OK, so there’s been a discussion about this on the cocoa-dev list of late, and no-one answered the question - I figure I’ll jump into the fray given that this is exactly what I’ve been working on with Hyperspaces.
There are two things you have to understand up front:
There is a notification “com.apple.switchSpaces” posted when a user chooses to switch spaces via the menu extra. This notification can also be (ab)used to switch spaces using your app as well - however…
There is no other notification that your active space has changed, and it is only posted by the menu extra. You need to check for changes from the CoreGraphicsServer - the easiest way to do that is as follows. It’s up to you to write the code that uses this, but attaching it to a simple NSTimer would probably work just fine.
I use this method in Hyperspaces, and it works just fine for my purposes (with the requisite delay associated with my application having to scan for changes). Apple’s Eric Schlegel explains the reasoning behind why there is no notification on the cocoa-dev list. Long story short, it sounds like Apple’s not quite done developing Spaces yet, and they don’t want people relying on APIs that aren’t finalised.