15.2 Resource Manager Matching Rules

The algorithm for determining which resource database entry matches a given query is the heart of the resource manager. All queries must fully specify the name and class of the desired resource (use of the characters ``*'' and ``?'' are not permitted). The library supports up to 100 components in a full name or class. Resources are stored in the database with only partially specified names and classes, using pattern matching constructs. An asterisk (*) is a loose binding and is used to represent any number of intervening components, including none. A period (.) is a tight binding and is used to separate immediately adjacent components. A question mark (?) is used to match any single component name or class. A database entry cannot end in a loose binding; the final component (which cannot be the character ``?'') must be specified. The lookup algorithm searches the database for the entry that most closely matches (is most specific for) the full name and class being queried. When more than one database entry matches the full name and class, precedence rules are used to select just one.

The full name and class are scanned from left to right (from highest level in the hierarchy to lowest), one component at a time. At each level, the corresponding component and/or binding of each matching entry is determined, and these matching components and bindings are compared according to precedence rules. Each of the rules is applied at each level before moving to the next level, until a rule selects a single entry over all others. The rules, in order of precedence, are:

  1. An entry that contains a matching component (whether name, class, or the character ``?'') takes precedence over entries that elide the level (that is, entries that match the level in a loose binding).

  2. An entry with a matching name takes precedence over both entries with a matching class and entries that match using the character ``?''. An entry with a matching class takes precedence over entries that match using the character ``?''.

  3. An entry preceded by a tight binding takes precedence over entries preceded by a loose binding.

To illustrate these rules, consider the following resource database entries:

xmh*Paned*activeForeground:		red	(entry A)
*incorporate.Foreground:		blue	(entry B)
xmh.toc*Command*activeForeground:	green	(entry C)
xmh.toc*?.Foreground:			white	(entry D)
xmh.toc*Command.activeForeground:	black	(entry E)

Consider a query for the resource:

xmh.toc.messagefunctions.incorporate.activeForeground	(name)
Xmh.Paned.Box.Command.Foreground			(class)

At the first level (xmh, Xmh), rule 1 eliminates entry B. At the second level (toc, Paned), rule 2 eliminates entry A. At the third level (messagefunctions, Box), no entries are eliminated. At the fourth level (incorporate, Command), rule 2 eliminates entry D. At the fifth level (activeForeground, Foreground), rule 3 eliminates entry C.

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Christophe Tronche, ch@tronche.com