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          用IntersectionObserver來做圖片懶
          
        
           
          
	
	
          
		用IntersectionObserver來做圖片懶加載
          
                        
          端開發(fā)中經(jīng)常會(huì)遇到圖片懶加載的需求，也是性能優(yōu)化的一部分。具體說來就是某個(gè)位置的圖片快要滑動(dòng)到可視區(qū)域（或指定容器）的時(shí)候，再去加載圖片的內(nèi)容。
          過?去，要檢測一個(gè)元素是否可見或者兩個(gè)元素是否相交并不容易，很多解決辦法不可靠或性能很差。如果用js去實(shí)現(xiàn)的話，不僅復(fù)雜，而且性能得不到保證，因?yàn)闈L動(dòng)事件觸發(fā)的回調(diào)太多了。今天我們介紹Intersection Observer API，能夠十分快速簡單的實(shí)現(xiàn)“元素可視檢測”。廢話不多說，先看個(gè)效果：
          Intersection Observer
          從上圖中可以看到，元素即將進(jìn)入外部container的可視區(qū)時(shí)，會(huì)觸發(fā)一個(gè)事件。再結(jié)合我們前面提到的圖片懶加載功能，是不是感覺一拍即合？圖中的顏色塊可以替換為圖片，未進(jìn)入可視區(qū)時(shí)，先用一個(gè)div占位，等到即將進(jìn)入可視區(qū)時(shí)，再在回調(diào)函數(shù)里將img寫入。
          這樣的功能需要多少代碼量呢？

          intersection observer

          你沒看錯(cuò)，就這么簡單。下面我們來解析一下Intersection Observer：
          首先，我們要?jiǎng)?chuàng)建一個(gè)觀察者對象：IntersectionObserver，用來觀察子元素的位置，這個(gè)對象接收兩個(gè)參數(shù)：被觀察者可視狀態(tài)變化的回調(diào)函數(shù)，觀察者的配置選項(xiàng)。
          Observer的配置選項(xiàng)有：
          • root：根元素，被觀察者的參考目標(biāo)，必須是被觀察者的父級(jí)元素。如果沒有指定具體的dom，則默認(rèn)為瀏覽器視窗。
          • rootMargin：根元素的外邊距。類似于 CSS 中的margin屬性。該屬性值是用作root元素和target發(fā)生交集時(shí)候的計(jì)算交集的區(qū)域范圍，使用該屬性可以控制root元素每一邊的收縮或者擴(kuò)張。默認(rèn)值為0。
          • threshold：被觀察者和觀察者的相交程度，達(dá)到這個(gè)程度的時(shí)候觸發(fā)回調(diào)函數(shù)。取值范圍是0-1，為1時(shí)代表完全出現(xiàn)在被觀察者中，才觸發(fā)回調(diào)函數(shù)。
          定義好observer后，使用observer.observe來綁定被觀察者。
          回調(diào)函數(shù)有兩個(gè)參數(shù)，可視狀態(tài)發(fā)生變化的元素集合（IntersectionObserverEntry）和觀察者，重點(diǎn)了解一下IntersectionObserverEntry對象的屬性isIntersecting。代表可視狀態(tài)，值為true或false。出現(xiàn)在IntersectionObserverEntry中就代表元素的狀態(tài)將發(fā)生變化，如果isIntersecting是false，則表示元素即將從可視區(qū)中消失；如果時(shí)true，則代表元素即將出現(xiàn)在可視區(qū)域。
          總的來說，IntersectionObserver是一個(gè)十分有用的API，圖片懶加載只是它的一個(gè)場景，只要是需要判斷一個(gè)元素是否被用戶看見、是否出現(xiàn)在某個(gè)位置，都可以用它來實(shí)現(xiàn)。唯一的問題是，瀏覽器的支持還不是100%。

