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Sams Teach Yourself XML in 21 Days

Sep 16, 2005

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Beyond Simple XLinks

So far, you've only taken a look at simple XLinks, where the xlink:type attribute is set to "simple". In fact, simple links are in practice the only XLinks supported in widely available software. However, more extended links will be supported sooner or later, so it's worth taking a look at them today.

Extended links can involve multiple resources, multiple paths between those resources, and multidirectional paths. Consequently, actual implementations of extended links are a little vague compared to those of simple links because no one has really determined how extended XLinks will really be supported. (In technical terms, an extended link is called a directed labeled graph, and they are very general constructs.)

An extended link can really only be characterized as being made up of connections between resources. Those resources may be local, which means they're actually part of the extended link element, or remote, which means they're not part of the extended link element (but this does not mean they are necessarily in another document). If a link contains resources, it's called an inline link; if it does not contain any resources at all, it's called an out-of-line link. Inline links havetheir xlink:type value set to "resource". Out-of-line links have their xlink:type attribute value set to "locator".

Listing 14.3 is an example that shows an extended link. The extended link element, which contains two inline and three out-of-line links, is the <link> element (although you could give it any name). This example links to the various <state> elements in the ch10_01.xml example from Day 1, "Welcome to XML"—California, Massachusetts, and New York. (You might note that to pick out the various states from ch10_01.xml, this example uses XPointers, and you'll see how those XPointers work in a few pages.)

Example 14.3. Extended XLinks (`ch14_03.xml`)

<?xml version="1.0" encoding="UTF-8"?>
<document>
    <link xmlns:xlink="http://www.w3.org/1999/xlink"

                xlink:type="extended" xlink:title="State Data">

           <title xlink:type="resource" xlink:role="Title">

               State Data

           </title>

           <date xlink:type="resource" xlink:role="Date">

               March 1, 2005

           </date>

           <state xmlns:xlink = "http://www.w3.org/1999/xlink"

               xlink:type = "locator"

               xlink:show = "embed"

               xlink:href = "http://www.XMLPowerCorp.com/ch10_01.xml#xpointer

                   (/descendant::state[position() = 1]">

               xlink:title="California"

               xlink:role="State Data"

           </state>

           <state xmlns:xlink = "http://www.w3.org/1999/xlink"

               xlink:type = "locator"

               xlink:show = "embed"

               xlink:href = "http://www.XMLPowerCorp.com/ch10_01.xml#xpointer

                   (/descendant::state[position() = 2]">

               xlink:title="Massachusetts"

               xlink:role="State Data"

           </state>

           <state xmlns:xlink = "http://www.w3.org/1999/xlink"

               xlink:type = "locator"

               xlink:show = "embed"

               xlink:href = "http://www.XMLPowerCorp.com/ch10_01.xml#xpointer

                   (/descendant::state[position() = 3]">

               xlink:title="New York"

               xlink:role="State Data"

           </state>

       </link>
</document>

This example is a start, but it still doesn't do much more than create an extended link with several inline and out-of-line links in it. You can do more if you use the xlink:from and xlink:to attributes, which allow us to create directed links—that is, arcs.

Creating Arcs

When you have a simple XLink, you don't need to worry about how it works; the xlink:href attribute tells us all you need to know. On the other hand, what about the extended link you just created? What happens when you activate it? There are many conceivable paths between the resources. To add more direction to what's going on, you can create an arc.

All the possible paths between resources are arcs. You represent those paths in XML elements by setting the xlink:type attribute to "arc". Arcs use xlink:from and xlink:to elements to specify traversal paths. The xlink:from attribute indicates what resource an arc comes from, and the xlink:to attribute indicates what resource it goes to. You set the values of xlink:from and xlink:to to match the xlink:role attribute of the source and target resources.

Listing 14.4 shows an example that includes three arc elements, <arc1>, <arc2>, and <arc3>. The first arc takes us from California to Massachusetts, the second arc takes us from Massachusetts to New York, and the third arc takes us from New York back to California.

