include-pattern

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Revision as of 16:29, 6 November 2007 by BenWard (talk | contribs) (Substantial update // Moved Hyperlink include above OBJECT include // Made examples consistent // Improved voice in some sections // Describes ‘replacement’ behaviour to address mkaply 's issue)
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Include Pattern

Initially developed as part of resume-brainstorming, the include pattern is a mechanism to include a portion of data from one area of a page into another area of the same page. The following is documentation for re-use of the pattern in other microformats, and for publishers working with it.

Editors
 : Tantek Çelik
 : Ben Ward

Background

hResume needed the ability to include a name from one hCard at the top of a resume — the person's contact details — into into the separate hCards used in the same person's employment history. Repeating so much data would be inconvenient to publishers, irritating to consumers and would not have matched the existing publishing techniques used in Resumes and Curriculum Vitæ. The include pattern is a mechanism to reference data from the same page, avoiding repetition.

Scope

The include pattern is strictly limited to the scope of the current page. It cannot be used to include content from other URLs.

Quick Reference

The include-pattern is a mechanism to include content from one microformat into another microformat elsewhere in the same document, using hyperlinks (recommended) or OBJECT. For example: 

<a class="include" href="#author">Ben Ward</a>
<object class="include" data="#author"></object>

In specs which cite the include-pattern, either of the above code snippets will cause a microformats parser to replace the A or OBJECT element with class name "include" with the content fragment with ID "author". Full examples follow.

Include Pattern in General

To reference includes, use an include element with class name "include" and a document fragment identifier. The content to be included should have an ID attribute set, and that ID should be referenced from the HREF or DATA attribute at the point of inclusion.

The include element with class name "include" indicates a reference to a sub-tree elsewhere in the document which must be included in-place by microformat parsers. That is, the element with class "include" is _replaced_ in the DOM by the referenced sub-tree.

To prevent infinite loops, if a class="include" refers to itself or to an ancestor in the parse tree, then it is ignored and has no effect on the parser.

Per the scope, the OBJECT 'data' attribute and hyperlink 'href' attribute MUST be local ID references when used as include pattern instances. External references (requiring a consuming application to load an external resource) are not supported by this method.

There are two HTML elements available to reference includes, hyperlink/Anchor and OBJECT. They are documented below.

These methods of property indirection via a hyperlink element can apply to any/all properties in class-based microformats, but should only be used where a microformat explicitly states that the include-pattern is a dependency. For example, XOXO does not reference the include-pattern at this time, so sub-trees cannot be included by reference in XOXO. hResume and hReview do reference the include pattern.

HYPERLINK include

The recommended way to reference includes within microformats is to use a hyperlink.

HYPERLINK include example

Here is an hCard from the beginning of a resume, shown here as a verbose hCard. 

<span class="vcard">
 <span class="fn n" id="j">
  <span class="given-name">James</span> <span class="family-name">Levine</span>
 </span>
</span>

Elsewhere on the page, a second hCard re-uses the "fn n" content from the first hCard: 

<span class="vcard">
 <a href="#j" class="include">Ben Ward</a>
 <span class="org">SimplyHired</span>
 <span class="title">Microformat Brainstormer</span>
</span>

A microformat parser must treat the second hCard as follows, with the hyperlink include element completely replaced (including inner-text) by the sub-tree that was referenced: 

<span class="vcard">
 <span class="fn n" id="j">
  <span class="given-name">James</span> <span class="family-name">Levine</span>
 </span>
 <span class="org">SimplyHired</span>
 <span class="title">Microformat Brainstormer</span>
</span>

HYPERLINK include notes and issues

Using the hyperlink include pattern causes no known issues in any web browser. However, it is necessary to supply inner-text for the hyperlink itself. This can require repeating a small piece of information (such as a person's name in an hCard), or including generic text suitable for the context of the page. The user experience of Assistive Technology can be severely degraded by the presence of hyperlinks which do not contain text.

In most cases, publishers hide the include hyperlink from most browsers using CSS.

OBJECT include

The OBJECT include pattern is semantically superior to the hyperlink include; it is being used to embed content into the page. OBJECT was the original developed include pattern. However, there are serious browser compatibility issues that can affect some implementation scenarios.

OBJECT include example

Here is the same hCard from the beginning of the resume in the previous example. 

<span class="vcard">
 <span class="fn n" id="j">
  <span class="given-name">James</span> <span class="family-name">Levine</span>
 </span>
</span>

Elsewhere on the page, a second hCard re-uses the "fn n" content from the first hCard: 

<span class="vcard">
 <object data="#j" class="include"></object>
 <span class="org">SimplyHired</span>
 <span class="title">Microformat Brainstormer</span>
</span>

A microformat parser must treat the second hCard as follows, with the OBJECT include element completely replaced by the sub-tree that it referenced: 

<span class="vcard">
 <span class="fn n" id="j">
  <span class="given-name">James</span> <span class="family-name">Levine</span>
 </span>
 <span class="org">SimplyHired</span>
 <span class="title">Microformat Brainstormer</span>
</span>

This method of hCard property indirection via an object element can apply to any/all properties in class-based microformats.

OBJECT include notes and issues

  • Unlike the hyperlink pattern, the OBJECT pattern does not require any textual fallback. This makes it invisible to Assistive Technology with no adverse affect.
  • Apple's Safari (WebKit-based) browser has some rendering issues with this use of OBJECT: You should set a width and height of "0" for the include elements to resolve this.
  • Bugs have been reported in some web browsers (Internet Explorer, Safari, Firefox) that OBJECT elements referencing fragments of the same document erroneously cause the browser to make additional HTTP requests. For scenarios where HTTP requests are a sensitive performance measure, this makes the hyperlink pattern inappropriate.
  • Versions of Microsoft Internet Explorer with ActiveX controls disabled will throw a warning prompt upon finding an OBJECT element.


Acknowledgements

Thanks to discussions and brainstorms with a bunch of folks: Ryan King, James Levine, the whole crowd at the Microformats specifications working session at MashupCamp, Brian Suda, Scott Reynen, Drew McLellan.

Specifications Using

Considering

Implementations

References

Normative

  • HTML 4.01
  • XHTML 1.0

Informative

Related Pages

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