jQuery.buildFragment使用方法及思路分析

var rnocache = /<(?:script|object|embed|option|style)/i,
rchecked = /checked\s*(?:[^=]|=\s*.checked.)/i,
rnoshimcache = new RegExp("<(?:" + nodeNames + ")[\\s/>]", "i");
jQuery.fragments = {};
jQuery.buildFragment = function( args, context, scripts ) {
var fragment, cacheable, cachehit,
first = args[ 0 ];
// Set context from what may come in as undefined or a jQuery collection or a node
// Updated to fix #12266 where accessing context[0] could throw an exception in IE9/10 &
// also doubles as fix for #8950 where plain objects caused createDocumentFragment exception
// 根据参数context值的不同,确保context为文档根节点document
// jQuery1.8.0版本以后代码相对于之前版本有很大改进,以下是改进地方:
// 针对context参数值为undefined, jQuery对象,DOM元素节点情况改进代码
// 解决了1.8.0版本中context参数为文档片段(#document-fragment)的bug
context = context || document;
context = !context.nodeType && context[0] || context;
context = context.ownerDocument || context;
// Only cache "small" (1/2 KB) HTML strings that are associated with the main document
// Cloning options loses the selected state, so don't cache them
// IE 6 doesn't like it when you put <object> or <embed> elements in a fragment
// Also, WebKit does not clone 'checked' attributes on cloneNode, so don't cache
// Lastly, IE6,7,8 will not correctly reuse cached fragments that were created from unknown elems #10501
// html字符串小于512字节
// 克隆option标签会丢失选中状态,因此不缓存
// IE 6不能把<object>,<embed>标签嵌入到文档碎片中
// WebKit浏览器(如：Safria)克隆拥有checked属性节点时,也会丢失选中状态,因此不缓存,google最新版本不存在该bug
// 最后,IE6,7、8不会正确地重用由html5标签元素创建的缓存片段
if ( args.length === 1 && typeof first === "string" && first.length < 512 && context === document &&
first.charAt(0) === "<" && !rnocache.test( first ) &&
// 如果浏览器能够克隆checked属性和支持html5,或者html字符串中不存在checked和html5标签元素
(jQuery.support.checkClone || !rchecked.test( first )) &&
(jQuery.support.html5Clone || !rnoshimcache.test( first )) ) {
// Mark cacheable and look for a hit
// 如果以上条件都满足，则打上可缓存标记
cacheable = true;
// 将数组项字符串(主要是html字符串)缓存到jQuery.fragment对象的属性列表中，并获取缓存值
// 如果clean函数已经处理过了第二次相同的字符串内容，缓存值则为clean函数处理的文档碎片，字符串解析可以略过clean处理
fragment = jQuery.fragments[ first ];
// 在clean函数处理了第一次字符串(与第二次相同)后，cachehit为true
cachehit = fragment !== undefined;
}
// 判断缓存值
if ( !fragment ) {
fragment = context.createDocumentFragment();
// 通过clean函数处理args数组，将数组每一项字符串都生成dom节点，
// 并且添加到提供的文档碎片(fragment)中，因此返回的对象中的fragment属性
// 保存了参数args数组项字符串生成的dom节点
jQuery.clean( args, context, fragment, scripts );
// Update the cache, but only store false
// unless this is a second parsing of the same content
// 当cachehit为true时，jQuery.fragment[first]为clean函数处理的文档碎片
if ( cacheable ) {
jQuery.fragments[ first ] = cachehit && fragment;
}
}
return { fragment: fragment, cacheable: cacheable };
};