jQuery:掌握选择器

2024-12-25 技术教程

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-selectorsCSS选择器W3C标准文档https://github.com/jquery/sizzle/wiki/Sizzle-DocumentationSizzle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><scriptlanguage="javascript"> var$myObj=jQuery('divb:nth-child(even)');//选择器 $myObj.each( function(i){ vartTemp=jQuery(this).text(); iJs.put(tTemp);//输出选择结果 }); </script>调试信息:
...测试2...
...测试4...