关于 jQuery lazyload 插件的基本介绍和使用请看上一篇文章。（水水一章啦-。-）
Overview
让我们先从整体上看看代码的结构：
(function ($, window, document, undefined) {
var $window = $(window);
/* lazyload 方法主体 */
$.fn.lazyload = function (options) {
// ...
}
/* 在jQuery命名空间内定义了便捷的方法，判断图片是否在容器视口范围内 */
$.belowthefold = function (element, settings) {...}
$.rightoffold = function (element, settings) {...}
$.abovethetop = function (element, settings) {...}
$.leftofbegin = function (element, settings) {...}
$.inviewport = function (element, settings) {...}
/* 自定义选择器 */
$.extend($.expr[":"], {...});
})(jQuery, window, document);可以看一下注释，就不多做解释了。
选项
在上一节可以看到，使用lazyload最简单的方法像这样：
$('img.lazy').lazyload(); 复杂一点的，我们可以传入一个选项对象自定义lazyload的表现。让我们从源码看一下所有可选的选项参数：
$.fn.lazyload = function (options) {
var elements = this;
var $container;
// 默认配置
var settings = {
threshold : 0,
failure_limit : 0,
event : "scroll.lazyload",
effect : "show",
container : window,
data_attribute : "original",
data_srcset : "srcset",
skip_invisible : false,
appear : null,
load : null,
placeholder : "data:image/gif;base64,R0lGODdhAQABAPAAAMPDwwAAACwAAAAAAQABAAACAkQBADs="
}
}分别解释一下：
threshold : 临界值
threshold: 0这个值是针对container容器的，即距离container容器视口的临界值
event : 事件
event: "scroll.lazyload"container容器默认绑定这个事件，在这个事件被触发时，会不断的判断img元素是否满足出发apper事件（即显示图片）的条件。默认的事件为scroll，你可以自定义这个事件。
effect :显示方法
effect: "show"默认为show，也可以设置为fadeIn，源码中隐藏了一个配置属性：effectspeed，用于设置动画运行的时间。
data_attribute : 图片地址属性
data_attribute: "original"img元素的一个data属性，用于存放图片的真实地址。
skip_invisible : 是否忽略隐藏的img元素
skip_invisible: false设置为true时会忽略处理隐藏的img元素。
placeholder : 图片占位符
placeholder : "data:image/gif;base64,R0lGODdhAQABAPAAAMPDwwAAACwAAAAAAQABAAACAkQBADs="
lazyload会将该属性的值设置为img元素的初始src属性值。
appear
appear: null在img触发load事件时执行的回调。
failure_limit
failure_limit: 0为了便于理解，我们先来看一段与其有关的源码：
var counter = 0;
elements.each(function() {
if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) {
// ...
} else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) {
// ...
} else {
if (++counter > settings.failure_limit) {
return false;
}
}
});如果找到的是第failure_limit个img元素，且不在container视口上方，左方及视口内（可以允许在视口下方，右方），则中断循环。
两个示例：
demo1 下拉滚动: http://jsfiddle.net/ddEPL/
demo2 Tab切换: http://jsfiddle.net/ddEPL/1/>…
源码分析
/*
* Lazy Load - jQuery plugin for lazy loading images
*
* Copyright (c) 2007-2013 Mika Tuupola
*
* Licensed under the MIT license:
* http://www.opensource.org/licenses/mit-license.php
*
* Project home:
* http://www.appelsiini.net/projects/lazyload
*
* Version: 1.9.3
*
*/
(function($, 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);