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- <h1>Property Expression Parsing</h1>
- <p>
- <font size="-2">by 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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