TITLE: Description of Model-View-Controller 

RESPONSE: rmartin@rational.com (Robert Martin), 2 Mar 93


MVC stands for Model-Controller-View.  It is an object oriented
technique for creating user interfaces.  The main idea is that each
element of a user interface can be separated into three parts.  Lets
look at each of these in turn.

The Model is the mathematical or logical model of the user interface
element.  It knows how the element behaves from a state and transition
point of view.  For example, if the user interface element is a push
button that you push once for "ON" and push again for "OFF", then the
Model object would accept "PUSH" events and turn the status of the
button ON or OFF as need be.

The View object communicates with the Model in order to represent the
status of the Model in some convenient form for the user.  If the user
interface is graphical, then the View may draw the button in its ON or
OFF state depending on what the Model tells it.  If the user interface
is a lightbulb, then the View would energize the light, again
depending upon what the Model tell it about the state of the button.
In fact, there may be many views for each model, each view
representing the state of the single model in a different form.  For
example, if the push button is on, one View may illuminate a light,
another view may ring an alarm, still another view might print "TURN
THAT BUTTON OFF" on a screen.

Controller objects interpret user gestures into events that the Model
can understand.  As we said before, the pushbutton Model responds to
the "PUSH" event.  It is the Controller objects which generate the
"PUSH" events.  There may be many controllers serving a single Model.
For example, there may be a real pushbutton somewhere.  The computer
software that detects that this button has been pushed and sends the
"PUSH" event to the Model is a Controller.  Another Controller may
intercept mouse clicks within the image of the button created by a
View and then send the "PUSH" event to the Model.

So, we have a single MODEL and multiple VIEWS and CONTROLLERS.
Whenever one of the CONTROLLERS detects appropriate input, it sends
events to the MODEL.  If the MODEL changes state, it notifies all its
VIEWs so that they can update their presentation of the MODEL to the
user.