[转]jquery实现图片延时加载

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原理：通过 jQuery 插件 lazyload 使得在浏览器可见区域外的图片不会被加载，当图片被用户滚动到浏览器可见区域时图片才进行加载，有点类似使用 Google 搜索图片时的效果。很明显，通过使用图片延时加载可以提高页面的加载速度。

实现过程：

首先是引入jquery文件和插件文件。jquery文件在这里我就不再赘述了。下面我贴出插件文件，我们将该文件命名为jquery.lazyload.js。

1 // JavaScript Document
  2 /*
  3  * Lazy Load - jQuery plugin for lazy loading images
  4  *
  5  * Copyright (c) 2007-2012 Mika Tuupola
  6  *
  7  * Licensed under the MIT license:
  8  *   http://www.opensource.org/licenses/mit-license.php
  9  *
 10  * Project home:
 11  *   http://www.appelsiini.net/projects/lazyload
 12  *
 13  * Version:  1.8.0
 14  *
 15  */
 16 (function($, window) {
 17     var $window = $(window);
 18 
 19     $.fn.lazyload = function(options) {
 20         var elements = this;
 21         var $container;
 22         var settings = {
 23             threshold       : 0,
 24             failure_limit   : 0,
 25             event           : "scroll",
 26             effect          : "show",
 27             container       : window,
 28             data_attribute  : "original",
 29             skip_invisible  : true,
 30             appear          : null,
 31             load            : null
 32         };
 33 
 34         function update() {
 35             var counter = 0;
 36       
 37             elements.each(function() {
 38                 var $this = $(this);
 39                 if (settings.skip_invisible && !$this.is(":visible")) {
 40                     return;
 41                 }
 42                 if ($.abovethetop(this, settings) ||
 43                     $.leftofbegin(this, settings)) {
 44                         /* Nothing. */
 45                 } else if (!$.belowthefold(this, settings) &&
 46                     !$.rightoffold(this, settings)) {
 47                         $this.trigger("appear");
 48                 } else {
 49                     if (++counter > settings.failure_limit) {
 50                         return false;
 51                     }
 52                 }
 53             });
 54 
 55         }
 56 
 57         if(options) {
 58             /* Maintain BC for a couple of versions. */
 59             if (undefined !== options.failurelimit) {
 60                 options.failure_limit = options.failurelimit; 
 61                 delete options.failurelimit;
 62             }
 63             if (undefined !== options.effectspeed) {
 64                 options.effect_speed = options.effectspeed; 
 65                 delete options.effectspeed;
 66             }
 67 
 68             $.extend(settings, options);
 69         }
 70 
 71         /* Cache container as jQuery as object. */
 72         $container = (settings.container === undefined ||
 73                       settings.container === window) ? $window : $(settings.container);
 74 
 75         /* Fire one scroll event per scroll. Not one scroll event per image. */
 76         if (0 === settings.event.indexOf("scroll")) {
 77             $container.bind(settings.event, function(event) {
 78                 return update();
 79             });
 80         }
 81 
 82         this.each(function() {
 83             var self = this;
 84             var $self = $(self);
 85 
 86             self.loaded = false;
 87 
 88             /* When appear is triggered load original image. */
 89             $self.one("appear", function() {
 90                 if (!this.loaded) {
 91                     if (settings.appear) {
 92                         var elements_left = elements.length;
 93                         settings.appear.call(self, elements_left, settings);
 94                     }
 95                     $("<img />")
 96                         .bind("load", function() {
 97                             $self
 98                                 .hide()
 99                                 .attr("src", $self.data(settings.data_attribute))
100                                 [settings.effect](settings.effect_speed);
101                             self.loaded = true;
102 
103                             /* Remove image from array so it is not looped next time. */
104                             var temp = $.grep(elements, function(element) {
105                                 return !element.loaded;
106                             });
107                             elements = $(temp);
108 
109                             if (settings.load) {
110                                 var elements_left = elements.length;
111                                 settings.load.call(self, elements_left, settings);
112                             }
113                         })
114                         .attr("src", $self.data(settings.data_attribute));
115                 }
116             });
117 
118             /* When wanted event is triggered load original image */
119             /* by triggering appear.                              */
120             if (0 !== settings.event.indexOf("scroll")) {
121                 $self.bind(settings.event, function(event) {
122                     if (!self.loaded) {
123                         $self.trigger("appear");
124                     }
125                 });
126             }
127         });
128 
129         /* Check if something appears when window is resized. */
130         $window.bind("resize", function(event) {
131             update();
132         });
133 
134         /* Force initial check if images should appear. */
135         update();
136         
137         return this;
138     };
139 
140     /* Convenience methods in jQuery namespace.           */
141     /* Use as  $.belowthefold(element, {threshold : 100, container : window}) */
142 
143     $.belowthefold = function(element, settings) {
144         var fold;
145         
146         if (settings.container === undefined || settings.container === window) {
147             fold = $window.height() + $window.scrollTop();
148         } else {
149             fold = $(settings.container).offset().top + $(settings.container).height();
150         }
151 
152         return fold <= $(element).offset().top - settings.threshold;
153     };
154     
155     $.rightoffold = function(element, settings) {
156         var fold;
157 
158         if (settings.container === undefined || settings.container === window) {
159             fold = $window.width() + $window.scrollLeft();
160         } else {
161             fold = $(settings.container).offset().left + $(settings.container).width();
162         }
163 
164         return fold <= $(element).offset().left - settings.threshold;
165     };
166         
167     $.abovethetop = function(element, settings) {
168         var fold;
169         
170         if (settings.container === undefined || settings.container === window) {
171             fold = $window.scrollTop();
172         } else {
173             fold = $(settings.container).offset().top;
174         }
175 
176         return fold >= $(element).offset().top + settings.threshold  + $(element).height();
177     };
178     
179     $.leftofbegin = function(element, settings) {
180         var fold;
181         
182         if (settings.container === undefined || settings.container === window) {
183             fold = $window.scrollLeft();
184         } else {
185             fold = $(settings.container).offset().left;
186         }
187 
188         return fold >= $(element).offset().left + settings.threshold + $(element).width();
189     };
190 
191     $.inviewport = function(element, settings) {
192          return !$.rightofscreen(element, settings) && !$.leftofscreen(element, settings) && 
193                 !$.belowthefold(element, settings) && !$.abovethetop(element, settings);
194      };
195 
196     /* Custom selectors for your convenience.   */
197     /* Use as $("img:below-the-fold").something() */
198 
199     $.extend($.expr[':'], {
200         "below-the-fold" : function(a) { return $.belowthefold(a, {threshold : 0}); },
201         "above-the-top"  : function(a) { return !$.belowthefold(a, {threshold : 0}); },
202         "right-of-screen": function(a) { return $.rightoffold(a, {threshold : 0}); },
203         "left-of-screen" : function(a) { return !$.rightoffold(a, {threshold : 0}); },
204         "in-viewport"    : function(a) { return !$.inviewport(a, {threshold : 0}); },
205         /* Maintain BC for couple of versions. */
206         "above-the-fold" : function(a) { return !$.belowthefold(a, {threshold : 0}); },
207         "right-of-fold"  : function(a) { return $.rightoffold(a, {threshold : 0}); },
208         "left-of-fold"   : function(a) { return !$.rightoffold(a, {threshold : 0}); }
209     });
210 
211 })(jQuery, window);

