You can basically read anything from controls, and trigger callbacks at will, as if you had actually clicked a button or written some text. This means that you can write "expect-like" software -- just start up the program, and have another program read input from it's text fields and issue commands to it.

I have actually done a lot of this, putting old sourceless win32 and win16 programs run in the background on virtual machines on the server and building new web-based interfaces on top of them.

Actually it ranges from simple event spoofing (user clicked here, user dragged there) to injecting a DLL + spawning a thread under your control.

Event spoofing is pretty limited. While having a thread under your control gives you full power as you have full access to the process' memory and can call any function you want.

AutoIt is a popular software to automate GUI applications.

http://www.autoitscript.com/site/autoit/

It's quite easy, see here for example: http://blogs.msdn.com/b/oldnewthing/archive/2013/04/08/10409...

I think he means the APIs that allow you to simulate mouseclicks and keyboard input on arbitrary windows and controls