Chapter 15: Resource Manager Functions
A program often needs a variety of options in the X environment
(for example, fonts, colors, icons, and cursors).
Specifying all of these options on the command line is awkward
because users may want to customize many aspects of the program
and need a convenient way to establish these customizations as
the default settings.
The resource manager is provided for this purpose.
Resource specifications are usually stored in human-readable files
and in server properties.
The resource manager is a database manager with a twist.
In most database systems,
you perform a query using an imprecise specification,
and you get back a set of records.
The resource manager, however, allows you to specify a large
set of values with an imprecise specification, to query the database
with a precise specification, and to get back only a single value.
This should be used by applications that need to know what the
user prefers for colors, fonts, and other resources.
It is this use as a database for dealing with X resources that
inspired the name ``Resource Manager,''
although the resource manager can be and is used in other ways.
For example,
a user of your application may want to specify
that all windows should have a blue background
but that all mail-reading windows should have a red background.
With well-engineered and coordinated applications,
a user can define this information using only two lines of specifications.
As an example of how the resource manager works,
consider a mail-reading application called xmh.
Assume that it is designed so that it uses a
complex window hierarchy all the way down to individual command buttons,
which may be actual small subwindows in some toolkits.
These are often called objects or widgets.
In such toolkit systems,
each user interface object can be composed of other objects
and can be assigned a name and a class.
Fully qualified names or classes can have arbitrary numbers of component names,
but a fully qualified name always has the same number of component names as a
fully qualified class.
This generally reflects the structure of the application as composed
of these objects, starting with the application itself.
For example, the xmh mail program has a name ``xmh'' and is one
of a class of ``Mail'' programs.
By convention, the first character of class components is capitalized,
and the first letter of name components is in lowercase.
Each name and class finally has an attribute
(for example ``foreground'' or ``font'').
If each window is properly assigned a name and class,
it is easy for the user to specify attributes of any portion
of the application.
At the top level,
the application might consist of a paned window (that is, a window divided
into several sections) named ``toc''.
One pane of the paned window is a button box window named ``buttons''
and is filled with command buttons.
One of these command buttons is used to incorporate
new mail and has the name ``incorporate''.
This window has a fully qualified name, ``xmh.toc.buttons.incorporate'',
and a fully qualified class, ``Xmh.Paned.Box.Command''.
Its fully qualified name is the name of its parent, ``xmh.toc.buttons'',
followed by its name, ``incorporate''.
Its class is the class of its parent, ``Xmh.Paned.Box'',
followed by its particular class, ``Command''.
The fully qualified name of a resource is
the attribute's name appended to the object's fully qualified
name, and the fully qualified class is its class appended to the object's
class.
The incorporate button might need the following resources:
Title string,
Font,
Foreground color for its inactive state,
Background color for its inactive state,
Foreground color for its active state, and
Background color for its active state.
Each resource is considered
to be an attribute of the button and, as such, has a name and a class.
For example, the foreground color for the button in
its active state might be named ``activeForeground'',
and its class might be ``Foreground''.
When an application looks up a resource (for example, a color),
it passes the complete name and complete class of the resource
to a look-up routine.
The resource manager compares this complete specification
against the incomplete specifications of entries in the resource
database, finds the best match, and returns the corresponding
value for that entry.
The definitions for the resource manager are contained in
X11/Xresource.h.
Christophe Tronche, [email protected]
