前言：
貌似以前自己也写过图片懒加载插件，但是新公司使用的是jQuery.lazyload插件，为了更好的运用，自己还是把源码看了遍，分别记录了如何使用，
插件原理，各个配置属性的完整解释，demo实例，源码分析(较简短)，源码分析可以配合使用，配置属性，原理进行阅读，如需转载，请注明出处
博客园 华子yjh
一、如何使用
// 最简单的使用，不带参数 $('img').lazyload(); // 带参数(配置对象)，下面配置对象中的各个属性值都是默认的 $('img').lazyload({ threshold : 0, failure_limit : 0, event : "scroll", effect : "show", container : window, data_attribute : "original", skip_invisible : true, appear : null, load : null, placeholder : "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC" });
二、内部原理
首先选中的img元素都绑定了一个appear事件(处理img显示真实的图片地址)，方便以后满足条件时触发该事件；
在配置对象中有一个container属性配置，默认为window，如果img元素在该container容器视口中，则触发appear事件；
为了判断img元素是否在container容器视口范围中，造了如下四个轮子：
$.belowthefold = function(element, settings) {}; // 在视口下方 $.rightoffold = function(element, settings) {}; // 在视口右方 $.abovethetop = function(element, settings) {}; // 在视口上方 $.leftofbegin = function(element, settings) {}; // 在视口左方
看看源码中是如何利用这四个轮子：
if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) { /* Nothing. */ } // 不满足在上方，左方；也不满足在下方，右方； 则触发appear事件 else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) { $this.trigger("appear"); }
三、配置对象中的其他属性
临界值，这个值是针对container容器的，即距离container容器视口的临界值
{ threshold: 0 }
事件，container容器默认绑定这个事件，在这个事件被触发时，会不断的判断img元素是否满足触发appear的条件，
因此当浏览器不停的滚动下来时，如果满足条件，则显示图片；
另外还有一点，如果这个事件不是scroll事件，则选中的img元素都会绑定这个事件，绑定的这个事件中同样会触发内部appear事件；
{ event: 'scroll' }
显示方法，默认为show，也可以设置为fadeIn，API中隐藏了一个配置属性effectspeed，动画运行的时间
{ effect: "show" }
img元素的一个data属性，用于存放图片的真实地址
{ data_attribute: "original", }
忽略隐藏的img元素
{ skip_invisible: true }
图片占位符，img元素默认src属性为1*1像素的透明图片
{ placeholder: "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC" }
在img触发appear事件时执行的回调
{ appear: null }
在img触发load事件时执行的回调
{ load: null }
最后一个配置属性failure_limit
{ failure_limit: 0 }
为了便于理解，我们先来看一段与其有关的源码：
var counter = 0; elements.each(function() { if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) { // ... } else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) { // ... } else { if (++counter > settings.failure_limit) { return false; } } });
什么意思呢，如果找到的是第 failure_limit 个img元素，且不在container视口上方，左方及视口内（可以允许在视口下方，右方），则中断循环
三、demo
看完原理和配置属性，是否觉得很简单呢，来看看几个demo吧
demo1 下拉滚动: http://jsfiddle.net/ddEPL/
demo2 Tab切换: http://jsfiddle.net/ddEPL/1/ http://jsfiddle.net/ddEPL/8/
四、源码分析
/* * Lazy Load - jQuery plugin for lazy loading images * * Copyright (c) 2007-2013 Mika Tuupola * * Licensed under the MIT license: * http://www.opensource.org/licenses/mit-license.php * * Project home: * http://www.appelsiini.net/projects/lazyload * * Version: 1.9.3 * */ (function($, window, document, undefined) { 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, placeholder : "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC" }; function update() { var counter = 0; elements.each(function() { var $this = $(this); // 如果图片隐藏，且忽略隐藏，则中断循环 if (settings.skip_invisible && !$this.is(":visible")) { return; } if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) { /* Nothing. */ } // img满足在container视口中，则显示 else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) { $this.trigger("appear"); /* if we found an image we'll load, reset the counter */ counter = 0; } // 如果找到的是第（failure_limit + 1）个img元素，且不在container视口上方，左方及视口内（可以允许在视口下方，右方）， // 则中断循环 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. */ // 为container绑定scroll事件 if (0 === settings.event.indexOf("scroll")) { $container.bind(settings.event, function() { return update(); }); } this.each(function() { var self = this; var $self = $(self); self.loaded = false; /* If no src attribute given use data:uri. */ // 设置占位符 if ($self.attr("src") === undefined || $self.attr("src") === false) { if ($self.is("img")) { $self.attr("src", settings.placeholder); } } /* When appear is triggered load original image. */ // one绑定appear，触发后则移除该事件 $self.one("appear", function() { if (!this.loaded) { // 存在回调则触发 if (settings.appear) { var elements_left = elements.length; settings.appear.call(self, elements_left, settings); } $("<img />") .bind("load", function() { var original = $self.attr("data-" + settings.data_attribute); $self.hide(); if ($self.is("img")) { $self.attr("src", original); } else { $self.css("background-image", "url('" + original + "')"); } $self[settings.effect](settings.effect_speed); self.loaded = true; /* Remove image from array so it is not looped next time. */ // 更新elements，过滤掉已经加载的img元素，避免下次在update中轮循 var temp = $.grep(elements, function(element) { return !element.loaded; }); elements = $(temp); // 存在回调则触发 if (settings.load) { var elements_left = elements.length; settings.load.call(self, elements_left, settings); } }) .attr("src", $self.attr("data-" + settings.data_attribute)); } }); /* When wanted event is triggered load original image */ /* by triggering appear. */ // 绑定不是scroll的事件，用于触发appear事件 if (0 !== settings.event.indexOf("scroll")) { $self.bind(settings.event, function() { if (!self.loaded) { $self.trigger("appear"); } }); } }); /* Check if something appears when window is resized. */ $window.bind("resize", function() { update(); }); /* With IOS5 force loading images when navigating with back button. */ /* Non optimal workaround. */ if ((/(?:iphone|ipod|ipad).*os 5/gi).test(navigator.appVersion)) { $window.bind("pageshow", function(event) { if (event.originalEvent && event.originalEvent.persisted) { elements.each(function() { $(this).trigger("appear"); }); } }); } /* Force initial check if images should appear. */ $(document).ready(function() { 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.innerHeight ? window.innerHeight : $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(); }; $.inviewport = function(element, settings) { return !$.rightoffold(element, settings) && !$.leftofbegin(element, settings) && !$.belowthefold(element, settings) && !$.abovethetop(element, settings); }; /* Custom selectors for your convenience. */ /* Use as $("img:below-the-fold").something() or */ /* $("img").filter(":below-the-fold").something() which is faster */ $.extend($.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, document);