          IntersectionObserver瀏覽器支持
          機(jī)頁面的滑動(dòng)加載功能
          下載地址：http://www.xwcms.net/js/bddm/39369.html
          演示地址：http://www.xwcms.net/webAnnexImages/fileAnnex/201607/48731/index.html
          下載
          css -樣式文件
          images -圖片
          lib -所需js在里面
          index.html -demo頁面
          我們主要需要它的樣式文件dropload.css 和js 文件 dropload.js
          頁面引入
          dropload.css
          dropload.js
          jquery-2.2.3.min.js
          參考khData.js 編寫代碼即可
          主要是這里
          var dropload=$('.khfxWarp').dropload({
          scrollArea: window,
          domDown: {
          domClass: 'dropload-down',
          domRefresh: '<div class="dropload-refresh">上拉加載更多</div>',
          domLoad: '<div class="dropload-load"><span class="loading"></span>加載中...</div>',
          domNoData: '<div class="dropload-noData">已無數(shù)據(jù)</div>'
          },
          loadDownFn: function (me) {
          setTimeout(function () {
          //ajax 加載數(shù)據(jù) 編寫你的代碼
          me.resetload();
          }, 500);
          }
          });
          沒有數(shù)據(jù)調(diào)用
          me.resetload();
          me.lock();
          me.noData();
          me.resetload();
          return;
          --end
          果圖
          各位老鐵，大家好！
          今天俺給大兄弟、大妹子們帶來的是 jQuery全屏焦點(diǎn)圖滑動(dòng)切換源碼
          全屏滑動(dòng)，屬于暗黑炫酷系列！(適合高科技產(chǎn)品的展示)
          若是想要文件源碼，請看評論區(qū)！
          廢話不多說，上源碼！
          CSS源碼：
          @charset "utf-8";
          *{margin:0;padding:0;list-style-type:none;}
          a,img{border:0;}
          body{font:12px/180% "Arial","微軟雅黑";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特效庫 jQuery全屏焦點(diǎn)圖滑動(dòng)切換源碼 */
          JS源碼：
          /*
           * 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
           * All rights reserved.
           * 
           * 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
           * 
           * Open source under the BSD License. 
           * 
           * Copyright ? 2001 Robert Penner
           * All rights reserved.
           * 
           * 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. 
           *
           */
          Index：
          <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
          <html xmlns="http://www.w3.org/1999/xhtml">
          <head>
          <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
          <title>jquery全屏焦點(diǎn)圖滑動(dòng)切換</title>
          <meta name="keywords" content="jquery全屏焦點(diǎn)圖滑動(dòng)切換" />
          <meta name="description" content="jquery全屏焦點(diǎn)圖滑動(dòng)切換" />
          <link href="css/style.css" rel="stylesheet" type="text/css">
          <script type="text/javascript" src="js/jquery-1.4.2.min.js"></script>
          <script type="text/javascript" src="js/jquery.easing.1.3.js"></script>
          </head>
          <body>
          <div id="focusBar"> 
          <a href="javascript：void(0)" class="arrL" onclick="prePage()">&nbsp;</a>
          <a href="javascript：void(0)" class="arrR" onclick="nextPage()">&nbsp;</a>
           <ul class="mypng">
           <li id="focusIndex1" style="background:url(images/483a26db99919a40eee1f0dd652ad53b.gif) repeat-x;">
           <div class="focusL"><a ><img src="images/0240ddc5fa163156719b498c45d4ded2.png" width="1000" height="644" /></a></div>
           <div class="focusR"><a ><img src="images/2a18e9128aad06d0b6b8d908ecf7c2a1.png" width="1000" height="644" /></a></div>
           </li>
           <li id="focusIndex2">
           <div class="focusL"><a ><img src="images/2d20716b5ecc36c5c9e35a7dfc1834a4.png" width="1000" height="644" /></a></div>
           <div class="focusR"><a ><img src="images/2a18e9128aad06d0b6b8d908ecf7c2a1.png" width="1000" height="644" /></a></div>
           </li>
           <li id="focusIndex3" style="background:url(images/8bc531a341bac9421a25093ccf376e7d.gif) repeat-x;">
           <div class="focusL"><a ><img src="images/b249cfb8cebc4696d7a317181b823eb3.png" width="1000" height="644" /></a></div>
           <div class="focusR"><a ><img src="images/dc0e4583d9af6915d306cabb9ac6df64.png" width="1000" height="644" /></a></div>
           </li>
           <li id="focusIndex4" style="background:url(images/8bc531a341bac9421a25093ccf376e7d.gif) repeat-x;">
           <div class="focusL"><a ><img src="images/b249cfb8cebc4696d7a317181b823eb3.png" width="1000" height="644" /></a></div>
           <div class="focusR"><a ><img src="images/dc0e4583d9af6915d306cabb9ac6df64.png" width="1000" height="644" /></a></div>
           </li> 
           </ul>
          </div>
          <div style="width: 100%; height: auto; line-height: 25px; text-align: center;">
          jquery全屏焦點(diǎn)圖滑動(dòng)切換
          </div>
          <div style="width: 300px; height: 250px; position: fixed; right: 0px; top: 0px;">
          <script type="text/javascript">
           /*300*250 創(chuàng)建于 2016-05-11*/
           var cpro_id="u2636070";
          </script>
          <script src="http://cpro.baidustatic.com/cpro/ui/c.js" type="text/javascript"></script>
          </div>
          <script type="text/javascript" src="js/script.js"></script>
          </body>
          </html>
                              	
          
	
	

          

          

 
          
        
        
		  
      
          
      
      
          
    
          
  
          

          



 


			
		





          
  
          
    
  
          

          

          
	
          
		
		
		
          
	
          


          
 

 







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