 

　　下面我们来看引入方式。

　　

       你必须改变你的HTML代码。将​​SRC​​属性的占位符图像。演示页面使用1×1像素的灰度GIF 的真实图像的URL必须投入​​原始数据的​​属性。

　　

　　下面我们来看在html代码中需要书写些什么，这是调用方式。

　　

1 <script type="text/javascript">      
2 $(document).ready(function() { 
3          $("img").lazyload({ 
4                 effect: "fadeIn"
5           });      
6 });  
7 </script>

 

  　　到这里，其实已经实现了我们想要的效果了，图片将会以 fadeIn 形式被载入。

　　但是，我们在这里还是不得不提一下，这个插件的其他的功能。

1.可以设置阀值来控制 灵敏度，下边代码把阀值设置成200 表明当图片没有看到之前先load 200像素。

1 $("img").lazyload({ threshold : 200 });

 

2.可以通过设置占位符图片和自定事件来触发加载图片事件

1 $("img").lazyload({
2       placeholder: "img/grey.gif",      event: "click" 
3  });

 

3.事件触发加载，事件可以是任何 jQuery 时间, 如: click 和 mouseover. 你还可以使用自定义的事件, 如: sporty 和 foobar. 默认情况下处于等待状态, 直到用户滚动到窗口上图片所在位置. 在灰色占位图片被点击之前阻止加载图片, 你可以这样做:

1 $("img").lazyload({
2       placeholder: "img/grey.gif",      event: "click" 
3  });

 

延迟加载图片，Lazy Load 插件的一个不完整的功能, 但是这也能用来实现图片的延迟加载. 下面的代码实现了页面加载完成后再加载. 页面加载完成 5 秒后, 指定区域内的图片会自动进行加载.

1 $(function() {
 2       $("img:below-the-fold").lazyload({                                    
 3           placeholder: "img/grey.gif",
 4           event: "sporty"
 5 });  
 6 });  
 7 $(window).bind("load", function() {
 8       var timeout = setTimeout(function() {
 9       $("img").trigger("sporty") }, 5000);  
10 });