Tag Archives: tutorial

More fun with dbus

Since my dbus post last week, I’ve been playing around more with dbus whenever I get a few minutes. The modern Linux desktop is pretty good, but with minor tweaks, you can make things even better. (all this code is now up in a mercurial repository called dbus-hackery).

Automating Inactivity

Pidgin makes sounds on every message to me, xchat makes sounds on certain key words. Without these cues, I’d never remember to go check these applications. Because I have a tendency to leave my laptop on overnight, I found that I’d often have xchat ringing away at midnight when someone was looking for me. If I forgot to mute my machine before that, it would often wake me up.

One of the programs sending signals on dbus is gnome-screensaver.

def connect_screensaver(session_bus)
    ss_dbus = session_bus.service("org.gnome.ScreenSaver")
    ss = ss_dbus.object("/org/gnome/ScreenSaver")
    if ss.has_iface? "org.gnome.ScreenSaver"
        ss.default_iface = "org.gnome.ScreenSaver"
        puts "Connected to screensaver"
    return ss

def mute()
    IO.popen("aumix -vq") {|r|
        r.read.scan(/(d+)/) {|m|
            @@vol = m
            puts "saved volume: #{@@vol}"
    puts "muting"
    system("aumix -v 0")

def unmute()
    puts "unmuting"
    system("aumix -v #{@@vol}")

ss = connect_screensaver(session_bus)

ss.on_signal("ActiveChanged") {|s|
    if s

The connect will look exactly as expected from the previous look at dbus. The ActiveChanged signal outputs a single parameter, a boolean, which is true when the screensave goes active, false when the screensaver is deactivated.

Volume control on the command line is most easily done with aumix (though if you are on Ubuntu Gutsy you’ll have issues until you rebuild aumix yourself. Hopefully they’ll fix that bug soon.) A little regex fun captures the current levels to a package variable, and restores them back on unmute.

Now I’ve got global mute when the screensaver fires, restored when I return.

Better Away with Pidgin

My screensaver being locked is a pretty clear indication that I’m away, though it being unlocked isn’t a clear indication that I’m back. Especially on weekends, I pop back for a quick check of something, then the computer is put away again.

def set_away(pidgin)
    puts "trying to set away"
    name = "screensaver"
    status = pidgin.PurpleSavedstatusFind(name)[0]
    if not status > 0
        status = pidgin.PurpleSavedstatusNew(name, 5)[0]
    puts "Status #{status}"

    pidgin.PurpleSavedstatusSetMessage(status, "screen saver auto away")

ss.on_signal("ActiveChanged") {|s|
    if s

In order to set a status with a message, it has to be a saved status. To prevent growing that to infinity, I first look to see if it is defined, creating a new saved status if not. 5 is a magic number here meaning STATUS_AWAY (reference the pidgin status.h for more info). Then we set the message on that status, and activate it. A single line change on our screen saver signal adds this into play.

Keep on Hacking

One of the things I’m hoping to impress in these posts on dbus is that with a highly functional language like ruby, linking applications on a modern gnome desktop can be done even by mere mortals. Linking sound to your screen saver is something that would have required a reasonable chunk of c code. Now you can do it in 20 lines of ruby, thanks to dbus.

I’ve thought about creating some sort of extended control panel to enable the features I’ve hacked together, but the reality is the code is so small, and so simple, it seems like overkill. With code this easy, you should just jump in and hack it to your own needs.

As I keep playing with dbus, I’ll post more bits here. Twitter integration is still on my list of things to do, and maybe something I’ll even manage to get to this week.

Pidgin, Network Manager, Dbus, Ruby, oh my!

A few weeks ago I was chatting with Dan about my one great annoyance with Pidgin: it takes up to 15 minutes to realize that it’s network connection isn’t valid any more and to automatically reconnect. With NetworkManager and dbus, this should be a reasonably simple feat. Dan got curious about this and wrote himself a python program that sets pidgin status in this way.


A bit of background. One of the Freedesktop.org standards is d-bus:

D-Bus is a message bus system, a simple way for applications to talk to one another. In addition to interprocess communication, D-Bus helps coordinate process lifecycle; it makes it simple and reliable to code a “single instance” application or daemon, and to launch applications and daemons on demand when their services are needed.

The basic idea is that d-bus allows applications to publish interfaces that other applications on the desktop can interface with. Right now, only a few applications really support d-bus in a significant way. Fortunately, one of those is pidgin, which has an extremely rich d-bus interface.

Ruby & DBUS

Here’s a place where I know I’ll draw Dan‘s ire. I’ve been on a Ruby kick recently, so my first reaction to Dan’s post was “great, I’ll have to figure out how to do this in ruby now.” It sounds like typical language bias, but I’ll try to justify it at least a little.

