Control Your Linux Desktop with D-Bus

If you want to do more complex tasks than calling a single method, you can write a shell script with dbus-send commands or use a higher-level language to simplify the task. There are D-Bus bindings for languages such as Python, Ruby and Java.

In this next example, I implement a Python script that sets your status on Pidgin to “Away from keyboard” if your screensaver activates. This shows two aspects of D-Bus: the script waits for a signal from the screensaver, and then it calls a method in Pidgin. The script is shown in Listing 1.

#!/usr/bin/env python

def pidgin_status_func(state):
    obj = bus.get_object("im.pidgin.purple.PurpleService",
                         "/im/pidgin/purple/PurpleObject")
    pidgin = dbus.Interface(obj, "im.pidgin.purple.PurpleInterface")
    status = pidgin.PurpleSavedstatusFind("afk")
    if status == 0:
        status = pidgin.PurpleSavedstatusNew("afk", 5)
    if state:
        pidgin.PurpleSavedstatusSetMessage(status,
                                           "Away from keyboard")
        pidgin.PurpleSavedstatusActivate(status)

import dbus, gobject
from dbus.mainloop.glib import DBusGMainLoop

dbus.mainloop.glib.DBusGMainLoop(set_as_default=True)
bus = dbus.SessionBus()

bus.add_signal_receiver(pidgin_status_func,
                        dbus_interface="org.gnome.ScreenSaver",
                        signal_name="ActiveChanged")

loop = gobject.MainLoop()
loop.run()

Let's dissect this script. The function pidgin_status_func sets your status in Pidgin. It gets the im/pidgin/purple/PurpleObject object and then the im.pidgin.purple.PurpleInterface interface from the session bus. Then, it calls this interface's methods. It creates a new “saved status” type by first checking if the status type with name “afk” exists, and if not, it creates it (“afk” stands for “Away From Keyboard”, and 5 is the “away” status type).

Then, the function checks the state variable that is an argument to the pidgin_status_func function call (I explain what this argument means later). If the argument is true, it sets the status message of the new “afk” status to “Away from keyboard” and activates the new status. The effect is that Pidgin shows your status as “afk” with the status message “Away from keyboard”.

Now you need to call this function when the screensaver activates. Therefore, start the dbus main loop and connect to the session bus. Then, add a signal receiver that listens to the signal ActiveChanged from the org.gnome.ScreenSaver interface. If/when the signal fires, it calls out pidgin_status_func function. As the ActiveChanged signal has a boolean argument that signifies the current state of the screensaver (1 for active and 0 for non-active), you have defined one argument called state in the pidgin_status_func function. To keep listening, let the loop run indefinitely, as long as the script is running.

Pidgin has an extremely rich D-Bus interface; you can do almost anything with it. So let this example give you some inspiration to do some creative tasks in Pidgin!

Let's look at another example, this time in Ruby. We're going to create a script that shows the currently playing song in Rhythmbox as your status in Pidgin (Listing 2).

#!/usr/bin/env ruby

require 'dbus'

bus = DBus::SessionBus.instance
rhythmbox = bus.service("org.gnome.Rhythmbox")
player = rhythmbox.object("/org/gnome/Rhythmbox/Player")
player.introspect
player.default_iface = "org.gnome.Rhythmbox.Player"

pidgin = bus.service("im.pidgin.purple.PurpleService")
purple = pidgin.object("/im/pidgin/purple/PurpleObject")
purple.introspect
purple.default_iface = "im.pidgin.purple.PurpleInterface"

player.on_signal("playingUriChanged") do |uri|
  status = purple.PurpleSavedstatusFind("rhythmbox").first
  if status == 0
    status = purple.PurpleSavedstatusNew("rhythmbox", 2).first
  end
  purple.PurpleSavedstatusSetMessage(status, uri.to_s)
  purple.PurpleSavedstatusActivate(status)
end

Here you see the same type of commands as I used in the Python script: open a D-Bus session, define D-Bus services, objects and interfaces, and I define a signal receiver. And, a loop runs indefinitely to keep listening to the D-Bus signals.

Of course, this could be tidied up a bit. For example, you now are showing only the file path of the song as the status message. I'll leave it to the reader to extract the relevant ID3 tags out of the file and show them instead of the file path.