Also, there must be a mechanism for creating an appropriate view given a subject; that is, there must be an association between a subject class and allowable view classes for that subject. This association is established by defining class identifiers for component views. These identifiers are formed by concatenating the identifier for a given component subject class with an identifier that specifies a ‘‘view category.’’ For example, the class identifier for the PostScript external view of a line component is the concatenation of the identifier for the line component subject class and POSTSCRIPT_VIEW identifier, which specifies the category of external views for generating PostScript. This makes it possible to create an instance of the appropriate PostScript external view subclass given a line component subject.
The class identifiers for the Unidraw base and predefined derived classes are specified in the classes.h include file. Applications that define new catalog-managed objects or component views should define unique class identifiers in a similar manner. Class identifiers for catalog-managed objects are simply integers with mnemonic macro definitions, usually the class name in all-uppercase. For example, the identifier for the LineComp component subject class is
#define LINE_COMP 9030
View class identifiers are specified using the Combine macro, which takes as its first argument the macro definition of the corresponding component subject identifier; its second argument is an identifier for the view category. The Combine macro itself is then defined with a mnemonic name. For example, the identifier for the PostScript external view class PSLine (which generates PostScript for the line component subject) is defined as
#define PS_LINE Combine(LINE_COMP, POSTSCRIPT_VIEW)
where the POSTSCRIPT_VIEW view category identifier is defined as
#define POSTSCRIPT_VIEW 9103
N.B.: All class identifiers in a given application must be unique.
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