48.4 How a Specifier Is Instanced

Instancing of a specifier in a particular window domain proceeds as follows:

• First, SXEmacs searches for a specification whose locale is the same as the window. If that fails, the search is repeated, looking for a locale that is the same as the window’s buffer. If that fails, the search is repeated using the window’s frame, then using the device that frame is on. Finally, the specification whose locale is the symbol global (if there is such a specification) is considered.
• The inst-pairs contained in the specification that was found are considered in their order in the inst-list, looking for one whose tag set matches the device that is derived from the window domain. (The tag set is an unordered list of zero or more tag symbols. For all tags that have predicates associated with them, the predicate must match the device.)
• If a matching tag set is found, the corresponding instantiator is passed to the specifier’s instantiation method, which is specific to the type of the specifier. If it succeeds, the resulting instance object is returned as the result of the instancing and the instancing is done. Otherwise, the operation continues, looking for another matching inst-pair in the current specification.
• When there are no more inst-pairs to be considered in the current specification, the search starts over, looking for another specification as in the first step above.
• If all specifications are exhausted and no instance object can be derived, the instancing fails. (Actually, this is not completely true. Some specifier objects for built-in properties have a fallback value, which is either an inst-list or another specifier object, that is consulted if the instancing is about to fail. If it is an inst-list, the searching proceeds using the inst-pairs in that list. If it is a specifier, the entire instancing starts over using that specifier instead of the given one. Fallback values are set by the C code and cannot be modified, except perhaps indirectly, using any Lisp functions. The purpose of them is to supply some values to make sure that instancing of built-in properties can’t fail and to implement some basic specifier inheritance, such as the fact that faces inherit their properties from the default face.)

It is also possible to instance a specifier over a frame domain or device domain instead of over a window domain. The C code, for example, instances the top-toolbar-height variable over a frame domain in order to determine the height of a frame’s top toolbar. Instancing over a frame or device is similar to instancing over a window except that specifications for locales that cannot be derived from the domain are ignored. Specifically, instancing over a frame looks first for frame locales, then device locales, then the global locale. Instancing over a device domain looks only for device locales and the global locale.