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      <h1>Property Expression Parsing</h1>
      <p>
        <font size="-2">by&nbsp;Peter B. West</font>
      </p>
      <ul class="minitoc">
        <li>
          <a href="#N10014">Property expression parsing</a>
          <ul class="minitoc">
            <li>
              <a href="#N10044">Data types</a>
            </li>
            <li>
              <a href="#N10252">Tokenizer</a>
            </li>
            <li>
              <a href="#N1029C">Parser</a>
            </li>
          </ul>
        </li>
      </ul>
      
      <a name="N10014"></a>
      <h3>Property expression parsing</h3>
      <p>
        The parsing of property value expressions is handled by two
        closely related classes: <a href=
        "javascript:window.top.displayCode(
        'PropertyTokenizer.html#PropertyTokenizerClass' )" ><span
        class= "codefrag" >org.apache.fop.fo.expr.PropertyTokenizer</span></a>
        and its subclass, <a href= "javascript:window.top.displayCode(
        'PropertyParser.html#PropertyParserClass' )" ><span
        class= "codefrag" >org.apache.fop.fo.expr.PropertyParser</span></a>,
        and by <span class= "codefrag" >refineParsing(int, FONode,
        PropertyValue)</span> methods in the individual property
        classes.  <span class= "codefrag" >PropertyTokenizer</span>, as
        the name suggests, handles the tokenizing of the expression,
        handing <a href= "javascript:window.top.displayCode(
        'PropertyTokenizer.html#EOF' )" ><em>tokens</em></a> back to
        its subclass, <span
        class= "codefrag" >PropertyParser</span>. <span
        class= "codefrag" >PropertyParser</span>, in turn, returns a <a
        href= "javascript:window.top.displayCode(
        'PropertyValueList.html#PropertyValueListClass' )" ><span
        class= "codefrag">PropertyValueList</span></a>, a list of <a
        href= "javascript:window.top.displayCode(
        'PropertyValue.html#PropertyValueInterface' )" ><span class=
        "codefrag">PropertyValue</span></a>s.
      </p>
      <p>
        The tokenizer and parser rely in turn on the datatype
        definitions from the <span
        class= "codefrag" >org.apache.fop.datatypes</span> package,
        which include the <a href= "javascript:window.top.displayCode(
        'PropertyValue.html#NO_TYPE' )" ><span class= "codefrag"
        >PropertyValue</span> datatype constant definitions</a>.
      </p>
      <a name="N10044"></a>
      <h4>Data types</h4>
      <p>
        The data types currently defined in
        <span class= "codefrag" >org.apache.fop.datatypes</span> include:
      </p>
      <table class="ForrestTable" cellspacing="1" cellpadding="4">
        
          <tr>
            <th colspan="2" rowspan="1">Numbers and lengths</th>
          </tr>
          
          <tr>
            <th colspan="1" rowspan="1">Numeric</th>
            <td colspan="3" rowspan="1">
              The fundamental length data type.  <em>Numerics</em> of
              various types are constructed by the classes listed
              below.
            </td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <th colspan="3"
                rowspan="1">Constructor classes for <em>Numeric</em></th>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <td colspan="1" rowspan="1">Ems</td>
            <td colspan="2" rowspan="1">Relative length in <em>ems</em></td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <td colspan="1" rowspan="1">IntegerType</td>
            <td colspan="1" rowspan="1"></td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <td colspan="1" rowspan="1">Length</td>
            <td colspan="2" rowspan="1">In centimetres(cm), millimetres(mm),
              inches(in), points(pt), picas(pc) or pixels(px)</td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <td colspan="1" rowspan="1">Percentage</td>
            <td colspan="1" rowspan="1"></td>
          </tr>

          <tr>
            <th colspan="1" rowspan="1">Other Numeric</th>
            <td colspan="3" rowspan="1">
              Other numeric vaues which do not interact with the
              lengths represented by <em>Numeric</em> values.
            </td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <td colspan="1" rowspan="1">Angle</td>
            <td colspan="2" rowspan="1">In degrees(deg), gradients(grad) or
              radians(rad)</td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <td colspan="1" rowspan="1">Frequency</td>
            <td colspan="2" rowspan="1">In hertz(Hz) or kilohertz(kHz)</td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <td colspan="1" rowspan="1">Time</td>
            <td colspan="1" rowspan="1">In seconds(s) or milliseconds(ms)</td>
          </tr>
          
