Here are some sample resource entries:

*font                         : 12x24
frog*Foreground               : blue
frog.button1.Background       : orange
frog.b3.foreground            : green
frog*Label*cursor             : gobbler 

Note that the resource pattern is to the left of the colon, and its value to the right. The pattern can be general (such as the first entry, which changes the default font for every X and Tk application) or specific (such as the fourth entry, which changes only the foreground color of the widget named b3 of the application frog).

Let's use the following program as an example. We create the MainWindow and disable its propagate feature, so it doesn't shrink-wrap itself around the widgets inside it. This lets us see its background color. Somewhere early on in this process, Tk::CmdLine::SetArguments is called to initialize the option database with command-line option/values. We print @ARGV so we can see what's left in the array after the call to MainWindow.

The Label's purpose is simply to show us the application's class and name, which affect how option database lookups work.

Now for the three Buttons, each designed for a special purpose. The first has a hardcoded background color, so it should remain yellow no matter what background-related resources are in the option database. The second button has no attributes set, so its background can be set from the option database. Finally, the third button has explicity set its name using the Name option. A widget's name can be changed only during widget creation; the name cannot be reconfigured afterwards. If the Name option is omitted, Tk automatically generates the name.

Lastly, we loop through all the widgets (ignoring Labels) and configure the Button's text to show its full pathname. It's the pathname we're interested in when creating resource entries.

my $mw = MainWindow->new;
$mw->packPropagate(0);

print "ARGV=@ARGV.\n";

my $l1 = $mw->Label(-text =>
    "Application class/name is\n'" . $mw->class . '/' . $mw->name . "'");
$l1->pack;
my $b1 = $mw->Button(-background => 'yellow')->pack;
my $b2 = $mw->Button->pack;
my $b3 = $mw->Button('Name' => 'B3')->pack;

foreach ($mw->children) {
    next if ref $_ eq 'Tk::Label';
    $_->configure(-text => "'" . $_->PathName . "'");
}

Let's assume the preceding code is saved in a file named frog. This means that the program's name is frog and its class is Frog.

For our test runs, we'll use the same resource entries that we saw before, which we've stored in our .Xdefaults file.

*font                         : 12x24
frog*Foreground               : blue
frog.button1.Background       : orange
frog.b3.foreground            : green
frog*Label*cursor             : gobbler 

This file always resides in our home directory. Tk uses the value of the environment variable $HOME to locate the .Xdefaults file. Under Unix, the variable is always defined, but not so for you Win32 users, where the concept of a home directory doesn't exist. Nevertheless, you can define an MS-DOS HOME environment variable pointing to any directory you choose and store your .Xdefaults file there. If you want the .Xdefaults file on your desktop, use this command:

set HOME=c:\windows\desktop

Alternatively, you can create a batch (.bat) file, which might look like this:

set HOME=C:\Program Files\PerlApps
perl "C:\Program Files\PerlApps\fontviewer.pl"

This allows you to keep various resource files lying about.

Figure 16-1. frog application with a huge font

It's immediately clear that the font is huge. That's due to a loose binding *font : 12x24 resource. This turns out to be a very unpleasant resource, as it trashes nearly every graphical application. Fortunately for us, we can still tell that the program's class and name are Frog and frog, respectively.

Although you can't tell from the grayscale image, the first Button (.button) has a yellow background with blue lettering. Indeed, the loose binding frog*Foreground : blue resource ensures that any widget attribute of class Foreground is set to blue, unless otherwise overridden.

The second Button (.button1) has an orange background and, again, a blue foreground.

The third Button (.b3) has the standard gray background with green lettering when inactive. In the figure, the Button is active, thus its background is blue and foreground is black. It's blue because a Button's -activebackground option is of class Foreground.

Let's remove the loose font resource binding by commenting it out. The option database comment character is the exclamation point.

!*font                         : 12x24
frog*Foreground                : blue
frog.button1.Background        : orange
frog.b3.foreground             : green
frog*Label*cursor              : gobbler 

Figure 16-2. frog with normal fonts again

What do we do if we want to change the options on some Buttons but not others? From what we've covered, we know that all Button widgets nominally belong to the class Tk::Button, unless we do something to change it. If we want to group all Buttons with the text Ok on them, we can put them in the 'OkButton' class:

$mw->Button(-text => 'Ok', -class => 'OkButton');

Even if you create a Button with the text Cancel, as long as the -class option uses OkButton, it belongs to the proper class.