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130 ” CSS Selector Libraries selector expressions as their input and translate them into equivalent userland operations, which can have an impact on performance. Those points aside, they do offer a syntax that is considered by some to be more intuitive. M o s t developers these days know at least some CSS and probably have a little experience with a J a v a S c r i p t library that uses CSS selectors for node selection on the client side, such as jQuery. Thus, knowledge of that particular area is transferable to the server side when libraries based around it are used. T osum this up, if you’re particularly concerned about performance or simplicity of the application with regard to the number of components on which it is dependent, it’s probably best to stick with something like XPath. Otherwise, CSS selector libraries are worthy of consideration for making use of a common existing skillset. Eve n if your decision is to stick with XPath, keep reading. This chapter will also show some XPath equivalents for each set of explained CSS selectors, which may help to further your understanding of XPath. N o t e that these comparions are not necessarily comprehensive and there may be multiple ways to express the same thing in any given case. i jQuery Examples The documentation for the jQuery library itself actually has excellent visual clientside examples of selectors. If you find you aren’t certain that you understand any of the descriptions of CSS selectors that follow, the jQuery demos and source code will likely prove to be helpful supplements. Y o ucan find them at http://api.jquery.com/category/selectors/. Basics Let’s look at a few basic selectors and their results when applied to a markup example. Home
CSS Selector Libraries ” 131 About Us Contact Us • #nav would select the div element because it has an id attribute value of nav. • li would select all li elements by their node name. • .horizontal would select the ul element because it has a class of horizontal. (Note that elements can have multiple classes.) • * would select all elements in the document. • li, a would select all li and a elements in the document by combining the two selectors li and a into a comma-delimited list. H e r e are the XPath equivalents along side their respective CSS counterparts. Aside from the .class selector, the XPath expressions are not significantly longer or more complex. Selector CSS XPath id #nav //*[@id=“nav”] element li //li class .horizontal //*[@class=“horizontal” or starts-with(@class, “horizontal ”) or contains(@class, “ horizontal ”) or ends-with(@class, “ horizontal”)] wildcard * //* multiple li, a //li|//a
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