Detecting events in iTunes

Question

I've only spent 30 days with AppleScript, so I'm still asking questions like "Can AppleScript get messages or does it only send them?"
Here's what I'm getting at. Is there a way for an AppleScript to detect when iTunes changes tracks, when it reaches the end of a playlist, or if the user changes iTune's playing state?
Similarly, can a script detect when the user opens a new movie in QuickTime (or selects a new movie).
I'm imagining in pseudo code:
on iTunesEvent theEvent
-- script here
end iTunesTimeEvent
on QuickTimeEvent theEvent
-- script here
end QuickTimeEvent
Anything like that?
In other words, does iTunes send a message when such event occurs. If not, then my script must regularly get the state of those properties from iTunes and determine if they've changed, correct?
I've received VERY helpful advice and insights on these forums! Many thanks to all! Through your help I've got a powerful tool I'll use almost every day in my work.
John

Answer 1

What you're asking for is a feature known as attachability - the abilty to attach a script to an application.
It is possible to do in AppleScript, but the catch is that the application developer has to specifically support it in his app. Like the vast majority of applications iTunes doesn't do this.
The only way you can do what you describe is to periodically poll the app to find out what it's doing. iTunes supports a 'player state' property that tells you what it's doing, plus a 'current track' property that identifies the current track being played (if any).
Something like this should work (untested):
<pre class=command>global lastTrack
on run
-- initialize a variable to store the current track
set lastTrack to missing value
end run
on idle
tell application "iTunes"
if (get player state) = playing then
set curTrack to (get current track)
if curTrack ≠ lastTrack then
-- we're not playing what we were playing last time we checked, so:
display dialog "Now playing " & name of curTrack giving up after 5
-- record the current track for prosperity
set lastTrack to curTrack
end if
end if
return 10 -- check again in 10 seconds
end idle</pre>