Smalltalk and Object Orientation: an Introduction - Free

How the update message is handled is the responsibility of the receiver (in this case the view). For
example, in order to find out what change has occurred, the view will need to request some information
maintained within the model. Thus the information model only needs to send the change method to
itself at the appropriate times, the views must then handle the update: message i ndependent of the
model.
depedents
model
model
view
controller
view
controller
model
Figure 26.6: The dependency relationship within the MVC
In Figure 26.6 notice that the view and the controller both have direct links to the model and to each
other. This means that the controller can send requests for services directly to the model while the view
can request state information directly from the model. Also note that the controller and view both
possess explicit links to each other which allows them to communicate directly. However, although the
model must know that something may be interested in it (i.e. it must send itself one of the changed
messages), it does not know what that thing is - it could be any t ype of object including a view. Also
remember that it is the view that would have had to make itself a dependent of the model and thus the
model developer need never know how the model would be used.
This arrangement means that:
• Views and controllers tend to be tightly coupled.
• Views and controllers come in pairs. To support this most views in VisualWorks know what
controller they should have and can instantiate it for you.
• More than one view can be associated with a single model.
This last situation is k nown as having multiple views and is illustrated in Figure 26.7. Thus the same
information can be displayed simultaneously by different views. All of which are updated as
appropriate, if and when required.
dependents
model
view1
controller
model
viewN
controller
model
view
controller1
model
view
controllerN
model
Figure 26.7: Multiple views
26.3 The MVC with the statement model
Using the financial manager application example described earlier in this book, we will look at how the
balance might be displayed and u pdated. The diagram illustrated in Figure 26.8 shows the three
elements of the MVC triad, the model (in this case an instance of statement - hence the use of statement
in brackets to indicate we mean a statement and not a general type of model object), the view and the
controller.
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