Smalltalk and Object Orientation: an Introduction - Free

and wit hdrawals and requests for the current balance. It could therefore be used with a view and a
controller as part of a graphic application.
In some versions of Smalltalk (for example VisualWorks) there is an explicit class Model from
which models can inherit model like behaviour. Figure 26.3 illustrates part of the Model class hierarchy.
It illustrates that classes such as Browser and Inspector are actually models, which hold some
data. However, there is no re ason why an object which will be used as part of the MVC triad must be a
subclass of Model.
The important point to note about an object, which will be used as a model within the MVC, is that
it must provide the application functionality independent of the type of user interface being used (ignore
the issue of printing the statement in the financial manager example here). In earlier chapters, we
explicitly sent messages to the financial manager application via the Workspace. However, these
messages could have been sent by another object, such as a banking system object, or by a controller or
view object.
Figure 26.3: Part of the Model class hierarchy
26.2.2.2 Views
The information held by the model is present ed to a user via a view. That is, a view obtains information
from the model and decides how to present that information to the user. This can be in the form of text,
graphics, widgets, or combinations of these etc. However, an important point to note is th at the view
should not possess any knowledge of what the data means, neither should it change it, initiate some
application process because of it, or process it (other than what is necessary for the user interface).
Notice that it is the view that decides how the data should be displayed independently of the model.
The result of this is that we could have one view which displays the balance after each transaction as
a table, and another as a graph. Depending on which view we connect to the model, we will ge t a
different user interface. However, we will not have had to change the model at all.
Figure 26.4 illustrates part of the View class hierarchy. This hierarchy illustrates how the various
elements typicall y found in a graphical interface are different types of view. For example, a
RadioButtonView is a subclass of LabelledButtonView, which in turn is a subclass of
BasicButtonView, which is a subclass of SimpleView, which is a direct subclass of View.
Similarly ScrollBar, MenuItemView and LauncherView are all direct subclasses of View.
This figure also illustrates one of the interesting facilities in VisualWorks. Consider the
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