Lazy Load 是一个用 JavaScript 编写的 jQuery 插件. 它可以延迟加载长页面中的图片. 在浏览器可视区域外的图片不会被载入, 直到用户将页面滚动到它们所在的位置. 这与图片预加载的处理方式正好是相反的.

/*   * 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);