t-=1)) * Math.sin( (t*d-s)*(2*Math.PI)/p )*.5 + c + b;

easeInBack: function (x, t, b, c, d, s) {

if (s==undefined) s=1.70158;

return c*(t/=d)*t*((s+1)*t - s) + b;

easeOutBack: function (x, t, b, c, d, s) {

if (s==undefined) s=1.70158;

return c*((t=t/d-1)*t*((s+1)*t + s) + 1) + b;

easeInOutBack: function (x, t, b, c, d, s) {

if (s==undefined) s=1.70158;

if ((t/=d/2) < 1) return c/2*(t*t*(((s*=(1.525))+1)*t - s)) + b;

return c/2*((t-=2)*t*(((s*=(1.525))+1)*t + s) + 2) + b;

easeInBounce: function (x, t, b, c, d) {

return c - jQuery.easing.easeOutBounce (x, d-t, 0, c, d) + b;

easeOutBounce: function (x, t, b, c, d) {

if ((t/=d) < (1/2.75)) {

return c*(7.5625*t*t) + b;

} else if (t < (2/2.75)) {

return c*(7.5625*(t-=(1.5/2.75))*t + .75) + b;

} else if (t < (2.5/2.75)) {

return c*(7.5625*(t-=(2.25/2.75))*t + .9375) + b;

} else {

return c*(7.5625*(t-=(2.625/2.75))*t + .984375) + b;

}

easeInOutBounce: function (x, t, b, c, d) {

if (t < d/2) return jQuery.easing.easeInBounce (x, t*2, 0, c, d) * .5 + b;

return jQuery.easing.easeOutBounce (x, t*2-d, 0, c, d) * .5 + c*.5 + b;

}

});

* TERMS OF USE - EASING EQUATIONS