How much logic should you put in the UI class?

Question

I'm not sure if this has been asked or not yet, but how much logic should you put in your UI classes?

When I started programming I used to put all my code behind events on the form which as everyone would know makes it an absolute pain in the butt to test and maintain. Overtime I have come to release how bad this practice is and have started breaking everything into classes.

Sometimes when refactoring I still have that feeling of "where should I put this stuff", but because most of the time the code I'm working on is in the UI layer, has no unit tests and will break in unimaginable places, I usually end up leaving it in the UI layer.

Are there any good rules about how much logic you put in your UI classes? What patterns should I be looking for so that I don't do this kind of thing in the future?

Answer 1

Just logic dealing with the UI.

Sometimes people try to put even that into the Business layer. For example, one might have in their BL:

if (totalAmount < 0)
     color = "RED";
else
     color = "BLACK";

And in the UI display totalAmount using color -- which is completely wrong. It should be:

if (totalAmount < 0)
     isNegative = true;
else
     isNegative = false;

And it should be completely up to the UI layer how totalAmount should be displayed when isNegative is true.