Property Expression Parsing

- Property expression parsing - - The following discussion of the experiments with alternate - property expression parsing is very much a work in progress, - and subject to sudden changes. - -

- The parsing of property value expressions is handled by two - closely related classes: PropertyTokenizer and its - subclass, PropertyParser. - PropertyTokenizer, as the name suggests, handles - the tokenizing of the expression, handing tokens - back to its subclass, - PropertyParser. PropertyParser, in - turn, returns a PropertyValueList, a list of - PropertyValues. -

- The tokenizer and parser rely in turn on the datatype - definition from the org.apache.fop.datatypes - package and the datatype static final int - constants from PropertyConsts. -

- Data types -

- The data types currently defined in - org.apache.fop.datatypes include: -

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Numbers and lengths
Numeric	- The fundamental numeric data type. Numerics of - various types are constructed by the classes listed - below. -
-	Constructor classes for Numeric
	Angle	In degrees(deg), gradients(grad) or - radians(rad)
	Ems	Relative length in ems
	Frequency	In hertz(Hz) or kilohertz(kHz)
	IntegerType	-
	Length	In centimetres(cm), millimetres(mm), - inches(in), points(pt), picas(pc) or pixels(px)
	Percentage	-
	Time	In seconds(s) or milliseconds(ms)
Strings
StringType	- Base class for data types which result in a String. -
	Literal	- A subclass of StringType for literals which - exceed the constraints of an NCName. -
	MimeType	- A subclass of StringType for literals which - represent a mime type. -
	UriType	- A subclass of StringType for literals which - represent a URI, as specified by the argument to - url(). -
	NCName	- A subclass of StringType for literals which - meet the constraints of an NCName. -
		Country	An RFC 3066/ISO 3166 country code.
		Language	An RFC 3066/ISO 639 language code.
		Script	An ISO 15924 script code.
Enumerated types
EnumType	- An integer representing one of the tokens in a set of - enumeration values. -
	MappedEnumType	- A subclass of EnumType. Maintains a - String with the value to which the associated - "raw" enumeration token maps. E.g., the - font-size enumeration value "medium" maps to - the String "12pt". -
Colors
ColorType	- Maintains a four-element array of float, derived from - the name of a standard colour, the name returned by a - call to system-color(), or an RGB - specification. -
Fonts
FontFamilySet	- Maintains an array of Strings containing a - prioritized list of possibly generic font family names. -
Pseudo-types
- A variety of pseudo-types have been defined as - convenience types for frequently appearing enumeration - token values, or for other special purposes. -
Inherit	- For values of inherit. -
Auto	- For values of auto. -
None	- For values of none. -
Bool	- For values of true/false. -
FromNearestSpecified	- Created to ensure that, when associated with - a shorthand, the from-nearest-specified-value() - core function is the sole component of the expression. -
FromParent	- Created to ensure that, when associated with - a shorthand, the from-parent() - core function is the sole component of the expression. -

- Tokenizer -

- The tokenizer returns one of the following token - values: -

- - 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 - ; - -

- Most of these tokens are self-explanatory, but a few need - further comment. -

AUTO: - Because of its frequency of occurrence, and the fact that - it is always the initial value 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. -
NONE: - Similarly to AUTO, NONE has been promoted to a pseudo-type - because of its frequency. -
BOOL: - There is a de facto 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. -
MIMETYPE: - The property content-type introduces this - complication. It can have two values of the form - content-type:mime-type - (e.g. content-type="content-type:xml/svg") or - namespace-prefix:prefix - (e.g. content-type="namespace-prefix:svg"). The - experimental code reduces these options to the payload - in each case: an NCName in the case of a - namespace prefix, and a MIMETYPE in the case of a - content-type specification. NCNames cannot - contain a "/". -

- Parser -

- The parser retuns a PropertyValueList, - necessary because of the possibility that a list of - PropertyValue elements may be returned from the - expressions of soem properties. -

- PropertyValueLists may contain - PropertyValues or other - PropertyValueLists. This latter provision is - necessitated for the peculiar case of of - text-shadow, 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. -

- Other special cases include the processing of the core - functions from-parent() and - from-nearest-specified-value() 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 - FromParent and - FromNearestSpecified 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.) -

- The experimental code is a simple extension of the existing - parser code, which itself borrowed heavily from James - Clark's XT processor. -