Running applescript in background (as a standalone app or iTunes script)

Question

Hiya,
I'm working on making an Apple script that syncs my currently playing track with Twitter, Facebook, and Last.fm. I have the syncing portion worked out, but the method in which the program receives information is not ideal.
I was wondering if there exist a set of commands that allows my program to work in the backhround, monitoring itunes for track-changes. Once a track changes, the syncing process will begin. Once the process ends, the program will once again become a background processes. This also beckons the question: is there a command for my program to launch when iTunes launches. I know Last.FM's official client has an option that allows the program to launch when iTunes launch. I would like to know if that's easy to implement.
Ideally, if anyone can point me to a page/book for tutorials on how to futz around with iTunes commands and background processes, I would be greatful!
Thanks!

Answer 1

The background element of your request is fairly straightforward - use an 'idle' handler.
In this case you want your idle handler to periodically check the state of iTunes and perform whatever actions you like once you know it's open.
The other element - having your script open when iTunes opens, is not so easy. iTunes doesn't have the option to run a script or perform any action when it's opened. Therefore there's no easy way to achieve this - at least not via AppleScript. The typical way of doing it is to write a daemon process that sits in the background, but this isn't easy in AppleScript, or write a plug-in (which also isn't possible in AppleScript).
As a result your best option may be to have an AppleScript app that's always running (e.g. launched at login) which sits around checking for iTunes, and kicking in once it detects iTunes has launched.
Something like:
-- variable to keep track of iTunes state last time we checked:
global iTunesWasRunning
on run
  set iTunesWasRunning to false
end run
on idle
  if application "iTunes" is running then
    -- it's running now, was it running before?
    if iTunesWasRunning then
      -- code here for checking track changes
    else
      -- it's just been launched, so do whatever you need on app launch
    end if
  end if
  -- remember the current app state for the next iteration
  set iTunesWasRunning to (application "iTunes" is running)
  -- check again in 10 seconds time
  return 10
end idle
Now you have three places you can run code - iTunes is still running, iTunes has just been launched, and itunes isn't running at all. From here you should be able to insert the relevant code for what you're trying to achieve.