The property datatypes are defined in the org.apache.fop.datatypes package, except Number and String which are java primitives. The FOP datatypes are:
The org.apache.fop.fo.Property class is the superclass for all
Property subclasses. There is a subclass for each kind of property
datatype. These are named using the datatype name plus the word
Property, resulting in NumberProperty, StringProperty, and so
on. There is also a class EnumProperty which uses an int
primitive to hold enumerated values. There is no corresponding Enum
datatype class.
The Property class provides a "wrapper" around any possible property value. Code manipulating property values (in layout for example) usually knows what kind (or kinds) of datatypes are acceptable for a given property and will use the appropriate accessor.
The base Property class defines accessor methods for all FO property datatypes, such as getNumber(), getColorType(), getSpace(), getEnum(), etc. It doesn't define accessors for SVG types, since these are handled separately (at least for now.) In the base Property class, all of these methods return null, except getEnum which returns 0. Individual subclasses return a value of the appropriate type, such as Length or ColorType. A subclass may also choose to return a reasonable value for other accessor types. For example, a SpaceProperty will return the optimum value if asked for a Length.
The Property class contains a nested class called Maker. This is the base class for all other property Makers. It provides basic framework functionality which is overridden by the code generated by properties.xsl from the *properties.xml files. In particular it provides basic expression evaluation, using PropertyParser class in the org.apache.fop.fo.expr package.
Other Property subclasses such as LengthProperty define their own nested Maker classes (subclasses of Property.Maker). These handle conversion from the Property subclass returned from expression evaluation into the appropriate subclass for the property.
For each generic or specific property definition in the properties.xml files, a new subclass of one of the Maker classes is created. Note that no new Property subclasses are created, only new PropertyMaker subclasses. Once the property value has been parsed and stored, it has no specific functionality. Only the Maker code is specific. Maker subclasses define such aspects as keyword substitutions, whether the property can be inherited or not, which enumerated values are legal, default values, corresponding properties and specific datatype conversions.
In the properties xml files, one can define generic property definitions which can serve as a basis for individual property definitions. There are currently several generic properties defined in foproperties.xml. An example is GenericColor, which defines basic properties for all ColorType properties. Since the generic specification doesn't include the inherited or default elements, these should be set in each property which is based on GenericColor. Here is an example:
<property type='generic'>
<name>background-color</name>
<use-generic>GenericColor</use-generic>
<inherited>false</inherited>
<default>transparent</default>
</property>
A generic property specification can include all of the elements defined for the property element in the DTD, including the description of components for compound properties, and the specification of keyword shorthands.
Generic property specifications can be based on other generic specifications. An example is GenericCondPadding template which is based on the GenericCondLength definition but which extends it by adding an inherited element and a default value for the length component.
Generic properties can specify enumerated values, as in the GenericBorderStyle template. This means that the list of values, which is used by 8 properties (the "absolute" and "writing-mode-relative" variants for each BorderStyle property) is only specified one time.
When a property includes a "use-generic" element and includes no other elements (except the "name" element), then no class is generated for the property. Instead the generated mapping will associate this property directly with an instance of the generic Maker.
A generic class may also be hand-coded, rather than generated from the
properties file.
Properties based on such a generic class are indicated by the
attribute ispropclass='true' on the
use-generic element.
This is illustrated by the SVG properties, most of which use one of the Property subclasses defined in the org.apache.fop.svg package. Although all of these properties are now declared in svgproperties.xml, no specific classes are generated. Classes are only generated for those SVG properties which are not based on generic classes defined in svg.
Properties may be defined for all flow objects or only for
particular flow objects. A PropertyListBuilder object will always look
first for a Property.Maker for the flow object before looking in the
general list. These are specified in the
element-property-list section of the properties.xml
files. The localname element children of this element specify for
which flow-object elements the property should be registered.
NOTE: All the properties for an object or set of objects must be specified in a single element-property-list element. If the same localname appears in several element lists, the later set of properties will hide the earlier ones! Use the ref functionality if the same property is to be used in different sets of element-specific mappings.
A property element may have a type attribute with the value
ref. The
content of the name child element is the name of the referenced
property (not its class-name!). This indicates that the property
specification has
already been given, either in this same specification file or in a
different one (indicated by the family attribute). The
value of the family attribute is XX where the file
XXproperties.xml defines the referenced property. For
example, some SVG objects may have properties defined for FO. Rather
than defining them again with a new name, the SVG properties simply
reference the defined FO properties. The generating mapping for the
SVG properties will use the FO Maker classes.
Some properties have both absolute and writing-mode-relative forms. In general, the absolute forms are equivalent to CSS properties, and the writing-mode-relative forms are based on DSSSL. FO files may use either or both forms. In FOP code, a request for an absolute form will retrieve that value if it was specified on the FO; otherwise the corresponding relative property will be used if it was specified. However, a request for a relative form will only use the specified relative value if the corresponding absolute value was not specified for that FO.
Corresponding properties are specified in the properties.xml files
using the element corresponding, which has at least one
propval child and may have a propexpr child,
if the corresponding
value is calculated based on several other properties, as for
start-indent.
NOTE: most current FOP code accesses the absolute variants of these properties, notably for padding, border, height and width attributes. However it does use start-indent and end-indent, rather than the "absolute" margin properties.
The XSL script propmap.xsl is used to generate
property mappings based on
both foproperties.xml and svgproperties.xml. The mapping classes
in the main fop packages simply load these automatically generated
mappings. The mapping code still uses the static
"maker" function of the generated object to obtain a Maker
object. However, for all generated classes, this method returns an
instance of the class itself (which is a subclass of Property.Maker)
and not an instance of a separate nested Maker class.
For most SVG properties which use the SVG Property classes directly, the generated mapper code calls the "maker" method of the SVG Property class, which returns an instance of its nested Maker class.
The property generation also handles element-specific property mappings as specified in the properties XML files.
For any property whose datatype is Enum or which
contains possible enumerated values, FOP code may need to access
enumeration constants. These are defined in the interfaces whose name
is the same as the generated class name for the property,
for example BorderBeforeStyle.NONE. These interface classes
are generated by the XSL script enumgen.xsl. A separate
interface defining the enumeration constants is always generated for
every property which uses the constants, even if the constants
themselves are defined in a generic class, as in BorderStyle.
If a subproperty or component of a compound property has enumerated
values, the constants are defined in a nested interface whose name is
the name of the subproperty (using appropriate capitalization
rules). For example,
the keep properties may have values of AUTO or FORCE or an integer
value. These are defined for each kind of keep property. For example,
the keep-together property is a compound property with the components
within-line, within-column and within-page. Since each component may
have the values AUTO or FORCE, the KeepTogether interface defines
three nested interfaces, one for each component, and each defines
these two constants. An example of a reference in code to the constant
is KeepTogether.WithinPage.AUTO.
Some XSL FO properties are specified by compound datatypes. In the FO file,
these are defined by a group of attributes, each having a name of the
form property.component, for example
space-before.minimum. These are several compound
datatypes:
These are described in the properties.xml files using the element
compound which has subproperty children. A subproperty element is much
like a property element, although it may not have an inherited child
element, as only a complete property object may be inherited.
Specific datatype classes exist for each compound property. Each
component of a compound datatype is itself stored as a Property
object. Individual components may be accessed either by directly
performing a get operation on the name, using the "dot" notation,
eg. get("space-before.optimum"); or by using an accessor on the compound
property, eg. get("space-before").getOptimum().
In either case,
the result is a Property object, and the actual value may be accessed
(in this example) by using the "getLength()" accessor.