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。

http://www.w3.org/TR/css3-selectors
CSS选择器W3C标准文档

https://github.com/jquery/sizzle/wiki/Sizzle-Documentation
Sizzle文档

http://api.jquery.com/category/selectors/
jQuery选择器官方文档

jquery1.9.0源代码有这样一行：

jQuery.find = Sizzle;

 

导入jquery.js和sizzle.js，可以看到jQuery.find和Sizzle确实是一回事。

 

iJs.showObjectNames("window.jQuery.find");
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(<a\shref="[^">]*">)[^<]*(</a>)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：

<div>
    <b>...测试1...</b>
    <b>...测试2...</b>
    <b>...测试3...</b>
    <b>...测试4...</b>
</div>

<script language="javascript">
    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
        });
</script>

调试信息:
     ...测试2...
     ...测试4...