[转]jquery实现图片延时加载

本文转自：http://www.cnblogs.com/wscdzl/archive/2012/09/03/2668838.html

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。

// JavaScript Document
/*
 * Lazy Load - jQuery plugin for lazy loading images
 *
 * Copyright (c) 2007-2012 Mika Tuupola
 *
 * Licensed under the MIT license:
 *   http://www.opensource.org/licenses/mit-license.php
 *
 * Project home:
 *   http://www.appelsiini.net/projects/lazyload
 *
 * Version:  1.8.0
 *
 */
(function($, window) {
    var $window = $(window);

    $.fn.lazyload = function(options) {
        var elements = this;
        var $container;
        var settings = {
            threshold       : 0,
            failure_limit   : 0,
            event           : "scroll",
            effect          : "show",
            container       : window,
            data_attribute  : "original",
            skip_invisible  : true,
            appear          : null,
            load            : null
        };

        function update() {
            var counter = 0;
      
            elements.each(function() {
                var $this = $(this);
                if (settings.skip_invisible && !$this.is(":visible")) {
                    return;
                }
                if ($.abovethetop(this, settings) ||
                    $.leftofbegin(this, settings)) {
                        /* Nothing. */
                } else if (!$.belowthefold(this, settings) &&
                    !$.rightoffold(this, settings)) {
                        $this.trigger("appear");
                } else {
                    if (++counter > settings.failure_limit) {
                        return false;
                    }
                }
            });

        }

        if(options) {
            /* Maintain BC for a couple of versions. */
            if (undefined !== options.failurelimit) {
                options.failure_limit = options.failurelimit; 
                delete options.failurelimit;
            }
            if (undefined !== options.effectspeed) {
                options.effect_speed = options.effectspeed; 
                delete options.effectspeed;
            }

            $.extend(settings, options);
        }

        /* Cache container as jQuery as object. */
        $container = (settings.container === undefined ||
                      settings.container === window) ? $window : $(settings.container);

        /* Fire one scroll event per scroll. Not one scroll event per image. */
        if (0 === settings.event.indexOf("scroll")) {
            $container.bind(settings.event, function(event) {
                return update();
            });
        }

        this.each(function() {
            var self = this;
            var $self = $(self);

            self.loaded = false;

            /* When appear is triggered load original image. */
            $self.one("appear", function() {
                if (!this.loaded) {
                    if (settings.appear) {
                        var elements_left = elements.length;
                        settings.appear.call(self, elements_left, settings);
                    }
                    $("<img />")
                        .bind("load", function() {
                            $self
                                .hide()
                                .attr("src", $self.data(settings.data_attribute))
                                [settings.effect](settings.effect_speed);
                            self.loaded = true;

                            /* Remove image from array so it is not looped next time. */
                            var temp = $.grep(elements, function(element) {
                                return !element.loaded;
                            });
                            elements = $(temp);

                            if (settings.load) {
                                var elements_left = elements.length;
                                settings.load.call(self, elements_left, settings);
                            }
                        })
                        .attr("src", $self.data(settings.data_attribute));
                }
            });

            /* When wanted event is triggered load original image */
            /* by triggering appear.                              */
            if (0 !== settings.event.indexOf("scroll")) {
                $self.bind(settings.event, function(event) {
                    if (!self.loaded) {
                        $self.trigger("appear");
                    }
                });
            }
        });

        /* Check if something appears when window is resized. */
        $window.bind("resize", function(event) {
            update();
        });

        /* Force initial check if images should appear. */
        update();
        
        return this;
    };

    /* Convenience methods in jQuery namespace.           */
    /* Use as  $.belowthefold(element, {threshold : 100, container : window}) */

    $.belowthefold = function(element, settings) {
        var fold;
        
        if (settings.container === undefined || settings.container === window) {
            fold = $window.height() + $window.scrollTop();
        } else {
            fold = $(settings.container).offset().top + $(settings.container).height();
        }

        return fold <= $(element).offset().top - settings.threshold;
    };
    
    $.rightoffold = function(element, settings) {
        var fold;

        if (settings.container === undefined || settings.container === window) {
            fold = $window.width() + $window.scrollLeft();
        } else {
            fold = $(settings.container).offset().left + $(settings.container).width();
        }

        return fold <= $(element).offset().left - settings.threshold;
    };
        
    $.abovethetop = function(element, settings) {
        var fold;
        
        if (settings.container === undefined || settings.container === window) {
            fold = $window.scrollTop();
        } else {
            fold = $(settings.container).offset().top;
        }

        return fold >= $(element).offset().top + settings.threshold  + $(element).height();
    };
    
    $.leftofbegin = function(element, settings) {
        var fold;
        
        if (settings.container === undefined || settings.container === window) {
            fold = $window.scrollLeft();
        } else {
            fold = $(settings.container).offset().left;
        }

        return fold >= $(element).offset().left + settings.threshold + $(element).width();
    };

    $.inviewport = function(element, settings) {
         return !$.rightofscreen(element, settings) && !$.leftofscreen(element, settings) && 
                !$.belowthefold(element, settings) && !$.abovethetop(element, settings);
     };

    /* Custom selectors for your convenience.   */
    /* Use as $("img:below-the-fold").something() */

    $.extend($.expr[':'], {
        "below-the-fold" : function(a) { return $.belowthefold(a, {threshold : 0}); },
        "above-the-top"  : function(a) { return !$.belowthefold(a, {threshold : 0}); },
        "right-of-screen": function(a) { return $.rightoffold(a, {threshold : 0}); },
        "left-of-screen" : function(a) { return !$.rightoffold(a, {threshold : 0}); },
        "in-viewport"    : function(a) { return !$.inviewport(a, {threshold : 0}); },
        /* Maintain BC for couple of versions. */
        "above-the-fold" : function(a) { return !$.belowthefold(a, {threshold : 0}); },
        "right-of-fold"  : function(a) { return $.rightoffold(a, {threshold : 0}); },
        "left-of-fold"   : function(a) { return !$.rightoffold(a, {threshold : 0}); }
    });

})(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。

　　

<script type="text/javascript">      
$(document).ready(function() { 
         $("img").lazyload({ 
                effect: "fadeIn"
          });      
});  
</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。

$("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

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

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:

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

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.

$(function() {
      $("img:below-the-fold").lazyload({                                    
          placeholder: "img/grey.gif",
          event: "sporty"
});  
});  
$(window).bind("load", function() {
      var timeout = setTimeout(function() {
      $("img").trigger("sporty") }, 5000);  
}); 

更多的方法和资料，请参见官网http://www.appelsiini.net/projects/lazyload。

这里需要特别说明一下，jquery的版本不要太低哦，1.3的就不会出效果的。

这是我写的一个小demo，拿去参考一下还是极好的。

下载地址：https://files.cnblogs.com/wscdzl/demo.rar