There is a family of Maker objects for each of the property datatypes,
such as Length, Number, Enumerated, Space, etc. But since each Property
has specific aspects such as whether it's inherited, its default value,
its corresponding properties, etc. there is usually a specific Maker for
each Property. All these Maker classes are created during the code
generation phase by processing (using XSLT) the XML property description
files to create Java classes.
The Maker first checks for "keyword" values for a property. These are
things like "thin, medium, thick" for the border-width property. The
datatype is really a Length but it can be specified using these keywords
whose actual value is determined by the "User Agent" rather than being
specified in the XSL standard. For FOP, these values are currently
still needs to be parsed as described next.
The Maker also checks to see if the property is an Enumerated type and
then checks whether the value matches one of the specified enumeration
Otherwise the Maker uses the property parser in the fo.expr package to
evaluate the attribute value and return a Property object. The parser
interprets the expression language and performs numeric operations and
function call evaluations.
If the returned Property value is of the correct type (specificed in
be able to convert the returned type into the correct type.
Some kinds of property values can't be fully resolved during FO tree
building because they depend on layout information. This is the case of
length values specified as percentages and of the special
proportional-column-width(x) specification for table-column widths.
These are stored as special kinds of Length objects which are evaluated
during layout. Expressions involving "em" units which are relative to
font-size _are_ resolved during the FO tree building however.