jQuery:掌握選擇器

選擇器至少可以追溯到“CSS選擇器”。jQuery的源代碼中內嵌了一個叫Sizzle的對象，其實就是選擇器了。在jQuery官網上顯示Sizzle屬於“Other jQuery Foundation Projects”，Sizzle能夠獨立爲一個單獨的項目，由此不難體會到選擇器的重要性。看看下面三個頁面，相比之下，jQuery選擇器官方文檔看起來是最“亂”的。

http://www.w3.org/TR/css3-selectors
CSS選擇器W3C標準文檔

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

http://api.jquery.com/category/selectors/
jQuery選擇器官方文檔

jquery1.9.0源代碼有這樣一行：

jQuery.find = Sizzle;

 

導入jquery.js和sizzle.js，可以看到jQuery.find和Sizzle確實是一回事。

 

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自稱"supports virtually all CSS 3 Selectors"，那麼不妨就參考下面W3C描述吧，再難找到更好的文檔片斷了（點擊鏈接可查看語法細節），不是麼？

 (備註：這個Table貼過來總是出現顯示問題，於是對html代碼進行了編輯，其中一個替換是(<a\shref="[^">]*">)[^<]*(</a>)替換爲$1更多$2)

語法	含義	鏈接	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()返回的對象類型是不一樣的，前者是jQuery.fn，後者是Sizzle。例如，jQuery('html body div#dbg');和jQuery.find('html body div#dbg');都是“選擇”了id爲dbg的div，但是前者表示爲jQuery.fn對象，後者表示爲Sizzle對象。

選擇器的語法是有標準的，逐一嘗試每一種寫法沒有必要，今天學會了也許明天就忘記，不如有文檔在手，用到時再翻閱。需要想一想的是選擇器的價值所在，從根本上來講，選擇器就是幫助我們避免了遍歷DOM（或者也包括XML?）的麻煩。一個兩個的遍歷代碼其實也不難寫，但是網頁上的互動多了就麻煩了，下面是一個隔行選取的代碼片斷，不難體會其中運用選擇器的奧妙所在：

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

<script language="javascript">
    var $myObj = jQuery('div b:nth-child(even)');//選擇器
    $myObj.each(
        function(i){
            var tTemp = jQuery(this).text();
            iJs.put(tTemp);//輸出選擇結果
        });
</script>

調試信息:
    ...測試2...
    ...測試4...