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<?xml version="1.0" encoding="ISO-8859-1"?>
<!-- $Id$ -->
<!--
<!DOCTYPE document SYSTEM "../xml-docs/dtd/document-v10.dtd">
-->
<document>
<header>
<title>Property Expression Parsing</title>
<authors>
<person id="pbw" name="Peter B. West" email="pbwest@powerup.com.au"/>
</authors>
</header>
<body>
<!-- one of (anchor s1) -->
<s1 title="Property expression parsing">
<note>
The following discussion of the experiments with alternate
property expression parsing is very much a work in progress,
and subject to sudden changes.
</note>
<p>
The parsing of property value expressions is handled by two
closely related classes: <code>PropertyTokenizer</code> and its
subclass, <code>PropertyParser</code>.
<code>PropertyTokenizer</code>, as the name suggests, handles
the tokenizing of the expression, handing <em>tokens</em>
back to its subclass,
<code>PropertyParser</code>. <code>PropertyParser</code>, in
turn, returns a <code>PropertyValueList</code>, a list of
<code>PropertyValue</code>s.
</p>
<p>
The tokenizer and parser rely in turn on the datatype
definition from the <code>org.apache.fop.datatypes</code>
package and the datatype <code>static final int</code>
constants from <code>PropertyConsts</code>.
</p>
<s2 title="Data types">
<p>
The data types currently defined in
<code>org.apache.fop.datatypes</code> include:
</p>
<table>
<tr><th colspan="2">Numbers and lengths</th></tr>
<tr>
<th>Numeric</th>
<td colspan="3">
The fundamental numeric data type. <em>Numerics</em> of
various types are constructed by the classes listed
below.
</td>
</tr>
<tr>
<td/>
<th colspan="3">Constructor classes for <em>Numeric</em></th>
</tr>
<tr>
<td/><td>Angle</td>
<td colspan="2">In degrees(deg), gradients(grad) or
radians(rad)</td>
</tr>
<tr>
<td/><td>Ems</td>
<td colspan="2">Relative length in <em>ems</em></td>
</tr>
<tr>
<td/><td>Frequency</td>
<td colspan="2">In hertz(Hz) or kilohertz(kHz)</td>
</tr>
<tr>
<td/><td>IntegerType</td><td/>
</tr>
<tr>
<td/><td>Length</td>
<td colspan="2">In centimetres(cm), millimetres(mm),
inches(in), points(pt), picas(pc) or pixels(px)</td>
</tr>
<tr>
<td/><td>Percentage</td><td/>
</tr>
<tr>
<td/><td>Time</td>
<td>In seconds(s) or milliseconds(ms)</td>
</tr>
<tr><th colspan="2">Strings</th></tr>
<tr>
<th>StringType</th>
<td colspan="3">
Base class for data types which result in a <em>String</em>.
</td>
</tr>
<tr>
<td/><th>Literal</th>
<td colspan="2">
A subclass of <em>StringType</em> for literals which
exceed the constraints of an <em>NCName</em>.
</td>
</tr>
<tr>
<td/><th>MimeType</th>
<td colspan="2">
A subclass of <em>StringType</em> for literals which
represent a mime type.
</td>
</tr>
<tr>
<td/><th>UriType</th>
<td colspan="2">
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/><th>NCName</th>
<td colspan="2">
A subclass of <em>StringType</em> for literals which
meet the constraints of an <em>NCName</em>.
</td>
</tr>
<tr>
<td/><td/><th>Country</th>
<td>An RFC 3066/ISO 3166 country code.</td>
</tr>
<tr>
<td/><td/><th>Language</th>
<td>An RFC 3066/ISO 639 language code.</td>
</tr>
<tr>
<td/><td/><th>Script</th>
<td>An ISO 15924 script code.</td>
</tr>
<tr><th colspan="2">Enumerated types</th></tr>
<tr>
<th>EnumType</th>
<td colspan="3">
An integer representing one of the tokens in a set of
enumeration values.
</td>
</tr>
<tr>
<td/><th>MappedEnumType</th>
<td colspan="2">
A subclass of <em>EnumType</em>. Maintains a
<em>String</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>String</em> "12pt".
</td>
</tr>
<tr><th colspan="2">Colors</th></tr>
<tr>
<th>ColorType</th>
<td colspan="3">
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">Fonts</th></tr>
<tr>
<th>FontFamilySet</th>
<td colspan="3">
Maintains an array of <em>String</em>s containing a
prioritized list of possibly generic font family names.
</td>
</tr>
<tr><th colspan="2">Pseudo-types</th></tr>
<tr>
<td colspan="4">
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>Inherit</th>
<td colspan="3">
For values of <em>inherit</em>.
</td>
</tr>
<tr>
<th>Auto</th>
<td colspan="3">
For values of <em>auto</em>.
</td>
</tr>
<tr>
<th>None</th>
<td colspan="3">
For values of <em>none</em>.
</td>
</tr>
<tr>
<th>Bool</th>
<td colspan="3">
For values of <em>true/false</em>.
</td>
</tr>
<tr>
<th>FromNearestSpecified</th>
<td colspan="3">
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>FromParent</th>
<td colspan="3">
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>
</s2>
<s2 title="Tokenizer">
<p>
The tokenizer returns one of the following token
values:
</p>
<source>
static final int
EOF = 0
,NCNAME = 1
,MULTIPLY = 2
,LPAR = 3
,RPAR = 4
,LITERAL = 5
,FUNCTION_LPAR = 6
,PLUS = 7
,MINUS = 8
,MOD = 9
,DIV = 10
,COMMA = 11
,PERCENT = 12
,COLORSPEC = 13
,FLOAT = 14
,INTEGER = 15
,ABSOLUTE_LENGTH = 16
,RELATIVE_LENGTH = 17
,TIME = 18
,FREQ = 19
,ANGLE = 20
,INHERIT = 21
,AUTO = 22
,NONE = 23
,BOOL = 24
,URI = 25
,MIMETYPE = 26
// NO_UNIT is a transient token for internal use only. It is
// never set as the end result of parsing a token.
,NO_UNIT = 27
;
</source>
<p>
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 <code>content-type</code> introduces this
complication. It can have two values of the form
<strong>content-type:</strong><em>mime-type</em>
(e.g. <code>content-type="content-type:xml/svg"</code>) or
<strong>namespace-prefix:</strong><em>prefix</em>
(e.g. <code>content-type="namespace-prefix:svg"</code>). The
experimental code reduces these options to the payload
in each case: an <code>NCName</code> in the case of a
namespace prefix, and a MIMETYPE in the case of a
content-type specification. <code>NCName</code>s cannot
contain a "/".
</dd>
</dl>
</s2>
<s2 title="Parser">
<p>
The parser retuns a <code>PropertyValueList</code>,
necessary because of the possibility that a list of
<code>PropertyValue</code> elements may be returned from the
expressions of soem properties.
</p>
<p>
<code>PropertyValueList</code>s may contain
<code>PropertyValue</code>s or other
<code>PropertyValueList</code>s. This latter provision is
necessitated for the peculiar case of of
<em>text-shadow</em>, which 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 <code>from-parent()</code> and
<code>from-nearest-specified-value()</code> 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
<code>FromParent</code> and
<code>FromNearestSpecified</code> 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>
</s2>
</s1>
</body>
</document>
|
