Run scripts when gnome-screensaver starts or stops in Ubuntu

For a project I have been playing with I need to be able to run a script when gnome-screensaver starts or stops on Ubuntu (10.04).

The gnome-screensaver FAQ provides detail on the dbus signal fired when gnome-screensaver activates and deactivates. The FAQ even has sample perl code to respond to the signal. Unfortunately the sample code polls dbus-monitor which is too inefficient for script I plan to run in the background on my laptop.

Python offers a great dbus library which allows signal receivers to be registered with dbus resulting in an efficient event listener.

On to the script.

Step 1: Setup the required files and directories

1. Make a directory for your work:
   

   sudo mkdir -p /opt/ssTrigger

2. Change yourself the owner of the new directory:
   

   sudo chown `whoami`:`whoami` /opt/ssTrigger

3. Create stubs for the scripts to be used
   

   touch /opt/ssTrigger/ssTrigger /opt/ssTrigger/ssStart /opt/ssTrigger/ssStop

4. Make the stub scripts executable:
   

   chmod 755 /opt/ssTrigger/*

Step 2: Edit the ssTrigger script

1. Open the ssTriger file created earlier:
   

   gedit /opt/ssTrigger/ssTrigger

2. Add the following to ssTrigger:
   

   #!/usr/bin/env python
   from gobject import MainLoop
   from dbus import SessionBus
   from dbus.mainloop.glib import DBusGMainLoop
   from subprocess import Popen
   
   class SSTrigger:
       def __init__(self):
           DBusGMainLoop(set_as_default=True)
           self.mem='ActiveChanged'
           self.dest='org.gnome.ScreenSaver'
           self.bus=SessionBus()
           self.loop=MainLoop()
           self.bus.add_signal_receiver(self.catch,self.mem,self.dest)
       def catch(self,ssOn):
           if ssOn == 1: #Screensaver turned on
               Popen(["/opt/ssTrigger/ssStart"])
           else: #Screensaver turned off
               Popen(["/opt/ssTrigger/ssStop"])
   
   SSTrigger().loop.run()
   

3. Save and close the file.

Step 3: Set ssTrigger to start when you login:

1. Open the Startup Applications menu:
   System -> Preferences -> Startup Applications
   
   
2. Click "Add" to create a new startup application.
   
   
3. Enter the following details:
   Name: Screensaver Trigger Script
   Command: /opt/ssTrigger/ssTrigger
   
   
4. Click "Add" to save your new application.
   
   
5. Restart your computer.

Finishing up:
Once you have restarted you can run:

ps aux | grep ssTrigger

Check the output to make sure the ssTrigger script has started at login.

Edit the ssStart and ssStop scripts to run commands as gnome-screensaver starts and stops respectively.