Is spying possible?

[Hugh Ho <hho321 yahoo com>]

Hi,

I'm new to X programming. I want to write a program which can intercept
other X apps' signal and events, such as keystroke or mouse clicks.  For
instance, I want my program to be able to detect whether another X app's
particular edit box is filled with text and be able to change that.  

Is there an X app that does something like Spy++ (MS Windows), so I can
take a look at the source?  Can someone point me to the right direction or
show me an sample app if what I want to do is possible?  Thanks.

-Hugh Ho