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      <h1>Implementing Properties</h1>
      <p>
        <font size="-2">by&nbsp;Peter B. West</font>
      </p>
      <ul class="minitoc">
        <li>
          <a href="#N10014">An alternative properties implementation</a>
          <ul class="minitoc">
            <li>
              <a href="#N10043">The history problem</a>
            </li>
            <li>
              <a href="#N10061">The construction hierarchy</a>
            </li>
            <li>
              <a href="#N10082">Representing properties: &lt;property&gt; classes</a>
            </li>
          </ul>
        </li>
      </ul>
      
      <a name="N10014"></a>
      <h3>An alternative properties implementation</h3>
      <div class="frame note">
        <div class="label">Note</div>
        <div class="content"> 
          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.
        </div>
      </div>
      <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>
      <div class="frame note">
        <div class="label">Note</div>
        <div class="content">
          
          <em>(XSL 1.0 Rec)</em> <strong>5.1.4 Inheritance</strong>
          ...The inheritable properties can be placed on any formatting
          object.
        </div>
      </div>
      <p>
        Even if the value is not inheritable, it may be accessed by
        its children through the <span class="codefrag">inherit</span>
        keyword or the <span class="codefrag">from-parent()</span>
        core function, and potentially by any of its descendents
        through the <span
        class="codefrag">from-nearest-specified-value()</span> 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>
      <a name="N10043"></a>
      <h4>The history problem</h4>
      <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>
      <a name="N10061"></a>
      <h4>The construction hierarchy</h4>
      <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>
      <a name="N10082"></a>
      <h4>Representing properties: &lt;property&gt; classes</h4>
      <a name="N10087"></a>
      <h4>Class vs instance</h4>
      <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 <span class="codefrag">PropertyValue</span> 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>
      <p>
        
        <strong>Next:</strong> <a href= "classes-overview.html"
                                  >Property classes overview.</a>
        
      </p>
      
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