Relationship between GUI and logic?

[David C. Ullrich]

On 23 May 2008 18:23:04 +0100 (BST), Matthew Woodcraft
<mattheww at chiark.greenend.org.uk> wrote:

>John Salerno  <johnjsal at gmailNOSPAM.com> wrote:
>> Basically, the question is this: can you write the logic behind a 
>> program (whether it be a game, an email client, a text editor, etc.) 
>> without having any idea of how you will implement the GUI?
>
>You probably could, but it's best not to unless you're somehow forced
>to.
>
>In practice, the interface that you want your 'logic' layer to provide
>to the GUI is likely to depend on some of the details of how the GUI
>works.
>
>If you did the lower levels first without having any idea at all of
>what you were going to do with the GUI, it's quite likely that you'd
>find you'd written a few functions which turned out not to be needed at
>all, or that some functionality that the GUI needs to use very
>frequently is rather slow to execute, or that some functionality that
>you thought belonged to the 'logic' is really better done in the GUI
>layer, and so on and so forth.
>
>For example, if you were writing the 'logic' for a chess program you
>might choose a way of modelling the board that can't represent a
>position with more than one black king. And then when you got round to
>doing the GUI you might find out that your users would like to be able
>to have this temporarily when setting up a position.

What you say may be true, but this doesn't seem to me like a good
example. Not that I see _why_ the player would want to have
two kings temporarily, but if so I wouldn't have the view report
this state to the model, I'd have the view wait until the player had
decided on the final configuration betore saying anything to the
model. The model should only deal with legal positions.

(Could be that it's not until we start actually playing the game 
through the GUI that we find the model can't deal with two 
black queens. But that's not an example either, that would just 
mean the model is wrong, not allowing every legal position.)

>-M-

David C. Ullrich