Example 14.4. Creating Extended XLinks (`ch14_04.xml`)

<?xml version="1.0" encoding="UTF-8"?>
<document>
    <link xmlns:xlink="http://www.w3.org/1999/xlink"
             xlink:type="extended" xlink:title="State Data">
        <title xlink:type="resource" xlink:role="Title">
            State Data
        </title>
        <date xlink:type="resource" xlink:role="Date">
            March 1, 2005
        </date>
        <state xmlns:xlink = "http://www.w3.org/1999/xlink"
            xlink:type = "locator"
            xlink:show = "embed"
            xlink:href = "http://www.XMLPowerCorp.com/ch10_01.xml#xpointer
                (/descendant::state[position() = 1]">
            xlink:title="California"
            xlink:role="California"
        </state>
        <state xmlns:xlink = "http://www.w3.org/1999/xlink"
            xlink:type = "locator"
            xlink:show = "embed"
            xlink:href = "http://www.XMLPowerCorp.com/ch10_01.xml#xpointer
                (/descendant::state[position() = 2]">
            xlink:title="Massachusetts"
            xlink:role="Massachusetts"
        </state>
        <state xmlns:xlink = "http://www.w3.org/1999/xlink"
            xlink:type = "locator"
            xlink:show = "embed"
            xlink:href = "http://www.XMLPowerCorp.com/ch10_01.xml#xpointer
                (/descendant::state[position() = 3]">
            xlink:title="New York"
            xlink:role="New York"
        </state>

        <arc1 xlink:type = "arc" xlink:from = "California"

               xlink:to = "Massachusetts" xlink:show="new"

               xlink:actuate="onRequest">

           </arc1>


           <arc2 xlink:type = "arc" xlink:from = "Massachusetts"

               xlink:to = "New York" xlink:show="new"

               xlink:actuate="onRequest">

           </arc2>


           <arc3 xlink:type = "arc" xlink:from = "New York"

               xlink:to = "California" xlink:show="new"

               xlink:actuate="onRequest">

           </arc3>
    </link>
</document>

The way these arcs are actually used or activated depends on the software you're working with or that you've created.

Creating Linkbases

When you place out-of-line links in their own documents, those documents are called linkbases. The set of out-of-line links in a linkbase is called a linkset. You typically have three types of links in a linkbase: extended links, locator links, and arcs. You cannot have any links that are of the resource type.

Here's an example that converts the extended link example into a linkbase:

<?xml version="1.0" encoding="UTF-8"?>
<document>
    <link xmlns:xlink="http://www.w3.org/1999/xlink"
             xlink:type="extended" xlink:title="State Data">
        <state xmlns:xlink = "http://www.w3.org/1999/xlink"
            xlink:type = "locator"
            xlink:show = "embed"
            xlink:href = "http://www.XMLPowerCorp.com/ch10_01.xml#xpointer
                (/descendant::state[position() = 1]">
            xlink:title="California"
            xlink:role="California"
        </state>
        <state xmlns:xlink = "http://www.w3.org/1999/xlink"
            xlink:type = "locator"
            xlink:show = "embed"
            xlink:href = "http://www.XMLPowerCorp.com/ch10_01.xml#xpointer
                (/descendant::state[position() = 2]">
            xlink:title="Massachusetts"
            xlink:role="Massachusetts"
        </state>
        <state xmlns:xlink = "http://www.w3.org/1999/xlink"
            xlink:type = "locator"
            xlink:show = "embed"
            xlink:href = "http://www.XMLPowerCorp.com/ch10_01.xml#xpointer
                (/descendant::state[position() = 3]">
            xlink:title="New York"
            xlink:role="New York"
        </state>

        <arc1 xlink:type = "arc" xlink:from = "California"
            xlink:to = "Massachusetts" xlink:show="new"
            xlink:actuate="onRequest">
        </arc1>

        <arc2 xlink:type = "arc" xlink:from = "Massachusetts"
            xlink:to = "New York" xlink:show="new"
            xlink:actuate="onRequest">
        </arc2>

        <arc3 xlink:type = "arc" xlink:from = "New York"
            xlink:to = "California" xlink:show="new"
            xlink:actuate="onRequest">
        </arc3>
    </link>
</document>

Up to this point, you haven't heard very much about the xlink:href attribute. You just know that it points at resources. But there's a great deal more to it. You can use this attribute to point to specific locations or sections of a document. And to do that, you use XPointers.

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