Scripting the Desktop

Example 14.1. A Background Color Changing Script with DCOP

Broadly speaking, each KDE application provides one or more DCOP interfaces, which in turn provide methods that another application can call. So, the first step in writing our script is to find the appropriate method to change the desktop background color. The easiest way to do this is using the kdcop frontend to the available DCOP methods.

Run kdcop from a Konsole or the minicli (the window which pops up on Alt+F2). The kdcop window shows the applications currently running which provide DCOP interfaces, using a tree view. In general, finding the correct method requires a little bit of searching through the tree view, but for this example, we'll short circuit that: the method we want is kdesktop->KBackgroundIface->setColor. The arguments and return type of the function are shown in the style of the C++ language. For setColor, the arguments are a color, c, which specifies the new background color, and a boolean value, isColorA, which specifies whether the color is the first or second (this is useful for setting gradients and so on).

To test that the function does what we expect, double-click on the setColor entry. To set the color c, click on the color selector button, and choose a color. Set whether the color should be color A with the checkbox. Click OK and the background color is set.

To access the DCOP method from your favorite scripting language, you can either use DCOP bindings, if available in the kdebindings module, or call the dcop command-line application. For the simple usage which we want, calling the dcop command-line application is sufficient. To call a DCOP method on the command line, we need to specify the application and interface owning the method, the method itself, and the arguments, in a form suitable for the shell. To use our setColor method on the command line, we use the following:

% dcop kdesktop KBackgroundIface setColor '#ffffff' false

We specify the application, interface and method in that order, followed by the arguments in the same order that they are shown in kdcop. To specify the color, we use the hexadecimal RGB value, as used in HTML. Note that it is enclosed in single quotes to protect the # from the shell. dcop has plenty of other options: take a look at the output of dcop --help.

So, that's all we need from DCOP, now it's just a case of writing a script around it. Here's a rough implementation in Perl:

$min=49;  # Minimum value of R, G, or B colour
$max=174; # Maximum value of R, G, or B colour
$step=5;  # Amount to step colour by on each step
$sleeptime=15; # Interval in seconds between each step

@start = ($max, $min, $min);
@colour = @start;

while (1) {
	foreach (0..5) {
		my $which = $_ % 3; # Which colour (R, G or B) to change
		my $updown = $_ % 2; # Whether to increase or decrease the colour value
		do {
			if ($updown == 0) { $colour[$which]+=$step; }
			if ($updown == 1) { $colour[$which]-=$step; }
			my $dcopcall=sprintf "dcop kdesktop KBackgroundIface setColor '#%x%x%x' true\n", @colour;
			system($dcopcall);
			sleep $sleeptime;
			} while (($colour[$which] >= $min) and ($colour[$which] <= $max));
		}
}

Just run the script with no arguments, and it will cycle the background colour through a slightly muted spectrum until it is killed. Voilà! Instant scripting pleasure!