Creating a Mac OS application that interacts with all text fields

What would be the first step to creating an application for Mac OS X that would take user interaction from any text field in the system? A couple examples of apps that do this are TextExpander and Typinator.

What I want to do would be using similar functions to access Text fields throughout the computer during user input to give other additional options, I'm just not sure where to even start in Xcode and with what type of project to use.

Any开发者_开发技巧 help or links would be greatly appreciated! Thanks!

---

I think you can probably capture what's being entered into text fields using an Input Manager, although these may be deprecated as of OS X 10.6. You might also want to look at SIMBL.