make AppleScript program that listens systemwide for shortcuts

Question

I'd like to create some kind of background process that listens to all keystrokes event and acts consequently (for example does some action if CMD-A is pressed while in Finder.app, or more complex things, like sequences to create shortcuts like in emacs..)

But how can I listen to keypresses systemwide on SnowLeopard? I thought of Applescript.. but found nothing..
(I'm good at ruby too, and intended to use rb-appscript actually)

thanks!

Answer 1

You can create a Service with Automator. If you want to run a ruby script, use the Run Shell Script action and select /usr/bin/ruby in the Shell: popup menu (no need for rb-appscript).

Services can be given a Keyboard Shortcut in the System Preferences -> Keyboard preference panel.

Answer 2

If you really want to capture all the keystrokes system-wide, you need CGEventTap. I don't think it's available directly to AppleScript, maybe on 10.6 you can use AppleScriptObjC. Surely it should be available to Ruby via Bridge Support.

However, a system-wide keyboard listener would be called very often. I would suggest to code in C or Obj-C instead, which would be more efficient.