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From: neil@comp.lancs.ac.uk (Neil Haddley)
Newsgroups: comp.lang.smalltalk
Subject: Holders
Message-ID: <709@dcl-csvax.comp.lancs.ac.uk>
Date: 10 Feb 89 11:12:36 GMT
Reply-To: neil@comp.lancs.ac.uk (Neil Haddley)
Organization: Department of Computing at Lancaster University, UK.
Lines: 82

The addition of 'Holder' to MVC
-------------------------------

Unfortunatley there is still a lack of useful reference  material
in the public domain to guide smalltalk programmers in attempting
to create complex MVC type tools.

To correct this in part I feel that it would be  useful  if  some
smalltalk  'experts'  could  provide  some simple 'tips' here. In
this spirit (Although I would not claim to be an  expert  myself)
there  follows some commonsense observations about the usefulness
of 'Holders' in creating complex MVC tools.

*WARNING* I am not attempting here to  teach  any  grandmother  to
suck eggs :-)

Simple MVC tools have no  independent  'state'.  Rather  what  is
displayed  in the tool corresponds quite easily to what is in the
instance variables of the underlying Model (The Real Object being
Manipulated).  More exactly what is displayed corresponds to what
can be got at via messages to  the  Model,  but  lets  not  split
hairs.

Complex MVC  tools  however  do  usually  have  some  indepentent
'state',  and  a  major  problem is  where this state can be put.
The view is a bad place, the Controller is a bad place,  and  the
Model is a very bad place.

The solution (used in the existing system code) is the  creation
of  a  Holder  Object. This object sits between the Model and the
View and Controllers.


                 --------------  Controller
             <---                    ^
      Model                          |
        .    <---                    V          Fig1 Simple MVC Tools
        .        -------------      View
        .                            ^
        ..............................
 
 
                            ---- Controller
                         <--         ^
      Model  <--- Holder             |
                    .    <--         V         Fig2 Complex MHVC Tools
                    .       ---     View
                    .                ^
                    ..................
 
        Key:
 
        ---> Pointer (Instance Variable)
        ...> Dependency
 

There are slight problems in that the dependency  is  messed  up,
and Controller Manipulation messages have to be redirected to the
Model, but it is not to difficult to get round these.

One could even introduce a new class Holder  [or  StandardSystem-
Holder :-)] to tidy things up ?

See  the  Form Editor is a simple  holder  example, here a  Form-
Holder  is  used  to hold a temp copy of the form being edited in
case 'cancel' is needed.  Although other examples exist  (String-
Holder etc).

Perhaps even the addition of a Holder is not enough, perhaps more
extra components are useful (but I hope not).

Finally I use MHVCs all the time
[and I've never had a single filling  :-)].

Hope this is of interest to some.

Neil

-- 
EMAIL:	neil@comp.lancs.ac.uk		| Post: University of Lancaster,
UUCP:	...!mcvax!ukc!dcl-cs!neil	|	Department of Computing,
					|	Bailrigg, Lancaster, UK.