Smalltalk and Object Orientation: an Introduction - Free

Figure 26.5: Part of the Controller class hierarchy
It might seem understandable that the m odel should be separated out from the user interface, but
why is the input separated from the output? This is for two reasons:
• Firstly, it allows the controllers and the views to inherit from different class hierarchies. This
allows the developer to overc ome the limitation of single inheritance enforced by Smalltalk (but
not all object oriented languages).
• Secondly, it allows different combinations of views and controllers. This may give different looks
(views) or feels (controllers).
26.2.3 How the MVC works
You may notice that in Figure 26.6 the arrows only point from the view and controller to the model.
There are no arrows from the model back to the view or controller. This is because the model does not
directly know anythi ng about its view or controller. This is partly because it does not need to know
anything about them and also because it enforces this separation. A common mistake for those new to
the MVC is to find that they have created instance variables in their model object which allow them to
link the model directly to the view and/or controller. This is extremely poor Smalltalk style!
So how do the view and the controller find out about changes in the model. Remember, the view
bases what it displays on the data held within the information model. In terms of Smalltalk relationships
this means that the view is a dependent of the model (see last chapter). The view, must therefore, record
itself as a dependent of the model ( Figure 26.6 illustrates how interaction occurs between the model,
view and controller). This means that it is the responsibility of the model to inform its dependents of
any changes to its contents (i.e. its view). This is done by the model sendin g a message to itself stating
that its contents has changed in some way. The changed method is actually defined in class Object
and is thus inherited by all classes in the system. This means that any object in the system can act as the
model in an MVC tria d 11 . This method causes an update: message to be sent to each of the views
referenced in the dependents collection.
11
This actually means that any object, irrespective of whether it is used in the MVC, can have dependents , and that those
dependents can be informed of changes to t he object is the same way that views are informed of changes in their associated
information model.
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