          <tr>
            <th colspan="2" rowspan="1">Strings</th>
          </tr>
          
          <tr>
            <th colspan="1" rowspan="1">StringType</th>
            <td colspan="3" rowspan="1">
              Base class for data types which result in a <em>String</em>.
            </td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <th colspan="1" rowspan="1">Literal</th>
            <td colspan="2" rowspan="1">
              A subclass of <em>StringType</em> for literals which
              exceed the constraints of an <em>NCName</em>.
            </td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <th colspan="1" rowspan="1">MimeType</th>
            <td colspan="2" rowspan="1">
              A subclass of <em>StringType</em> for literals which
              represent a mime type.
            </td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <th colspan="1" rowspan="1">UriType</th>
            <td colspan="2" rowspan="1">
              A subclass of <em>StringType</em> for literals which
              represent a URI, as specified by the argument to
              <em>url()</em>.
            </td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <th colspan="1" rowspan="1">NCName</th>
            <td colspan="2" rowspan="1">
              A subclass of <em>StringType</em> for literals which
              meet the constraints of an <em>NCName</em>.
            </td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <td colspan="1" rowspan="1"></td>
            <th colspan="1" rowspan="1">Country</th>
            <td colspan="1" rowspan="1">An RFC 3066/ISO 3166 country code.</td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <td colspan="1" rowspan="1"></td>
            <th colspan="1" rowspan="1">Language</th>
            <td colspan="1" rowspan="1">An RFC 3066/ISO 639 language code.</td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <td colspan="1" rowspan="1"></td>
            <th colspan="1" rowspan="1">Script</th>
            <td colspan="1" rowspan="1">An ISO 15924 script code.</td>
          </tr>
          
          <tr>
            <th colspan="2" rowspan="1">Enumerated types</th>
          </tr>
          
          <tr>
            <th colspan="1" rowspan="1">EnumType</th>
            <td colspan="3" rowspan="1">
              An integer representing one of the tokens in a set of
              enumeration values.
            </td>
          </tr>
          
          <tr>
            <td colspan="1" rowspan="1"></td>
            <th colspan="1" rowspan="1">MappedNumeric</th>
            <td colspan="2" rowspan="1">
              A subclass of <em>EnumType</em>.  Maintains a
              <em>Numeric</em> with the value to which the associated
              "raw" enumeration token maps.  E.g., the
              <em>font-size</em> enumeration value "medium" maps to
              the <em>Numeric</em> "12pt".
            </td>
          </tr>
          
          <tr>
            <th colspan="2" rowspan="1">Colors</th>
          </tr>
          
          <tr>
            <th colspan="1" rowspan="1">ColorType</th>
            <td colspan="3" rowspan="1">
              Maintains a four-element array of float, derived from
              the name of a standard colour, the name returned by a
              call to <em>system-color()</em>, or an RGB
              specification.
            </td>
          </tr>
          
          <tr>
            <th colspan="2" rowspan="1">Fonts</th>
          </tr>
          
          <tr>
            <th colspan="1" rowspan="1">FontFamilySet</th>
            <td colspan="3" rowspan="1">
              Maintains an array of <em>String</em>s containing a
              prioritized list of possibly generic font family names.
            </td>
          </tr>
          
          <tr>
            <th colspan="2" rowspan="1">Pseudo-types</th>
          </tr>
          
          <tr>
            <td colspan="4" rowspan="1">
              A variety of pseudo-types have been defined as
              convenience types for frequently appearing enumeration
              token values, or for other special purposes.
            </td>
          </tr>
          
          <tr>
            <th colspan="1" rowspan="1">Inherit</th>
            <td colspan="3" rowspan="1">
              For values of <em>inherit</em>.
            </td>
          </tr>
          
          <tr>
            <th colspan="1" rowspan="1">Auto</th>
            <td colspan="3" rowspan="1">
              For values of <em>auto</em>.
            </td>
          </tr>
          
          <tr>
            <th colspan="1" rowspan="1">None</th>
            <td colspan="3" rowspan="1">
              For values of <em>none</em>.
            </td>
          </tr>
          
          <tr>
            <th colspan="1" rowspan="1">Bool</th>
            <td colspan="3" rowspan="1">
              For values of <em>true/false</em>.
            </td>
          </tr>
          
