Tk::option - Using the option database in Perl/Tk
The option database (also known as the resource database or the application defaults database) is a set of rules for applying default options to widgets. Users and system administrators can set up these rules to customize the appearance of applications without changing any application code; for example, a user might set up personal foreground and background colors, or a site might use fonts associated with visual or language preferences. Different window managers (and implementations of them) have implemented the database differently, but most Xt-based window managers use the .Xdefaults file or the xrdb utility to manage user preferences; some use both, and/or implement a more complex set of site, user and application databases. Check your site documentation for these topics or your window manager's RESOURCE_MANAGER property.
For most applications, the option database "just works." The option... methods are for applications that need to do something unusual, such as add new rules or test an option's default. Even in such cases, the application should provide for user preferences. Do not hardcode widget options without a very good reason. All users have their own tastes and they are all different. They choose a special font in a special size and have often spend a lot of time working out a color scheme that they will love until death. When you respect their choices they will enjoy working with your applications much more. Don't destroy the common look and feel of a personal desktop.
All widgets in an application are identified hierarchically by pathname, starting from the MainWindow and passing through each widget used to create the endpoint. The path elements are widget names, much like the elements of a file path from the root directory to a file. The rules in the option database are patterns that are matched against a widget's pathname to determine which defaults apply. When a widget is created, the Name option can be used to assign the widget's name and thus create a distinctive path for widgets in an application. If the Name option isn't given, Perl/Tk assigns a default name based on the type of widget; a MainWindow's default name is the appname. These defaults are fine for most widgets, so don't feel you need to find a meaningful name for every widget you create. A widget must have a distinctive name to allow users to tailor its options independently of other widgets in an application. For instance, to create a Text widget that will have special options assigned to it, give it a name such as:
$text = $mw->Text(Name => 'importantText');
You can then tailor the widget's attributes with a rule in the option database such as:
The class attribute identifies groups of widgets, usually within an application but also to group similar widgets among different applications. One typically assigns a class to a TopLevel or Frame so that the class will apply to all of that widget's children. To extend the example, we could be more specific about the importantText widget by giving its frame a class:
$frame = $mw->Frame(-class => 'Urgent'); $text = $frame->Text(Name => 'importantText');
Then the resource pattern can be specified as so:
Similarly, the pattern
*Urgent*background: cyan would apply to all widgets in the frame.
Identify a new widget with name and/or class. Name specifies the path element for the widget; names generally begin with a lowercase letter. -class specifies the class for the widget and its children; classes generally begin with an uppercase letter. If not specified, Perl/Tk will assign a unique default name to each widget. Only MainWindow widgets have a default class, made by uppercasing the first letter of the application name.
The PathName method returns the widget's pathname, which uniquely identifies the widget within the application.
The optionAdd method adds a new option to the database. Pattern contains the option being specified, and consists of names and/or classes separated by asterisks or dots, in the usual X format. Value contains a text string to associate with pattern; this is the value that will be returned in calls to the optionGet method. If priority is specified, it indicates the priority level for this option (see below for legal values); it defaults to interactive. This method always returns an empty string.
The optionClear method clears the option database. Default options (from the RESOURCE_MANAGER property or the .Xdefaults file) will be reloaded automatically the next time an option is added to the database or removed from it. This method always returns an empty string.
The optionGet method returns the value of the option specified for $widget under name and class. To look up the option, optionGet matches the patterns in the resource database against $widget's pathname along with the class of $widget (or its parent if $widget has no class specified). The widget's class and name are options set when the widget is created (not related to class in the sense of bless); the MainWindow's name is the appname and its class is (by default) derived from the name of the script.
If several entries in the option database match $widget's pathname, name, and class, then the method returns whichever was created with highest priority level. If there are several matching entries at the same priority level, then it returns whichever entry was most recently entered into the option database. If there are no matching entries, then the empty string is returned.
The optionReadfile method reads fileName, which should have the standard format for an X resource database such as .Xdefaults, and adds all the options specified in that file to the option database. If priority is specified, it indicates the priority level at which to enter the options; priority defaults to interactive.
The priority arguments to the option methods are normally specified symbolically using one of the following values:
Level 20. Used for default values hard-coded into widgets.
Level 40. Used for options specified in application-specific startup files.
Level 60. Used for options specified in user-specific defaults files, such as .Xdefaults, resource databases loaded into the X server, or user-specific startup files.
Level 80. Used for options specified interactively after the application starts running. If priority isn't specified, it defaults to this level.
Any of the above keywords may be abbreviated. In addition, priorities may be specified numerically using integers between 0 and 100, inclusive. The numeric form is probably a bad idea except for new priority levels other than the ones given above.
The priority scheme used by core Tk is not the same as used by normal Xlib routines. In particular is assumes that the order of the entries is defined, but user commands like xrdb -merge can change the order.
database, option, priority, retrieve