使用jquery实现的一个图片延迟加载插件（含图片延迟加载原理）

<img src="images/placeholder.png"  lazy-src="images/realimg.jpg" />

<img src="images/placeholder.png"  lazy-src="images/realimg.jpg" alt="" />
<noscript><img src="images/realimg.jpg"  alt="" /></noscript>

(function( $ ){
$.fn.imglazyload = function( options ){
 var o = $.extend({
    attr  :   'lazy-src',
    container  :  window, 
    event   :  'scroll',    
    fadeIn      :   false,    
    threshold  :  0, 
    vertical  :  true 
   }, options ),

  event = o.event,
  vertical = o.vertical,
  container = $( o.container ),
  threshold = o.threshold, 
  // 将jQuery对象转换成DOM数组便于操作
  elems = $.makeArray( $(this) ),  
  dataName = 'imglazyload_offset',   
  OFFSET = vertical ? 'top' : 'left',
  SCROLL = vertical ? 'scrollTop' : 'scrollLeft',   
  winSize = vertical ? container.height() : container.width(),
  scrollCoord = container[ SCROLL ](),
  docSize = winSize + scrollCoord;

 // 延迟加载的触发器 
 var trigger = {

  init : function( coord ){
   return coord >= scrollCoord &&
                            coord <= ( docSize + threshold );
  },

  scroll : function( coord ){
   var scrollCoord = container[ SCROLL ]();
   return coord >= scrollCoord &&
                    coord <= ( winSize + scrollCoord + threshold );
  },

  resize : function( coord ){
   var scrollCoord = container[ SCROLL ](),
    winSize = vertical ?
                            container.height() :
                            container.width();
   return coord >= scrollCoord &&
                   coord <= ( winSize + scrollCoord + threshold );
  }
 };

 var loader = function( triggerElem, event ){
  var i = 0,
   isCustom = false,
   isTrigger, coord, elem, $elem, lazySrc;

  // 自定义事件只要触发即可，无需再判断
  if( event ){
   if( event !== 'scroll' && event !== 'resize' ){
    isCustom = true;
   }
  }
  else{
   event = 'init';
  }

  for( ; i < elems.length; i++ ){ 
   isTrigger = false;
   elem = elems[i];
   $elem = $( elem );
   lazySrc = $elem.attr( o.attr );

   if( !lazySrc || elem.src === lazySrc ){
    continue;
   }
   // 先从缓存获取offset值，缓存中没有才获取计算值,
   // 将计算值缓存，避免重复获取引起的reflow
   coord = $elem.data( dataName );

   if( coord === undefined ){
    coord = $elem.offset()[ OFFSET ];
    $elem.data( dataName, coord );
   }

   isTrigger = isCustom || trigger[ event ]( coord );   

   if( isTrigger ){
    // 加载图片
    elem.src = lazySrc;
    if( o.fadeIn ){
     $elem.hide().fadeIn();
    }
    // 移除缓存
    $elem.removeData( dataName );
    // 从DOM数组中移除该DOM
    elems.splice( i--, 1 );
   }
  }

  // 所有的图片加载完后卸载触发事件
  if( !elems.length ){
   if( triggerElem ){
    triggerElem.unbind( event, fire );
   }
   else{
    container.unbind( o.event, fire );
   }
   $( window ).unbind( 'resize', fire );
   elems = null;
  }

 };

 var fire = function( e ){
  loader( $(this), e.type );
 };

 // 绑定事件
 container = event === 'scroll' ? container : $( this ); 
 container.bind( event, fire );
 $( window ).bind( 'resize', fire );

 // 初始化
 loader();

 return this;
};

})( jQuery );

$( 'img' ).imglazyload({
 event : 'scroll',
 attr : 'lazy-src'
});

$( 'img' ).imglazyload();