XML Made Simple (Made Simple Programming)
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Not sure? Well, put bluntly, the Web has reached a point at which just about anything will fly when it comes to HTML documents. Take a look at the following snippet:. Believe it or not, that snippet will render without a problem in most Web browsers. And so will this:.
But, exactly what does this mean? Use this with CSS to minimize presentational clutter. Use this when you want to use frames to partition the screen. XML Namespaces were invented to rectify a common problem: the collision of documents using identical element names for different data. Imagine you were running a bookstore and had an inventory file called inventory. A human being could probably figure out that one title has nothing to do with the other, but an application that tried to sort it out would go nuts.
We need to have a way to distinguish between the two different semantic universes in which these identical terms exist. Your inventory file stores information about books on the shelf, but the sales file stores information about books that have been bought by customers. In either situation, regardless of the chasm that lies between the contexts of these identical terms, we need a way to properly label each context.
Namespaces to the rescue! To use and declare a namespace, we must first tie the namespace to a URI.
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URIs can take the following forms:. Uniform Resource Locator: a specific protocol, machine address, and file path e. For example, all published books have an ISBN. However, armed with the ISBN, you could walk into the store, ask an employee to search for you, and they could take you right to the book provided, of course, that it was in stock. We want to use our namespace throughout our XML documents, though, and the last thing we want to do is type out an entire URI every time we need to distinguish one context from another.
So, we define a prefix to represent our namespace to ease the strain on our typing fingers:. The agreed way to do that is to prefix the namespace declaration with xmlns :, like this:. At this point, we have something useful. If we needed to, we could add our prefix to appropriate elements to disambiguate I love that term! In most cases, placing your namespace declarations will be rather easy.
Please note, however, that namespaces have scope. Namespaces affect the element in which they are declared, as well as all the child elements of that element. However, if you want to limit your namespace scope to a certain part of a document, feel free to do so — remembering, of course, that this can get pretty tricky. Using Default Namespaces. It would become pretty tiresome to have to type a prefix for every single element in a document.
On the other side of the coin, all XSLT elements must be given the xsl: prefix. This document contains a root element letter that contains three other elements to , from , and message , each of which contains text. When you display your XML document, you should see something similar to Figure 2. Figure 2. As you can see, CSS did a marvelous job of rendering a nicely shaded box around the entire letter, setting fonts, and even displaying things like margins and padding. Strictly speaking, the CSS standard does allow for this sort of thing with the content property, which can produce generated text before and after document elements.
Think of it as a tool that you can use to transform your XML documents into other documents. Here are some of the possibilities:. XSLT is a rules-based, or functional language. Because XSLT can be a little bewildering even for veteran programmers, the best way to tackle it is to walk through a series of examples. Keeping both these elements simple will give us the opportunity to step through the major concepts involved.
They must therefore follow the rules that apply to all XML documents: an XSL file must contain a root element, all attribute values must be quoted, and so on. The version attribute is required. The xmlns:xsl attribute is used to declare an XML namespace with the prefix xsl. In our example, we will use an xsl prefix on all the stylesheet-related tags in our XSL documents to associate them with this namespace. The next element will be the output element, which is used to define the type of output you want from the XSL file. Now we come to the heart of XSLT — the template and apply-templates elements.
Together, these two elements make the transformations happen. Put simply, the XSLT processor for our immediate purposes, the browser starts reading the input document, looking for elements that match any of the template elements in our style sheet. When one is found, the contents of the corresponding template element tells the processor what to output before continuing its search.
Where a template contains an apply-templates element, the XSLT processor will search for XML elements contained within the current element and apply templates associated with them. The first thing we want to do is match the letter element that contains the rest of our document. This is fairly straightforward:. Were the value simply letter , the template would match letter elements throughout the document. By default, apply-templates will match not only elements, but text and even whitespace between the elements as well.
XSLT processors have a set of default, or implicit templates, one of which simply outputs any text or whitespace it encounters. Each of these templates matches one of the elements we expect to find inside the letter element: to , from , and message. In each case, we output a text label e. TO: and then use apply-templates to output the contents of the tag remember, in the absence of a select attribute that says otherwise,apply-templates will output any text contained in the tags automatically.
The last thing we have to do in the XSL file is close off the stylesheet element that began the file:. Left this way, the output would look something like this:. Each of our three main templates begins with a line break and then some whitespace before the label, which is being carried through to the output. But wait — what about the line break and whitespace that ends each template?
Well by default, the XSLT standard mandates that whenever there in only whitespace including line breaks between two tags, the whitespace should be ignored. But when there is text between two tags e. TO: , then the whitespace in and around that text should be passed along to the output. The vast majority of XML books and tutorials out there completely ignore these whitespace treatment issues.
Best to get a good grasp of it now, rather than waiting for insanity to set in when you least expect it. All it does is output the text it contains, even if it is just whitespace. Notice how each template now outputs its label e. TO: followed by a single space, then finishes off with a line break. This gives us the fine control over formatting that we need when outputting a plain text file.
Are we done yet? Not quite. When you view the XML document in Firefox, you should see something similar to the result pictured in Figure 2. Internet Explorer interprets the result as HTML code, even when the style sheet clearly specifies that it will output text. As a result, whitespace is collapsed and our whole document appears on one line.
For this reason, it is not yet practical to rely on browser support for XSLT in a real-world website. You should see something similar to Figure 2. What happens if you need to transform your own XML document into an XML document that meets the needs of another organization or person? Not to worry — XSLT will save the day! You see, Web browsers only supply collapsible tree formatting for XML documents without style sheets. XML documents that result from a style sheet transformation are displayed without any styling at all, or at best are treated as HTML — not at all the desired result.
There are several things that need to be added to your style sheet to signal to the browser that the document is more than a plain XML file, though. Here we have declared a default namespace for tags without prefixes in the style sheet. Next up, we can flesh out the output element to more fully describe the output document type:.
Internet Explorer for Windows displays XHTML documents in Quirks Mode when this declaration is present, so by omitting it we can ensure that this browser will display it in the more desirable Standards Compliance mode. The rest of the style sheet is as it was for the HTML output example we saw above. Now, we need to identify exactly what we need for our news items, binary files, and Web copy.