Right now I’m writing code on a weekly basis in 3 languages: C# (OpenSim), Java (Grad School Project), and Ruby (side web projects done in Rails). 3 languages is a lot to remain fluent in, and causes some interesting syntax errors when jumping back and forth between them. One less context switch seemed like a good idea. At some point I’ll bother writing up why ruby has seduced me, but that is for another day.

While finding python-dbus bindings are extremely straight forward, the ruby-dbus front is a little more of a wandering path. After finding a couple of abandoned efforts, I finally came to the active ruby-dbus project. Make sure to grab the latest and greatest code from there before proceeding.

Connecting to DBUS

Before I get into actually using dbus, I need to set up connections to it. There are 2 different buses, the system bus (which is shared for all users), and the session bus, which is unique per login session. Pidgin uses the session bus: it’s a user application. Network Manager uses the system bus: it’s events are system wide and affect all users.


require "dbus"

bus = DBus::SystemBus.instance
session_bus = DBus::SessionBus.instance

# Get the Pidgin Service
pidgin_dbus = session_bus.service("im.pidgin.purple.PurpleService")

# Get the object from this service
pidgin = pidgin_dbus.object("/im/pidgin/purple/PurpleObject")

# Introspect it
if pidgin.has_iface? "im.pidgin.purple.PurpleInterface"
    pidgin.default_iface = "im.pidgin.purple.PurpleInterface"
    puts "We have Pidgin interface"

n_dbus = bus.service("org.freedesktop.NetworkManager")
netman = n_dbus.object("/org/freedesktop/NetworkManager")
# Establish a proxy interface object for NetworkManager as it doesn't support introspection
poi = DBus::ProxyObjectInterface.new(netman, "org.freedesktop.NetworkManager")

Basically the pattern is clear. Get a service handle, get an object definition from that service handle, then get an interface from that object. There are 2 flavors for this, one where we’ve got introspection information, and one where we’ve got to go blind because introspection isn’t supported.

DBUS Introspection

DBUS interfaces come in 2 flavors, those that support introspection, and those that don’t. If an interface supports introspection you can get an interface definition off the dbus wire itself, otherwise you need to know the interface a priori. Pidgin supports introspection, Network Manager does not. The major short coming of the ars technica article on Pidgin and DBUS was the lack of information on using introspection to show all the other pidgin interface functions (of which there are > 600). Python dbus introspection throws an exception on my Ubuntu 7.10 environment, so it wasn’t helpful here. However, ruby, as usual, came to the rescue.

The ruby-dbus code contains an example application called gd-bus, which performs introspection on all dbus interfaces it can find, and prints them out nicely. The cheat sheet goes something like so:

M PurpleAccountsFind(in name:s, in protocol:s, out RESULT:i)

  • M – DBUS Method, aka Function (might also be S meaning it’s a Signal you can register to listen to)
  • PurpleAccountsFind – the method name
  • in / out – whether this is an input or output parameter
  • NAME:type – the name and type of the parameter. Types can be s – string, i – integer, u – unsigned int, ai – array of integers

Note: the ruby interface seems to always wrap arrays around the output parameters. I have no idea why, but it’s consistent, so a few extra “[0]”s get you a long ways. If you know why, please comment.

The translation of this interface specification into ruby gives you something like this:

def recycle_pidgin(pidgin)
    accounts = pidgin.PurpleAccountsGetAll
    for account in accounts[0]
        if pidgin.PurpleAccountIsConnected(account)[0] > 0

This cycles through all pidgin accounts, disconnects all the connected ones, and then attempts to connect all accounts. This is effectively what I end up doing by hand every time I switch networks with my laptop.

Bringing it all together

I now had connections to the 2 buses, and code to cycle the pidgin accounts. Last bit is actually watching for the Network Manager signal that I’ve got a new active network device.

poi.on_signal(bus, "DeviceNowActive") {

main = DBus::Main.new
main << bus

Network manager doesn’t support introspection. However, it does have pretty decent docs to figure out what the interface is. I’m still sad it doesn’t show up nicely in gd-bus though.

The proxy interface object sets up a signal using 2 parameters and a code block. Every time there is a DeviceNowActive signal on the system bus, I recycle pidgin. Pretty straight forward.

The last little bit is making this thing go into a loop. Ruby dbus contains it’s own main loop for just this task. I created a new main loop, tell it to watch the system bus, and then start it. And, we’re done.

The future’s so bright…

DBUS has been on my list of “I need to go figure this out” for a while. A morning of reading docs, hacking a bit, and crashing network manager a few times, and voila, you’ve got this blog post.

I’ve got lots of ideas floating around in my head now for other things that I can do with dbus to make my applications work better for me. As I bang a few of those out into code, expect to see more here about it. Pidgin is an especially target rich environment given how rich and interface they expose (nice job guys!).