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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Property Expression Parsing</title>
<link type="text/css" href="../../../page.css" rel="stylesheet">
</head>
<body marginheight="0" marginwidth="0" topmargin="0" leftmargin="0" text="#000000" bgcolor="#FFFFFF">
<div class="content">
<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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