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+<!DOCTYPE document PUBLIC "-//APACHE//DTD Documentation V1.1//EN"
+ "http://cvs.apache.org/viewcvs.cgi/*checkout*/xml-forrest/src/resources/schema/dtd/document-v11.dtd">
+
+<document>
+ <header>
+ <title>Implementing Properties</title>
+ <authors>
+ <person id="pbw" name="Peter B. West" email="pbwest@powerup.com.au"/>
+ </authors>
+ </header>
+ <body>
+ <section>
+ <title>An alternative properties implementation</title>
+ <note>
+ The following discussion focusses on the relationship between
+ Flow Objects in the Flow Object tree, and properties. There
+ is no (or only passing) discussion of the relationship between
+ properties and traits, and by extension, between properties
+ and the Area tree.
+ </note>
+ <p>
+ Property handling is complex and expensive. Varying numbers of
+ properties <strong>apply</strong> to individual Flow Objects
+ <strong>(FOs)</strong> in the <strong>FO tree </strong> but
+ any property may effectively be assigned a value on any
+ element of the tree. If that property is inheritable, its
+ defined value will then be available to any children of the
+ defining FO.
+ </p>
+ <note>
+ <em>(XSL 1.0 Rec)</em> <strong>5.1.4 Inheritance</strong>
+ ...The inheritable properties can be placed on any formatting
+ object.
+ </note>
+ <p>
+ Even if the value is not inheritable, it may be accessed by
+ its children through the <code>inherit</code> keyword or the
+ <code>from-parent()</code> core function, and potentially by
+ any of its descendents through the
+ <code>from-nearest-specified-value()</code> core function.
+ </p>
+ <p>
+ In addition to the assigned values of properties, almost every
+ property has an <strong>initial value</strong> which is used
+ when no value has been assigned.
+ </p>
+ <section>
+ <title>The history problem</title>
+ <p>
+ The difficulty and expense of handling properties comes from
+ this univeral inheritance possibility. The list of properties
+ which are assigned values on any particular <em>FO</em>
+ element will not generally be large, but a current value is
+ required for each property which applies to the <em>FO</em>
+ being processed.
+ </p>
+ <p>
+ The environment from which these values may be selected
+ includes, for each <em>FO</em>, <strong>for each applicable
+ property</strong>, the value assigned on this <em>FO</em>,
+ the value which applied to the parent of this <em>FO</em>,
+ the nearest value specified on an ancestor of this element,
+ and the initial value of the property.
+ </p>
+ </section>
+ <section>
+ <title>The construction hierarchy</title>
+ <p>
+ Properties are resoved in the <strong>FO tree</strong> in a
+ strictly hierarchical manner. Nodes are detected in the
+ input in a <strong>pre-order</strong> traversal, and are
+ built in the same order. This imples that there are two
+ phases, or states, of property resolution and construction.
+ Any particular FO node is either in a state of constructing
+ its own subtree, or in a stable state where the subtree
+ construction is complete. These states have differenct data
+ requirements.
+ </p>
+ <dl>
+ <dt>Subtree building</dt>
+ <dd>
+ In this state, all properties defined on this node, or any
+ of its ancestors must be available to the subtree. In
+ effect, any property defined on this node must be
+ available to its descendants, as all properties defined on
+ any ancestor are available to this node.
+ </dd>
+ <dt>Stable: subtree building complete</dt>
+ <dd>
+ In this state, only the properties <strong>applicable to
+ this node</strong> need be available.
+ </dd>
+ </dl>
+ </section>
+ <section>
+ <title>Representing properties: <property> classes</title>
+ <section>
+ <title>Class vs instance</title>
+ <p>
+ What information is required of property objects?
+ More particularly, what information is particular to the
+ property classes, and what to the instantiated
+ objects? The answer to this question depend largely on
+ how the property objects are used in the context
+ of layout and Area tree construction. The approach taken
+ in this implementation is that properties are simply flags
+ on certain data values associated with FOs. The semantics
+ of these flags are determined within the layout engine.
+ </p>
+ <p>
+ Certain constant information attaches to individual
+ property classes. This information is detailed in
+ the descriptions of individual properties in <em>Section
+ 7</em> of the specification. Such information is
+ represented in <strong>class</strong> fields and data
+ structures within the classes.
+ </p>
+ <p>
+ The "instance" information content of a property
+ is:
+ </p>
+ <ul>
+ <li>
+ explicitly, the <code>PropertyValue</code> datum of
+ the property, and
+ </li>
+ <li>
+ implicitly, the <strong>Flow Object</strong> to which
+ the property is attached.
+ </li>
+ </ul>
+ <p>
+ Properties, then, serve essentially to link <em>FO
+ instances</em> with <em>PropertyValue instances</em>,
+ attaching certain invariant semantic markers to the
+ PropertyValues in the process. In this implementation,
+ these functions can be realised entirely within the
+ property <strong>classes</strong> themselves,
+ without the need to instantiate any objects. In practice,
+ <strong>property singletons</strong> are
+ instantiated to make access to some invariants simpler.
+ </p>
+ </section>
+ </section>
+ <p>
+ <strong>Next:</strong> <link href="classes-overview.html"
+ >property classes overview.</link>
+ </p>
+ </section>
+ </body>
+</document>
+
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