User Interface in one class, processing in a different class

Question

Hi there,

My application has an option in the menu that needs to put the text of a textbox into a file.
Because in fact there is much more to my application, I wanted to isolate the event calls from the actual handlings. To do that, my event on my form class calls a method that lives in a different class (named Processing). How can I grab the text of my form class when I am in my Processing class? Should I pass it as an argument? Nah... How about some kind of constructor, or other class...?
How do I do this?

Thanks,
Emmi