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jQuery:掌握选择器

选择器至少可以追溯到“CSS选择器”。jQuery的源代码中内嵌了一个叫Sizzle的对象，其实就是选择器了。在jQuery官网上显示Sizzle属于“Other jQuery Foundation Projects”，Sizzle能够独立为一个单独的项目，由此不难体会到选择器的重要性。看看下面三个页面，相比之下，jQuery选择器官方文档看起来是最“乱”的。

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  1. http://www.w3.org/TR/css3-selectors
  2. CSS选择器W3C标准文档
  3.  
  4. https://github.com/jquery/sizzle/wiki/Sizzle-Documentation
  5. Sizzle文档
  6.  
  7. http://api.jquery.com/category/selectors/
  8. jQuery选择器官方文档

jquery1.9.0源代码有这样一行：

  1. jQuery.find = Sizzle; 
 
导入jquery.js和sizzle.js，可以看到jQuery.find和Sizzle确实是一回事。
 
  1. iJs.showObjectNames("window.jQuery.find");  
  2. iJs.showObjectNames("window.Sizzle"); 

    [Object] window.jQuery.find
        |--[function] attr
        |--[function] compile
        |--[function] contains
        |--[function] error
        |--[function] getText
        |--[function] isXML
        |--[function] matches
        |--[function] matchesSelector
        |--[function] setDocument
        |--[function] uniqueSort
        |--[object] selectors
    
    [Object] window.Sizzle
        |--[function] attr
        |--[function] compile
        |--[function] contains
        |--[function] error
        |--[function] getText
        |--[function] isXML
        |--[function] matches
        |--[function] matchesSelector
        |--[function] setDocument
        |--[function] uniqueSort
        |--[object] selectors
 
既然Sizzle自称"supports virtually all CSS 3 Selectors"，那么不妨就参考下面W3C描述吧，再难找到更好的文档片断了（点击链接可查看语法细节），不是么？
 (备注：这个Table贴过来总是出现显示问题，于是对html代码进行了编辑，其中一个替换是(]*">)[^<]*()替换为$1更多$2)
语法 含义 链接 版本
* any element 更多 2
E an element of type E 更多 1
E[foo] an E element with a "foo" attribute 更多 2
E[foo="bar"] an E element whose "foo" attribute value is exactly equal to "bar" 更多 2
E[foo~="bar"] an E element whose "foo" attribute value is a list of whitespace-separated values, one of which is exactly equal to "bar" 更多 2
E[foo^="bar"] an E element whose "foo" attribute value begins exactly with the string "bar" 更多 3
E[foo$="bar"] an E element whose "foo" attribute value ends exactly with the string "bar" 更多 3
E[foo*="bar"] an E element whose "foo" attribute value contains the substring "bar" 更多 3
E[foo|="en"] an E element whose "foo" attribute has a hyphen-separated list of values beginning (from the left) with "en" 更多 2
E:root an E element, root of the document 更多 3
E:nth-child(n) an E element, the n-th child of its parent 更多 3
E:nth-last-child(n) an E element, the n-th child of its parent, counting from the last one 更多 3
E:nth-of-type(n) an E element, the n-th sibling of its type 更多 3
E:nth-last-of-type(n) an E element, the n-th sibling of its type, counting from the last one 更多 3
E:first-child an E element, first child of its parent 更多 2
E:last-child an E element, last child of its parent 更多 3
E:first-of-type an E element, first sibling of its type 更多 3
E:last-of-type an E element, last sibling of its type 更多 3
E:only-child an E element, only child of its parent 更多 3
E:only-of-type an E element, only sibling of its type 更多 3
E:empty an E element that has no children (including text nodes) 更多 3
E:link
E:visited		 an E element being the source anchor of a hyperlink of which the target is not yet visited (:link) or already visited (:visited) 更多 1
E:active
E:hover
E:focus		 an E element during certain user actions 更多 1 and 2
E:target an E element being the target of the referring URI 更多 3
E:lang(fr) an element of type E in language "fr" (the document language specifies how language is determined) 更多 2
E:enabled
E:disabled		 a user interface element E which is enabled or disabled 更多 3
E:checked a user interface element E which is checked (for instance a radio-button or checkbox) 更多 3
E::first-line the first formatted line of an E element 更多 1
E::first-letter the first formatted letter of an E element 更多 1
E::before generated content before an E element 更多 2
E::after generated content after an E element 更多 2
E.warning an E element whose class is "warning" (the document language specifies how class is determined). 更多 1
E#myid an E element with ID equal to "myid". 更多 1
E:not(s) an E element that does not match simple selector s 更多 3
E F an F element descendant of an E element 更多 1
E > F an F element child of an E element 更多 2
E + F an F element immediately preceded by an E element 更多 2
E ~ F an F element preceded by an E element 更多 3
 
需注意，jQuery()和jQuery.find()返回的对象类型是不一样的，前者是jQuery.fn，后者是Sizzle。例如，jQuery('html body div#dbg');和jQuery.find('html body div#dbg');都是“选择”了id为dbg的div，但是前者表示为jQuery.fn对象，后者表示为Sizzle对象。

选择器的语法是有标准的，逐一尝试每一种写法没有必要，今天学会了也许明天就忘记，不如有文档在手，用到时再翻阅。需要想一想的是选择器的价值所在，从根本上来讲，选择器就是帮助我们避免了遍历DOM（或者也包括XML?）的麻烦。一个两个的遍历代码其实也不难写，但是网页上的互动多了就麻烦了，下面是一个隔行选取的代码片断，不难体会其中运用选择器的奥妙所在：

  1.  
  2.     ...测试1... 
  3.     ...测试2... 
  4.     ...测试3... 
  5.     ...测试4... 
 
  •  
  •    
  •     var $myObj = jQuery('div b:nth-child(even)');//选择器  
  •     $myObj.each(  
  •         function(i){  
  •             var tTemp = jQuery(this).text();  
  •             iJs.put(tTemp);//输出选择结果  
  •         });  
  •  
    • 调试信息:
        ...测试2...
        ...测试4...

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