          <tr>
            <th colspan="1" rowspan="1">FromNearestSpecified</th>
            <td colspan="3" rowspan="1">
              Created to ensure that, when associated with
              a shorthand, the <em>from-nearest-specified-value()</em>
              core function is the sole component of the expression.
            </td>
          </tr>
          
          <tr>
            <th colspan="1" rowspan="1">FromParent</th>
            <td colspan="3" rowspan="1">
              Created to ensure that, when associated with
              a shorthand, the <em>from-parent()</em>
              core function is the sole component of the expression.
            </td>
          </tr>
          
      </table>
      <a name="N10252"></a>
      <h4>Tokenizer</h4>
      <p>
        As mentioned above, the <span class= "codefrag"
        >PropertyTokenizer</span> hands <a href=
        "javascript:window.top.displayCode( 'PropertyTokenizer.html#EOF'
        )" ><em>tokens</em></a> back to its subclass, <span class=
        "codefrag" >PropertyParser</span>.  Most of these tokens are
        self-explanatory, but a few need further comment.
      </p>
      <dl>
        
        <dt>AUTO</dt>
        
        <dd>
          Because of its frequency of occurrence, and the fact that it
          is always the <em>initial value</em> for any property which
          supports it, AUTO has been promoted into a pseudo-type with
          its on datatype class.  Therefore, it is also reported as a
          token.
        </dd>
        
        <dt>NONE</dt>
        
        <dd>
          Similarly to AUTO, NONE has been promoted to a pseudo-type
          because of its frequency.
        </dd>
        
        <dt>BOOL</dt>
        
        <dd>
          There is a <em>de facto</em> boolean type buried in the
          enumeration types for many of the properties.  It had been
          specified as a type in its own right in this code.
        </dd>
        
        <dt>MIMETYPE</dt>
        
        <dd>
          The property <span class= "codefrag" >content-type</span>
          introduces this complication.  It can have two values of the
          form <strong>content-type:</strong><em>mime-type</em>
          (e.g. <span class= "codefrag"
          >content-type="content-type:xml/svg"</span>) or
          <strong>namespace-prefix:</strong><em>prefix</em>
          (e.g. <span class= "codefrag"
          >content-type="namespace-prefix:svg"</span>).  The
          experimental code reduces these options to the payload in
          each case: an <span class= "codefrag" >NCName</span> in the
          case of a namespace prefix, and a MIMETYPE in the case of a
          content-type specification.  <span class= "codefrag"
          >NCName</span>s cannot contain a "/".
        </dd>
        
      </dl>
      <a name="N1029C"></a>
      <h4>Parser</h4>
      <p>
        The parser returns a <a href= "javascript:window.top.displayCode(
        'PropertyValueList.html#PropertyValueListClass' )" ><span
        class= "codefrag" >PropertyValueList</span ></a>, necessary
        because of the possibility that a list of <a href=
        "javascript:window.top.displayCode(
        'PropertyValue.html#PropertyValueInterface' )" ><span class=
        "codefrag" >PropertyValue</span ></a> elements may be returned
        from the expressions of some properties.
      </p>
      <p>
        
        <span class= "codefrag" >PropertyValueList</span>s may contain
        <span class= "codefrag" >PropertyValue</span>s or other <span
        class= "codefrag" >PropertyValueList</span>s.  This latter
        provision is necessitated by some of the more peculiar
        expression possibilities, <em>e.g.</em> <em>font</em> and
        <em>text-shadow</em>.  <em>text-shadow</em> may contain whitespace
        separated sublists of either two or three elements, separated
        from one another by commas.  To accommodate this peculiarity,
        comma separated elements are added to the top-level list,
        while whitespace separated values are always collected into
        sublists to be added to the top-level list.
      </p>
      <p>
        Other special cases include the processing of the core
        functions <span class= "codefrag" >from-parent()</span> and
        <span class= "codefrag" >from-nearest-specified-value()</span>
        when these function calls are assigned to a shorthand
        property, or used with a shorthand property name as an
        argument.  In these cases, the function call must be the sole
        component of the expression.  The pseudo-element classes <span
        class= "codefrag" >FromParent</span> and <span
        class= "codefrag" >FromNearestSpecified</span> are generated in
        these circumstances so that an exception will be thrown if
        they are involved in expression evaluation with other
        components. (See Rec. Section 5.10.4 Property Value
        Functions.)
      </p>
      <p>
        The experimental code is a simple extension of the existing
        parser code, which itself borrowed heavily from James
        Clark's XT processor.
      </p>
      
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