vb.net barcode Crawling the DOM Tree in Java

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Crawling the DOM Tree
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In 7, we wrote a helper function named traverseTree() to crawl the DOM tree, which we ll have to do again in this chapter. The code for traverseTree() appears here: function traverseTree(node, func) { func(node); node = node.firstChild; while (node !== null) { arguments.callee(node, func); node = node.nextSibling; } } Coding traverseTree() to crawl the DOM by way of firstChild and nextSibling rather than iterating over childNodes is more than 100 times faster in Internet Explorer. So, our take on traverseTree() is already optimized relative to that Internet Explorer bug. Note that Firefox, Safari, and Opera crawl the DOM just as fast by iterating over childNodes. However, in Internet Explorer 9, Firefox, Safari, Chrome, and Opera,, traverseTree() has to crawl through Text nodes representing formatting whitespace in our XHTML markup. So if we could eliminate that ridiculous bit of work, traverseTree() would be much snappier in Internet Explorer 9, Firefox, Safari, Chrome, and Opera. DOM 3 defines an ElementTraversal interface that enables us to do just that. ElementTraversal provides the following members that we can use in place of firstChild, lastChild, previousSibling, nextSibling, and childNodes.length: firstElementChild lastElementChild previousElementSibling nextElementSibling childElementCount
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CHAPTER 9 LISTENING FOR EVENTS
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As its name implies, ElementTraversal is designed for traversing Element nodes. So, firstElementChild, lastElementChild, previousElementSibling, and nextElementSibling will contain an Element node or null (and never a Text, Comment, or any other kind of node). Note that childElementCount differs from childNodes.length in that it contains just the number of child Element nodes rather than the overall number of child nodes. Therefore, for Internet Explorer 9, Firefox, Safari, Chrome, and Opera, we can rework traverseTree() with firstElementChild and nextElementSibling to replace firstChild and nextSibling. The two function literals in the : expression differ only by those identifiers: var traverseTree = document.documentElement.firstElementChild function traverseTree (node, func) { func(node); node = node.firstElementChild; while (node !== null) { traverseTree(node, func); node = node.nextElementSibling; } } : function traverseTree (node, func) { func(node); node = node.firstChild; while (node !== null) { traverseTree(node, func); node = node.nextSibling; } } ; Note that the boolean expression, document.documentElement.firstElementChild, queries firstElementChild for the <html> Element node. That would be the <head> Element node. Why not simply query the firstElementChild of the Document node We could see whether document.firstElementChild refers to the <html> Element node. That s a bad idea: ElementTraversal is implemented only by Element nodes. So although all 12 node types have a firstChild member, only an Element node has a firstElementChild member, too. Hmm. If it were up to me, a Document node would implement ElementTraversal, too. But I do not write the standards, just about them.
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Note If you want to wade through the details of the ElementTraversal interface, visit www.w3.org/TR/ElementTraversal/.
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Finding an Element by Class
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The next helper function we want to rework is findClass(), which we wrote in 7 to help us find an Element node by its class attribute. DOM 3 adds a NodeSelector interface that defines two methods, querySelectorAll() and querySelector(). Those provide a way to query Element nodes with CSS selectors. That is to say, querySelectorAll() and querySelector() enable you to query elements in the same way that you would target them in a CSS rule. So for example, to query the <h4> element in our markup, we could invoke querySelectorAll() in any of the following ways in Firebug:
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document.querySelectorAll("h4.drag")[0]; // <h4 class="drag"> document.querySelectorAll("div#running h4")[0]; // <h4 class="drag"> document.getElementById("running").querySelectorAll("h4")[0]; // <h4 class="drag"> As the previous samples illustrate, NodeSelector is implemented by both Document and Element nodes. So you may query the whole DOM tree or just a branch, just like with getElementsByTagName().
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Note DocumentFragment nodes also implement NodeSelector.
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Like getElementsByTagName(), querySelectorAll() returns a NodeList. But unlike getElementsByTagName(), querySelectorAll() does not return a live DOM query, just a copy of the matching Element nodes. This is a terrific feature as far as script speed is concerned. It s sort of like optimizing some code by copying the Element nodes returned by getElementsByTagName() into an array so that you can work solely in ECMAScript. By the way, querySelector() returns the first Element node in the DOM tree matching a CSS selector. Since this is totally dependent on XHTML content, which is always being updated, I d discourage you from using querySelector(). If the element you want is no longer the first one in source code order months down the road, your script won t work. Just after the var statement for traverseTree, write one for findClass. The first operand to the : operator will be document.querySelectorAll, which will convert to true in Internet Explorer 8, Firefox 3.5, Safari 3.1, Chrome 4, and Opera 10. So, those versions or later will assign the first function literal to findClass. This one takes two parameters: name is the name of the class to find. root is where we begin descending the DOM tree from. root may be a Document or Element node. However, root is optional. In the event that root is undefined, we will simply assign document.documentElement to the parameter, which is the <html> element.
var findClass = document.querySelectorAll function (name, root) { root = root || document.documentElement; } : function() { } ; Next, call querySelectorAll() on the root parameter. Insofar as querySelectorAll() works with a CSS selector, we need to prefix a . on the class string in name. Let s do so and then return the value of calling querySelectorAll(): var findClass = document.querySelectorAll function (name, root) { root = root || document.documentElement; return root.querySelectorAll("." + name); } : function() { } ;
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