Smalltalk and Object Orientation: an Introduction - Free

• Views control what is displayed on the screen.
• Controllers handle user interaction.
• Models hold the domain data to be displayed by the view.
Therefore it is in the view class that the developer must define a method which will specify what the
custom view widget will display. This method is the displayOn: method. It takes one parameter
which is a GraphicsContext. GraphicsContexts are objects which work with windows to display
graphical objects (determining how the graphic object should be rendered).
The displayOn: method is sent by the system to the view whenever a change has occurred in one
of the objects on which it depends (such as the model) or when the screen needs to be updated (for
example, after a window which has been partially covering the view is closed).
For the view to determine what it should display it needs to communicate with the model which
holds the data. It must therefore have a link to the model (whether it is a separate object or the instance
of application model which holds the parent window).
Finally, the view must also work with a controller. This controller will determine what action the
application should take whenever the user interacts with the view widget. Note that this controller does
not attempt to control how the user interacts with the whole window, rather it is only concerned with the
user’s interaction with the view. For example, what happens when the user clicks in the view etc.
We shall now consider how the SmallDraw application is structured.
29.3 The structure of the application
Figure 29.4 illustrates the inheritance structure between the classes used in the SmallDraw application.
This class hierarchy is typical of an application which incorporates a custom view. For example, we
have an ApplicationModel hierarchy with the main SmallDraw application. We have a
Controller subclass ( DrawingController), a Model subclass ( DrawingModel) and a
View subclass ( DrawingView). In addition we have three types of object which can be drawn in a
SmallDraw drawing: BoxWidget, CircleWidget and PointWidget. The only difference
between these three classes is what is drawn on to the graphics context representing the view. The
DrawingWidget class from which they all inherit defines common attributes used by all objects
displayed within a DrawingView (such as t heir origin, the extent of their boundary and the size of
their border). Each of these classes will be considered in more detail in a later section in this chapter.
Object
Model
DrawingModel ApplicationModel
Controller
view
model
View
model
controller
DrawingWidget
origin
extent
border
style
nodes
SmallDraw
drawingView
ControllerWithMenu
DrawingView
defaultController
BoxWidget
CircleWidget
PointWidget
diameter
drawingModel
DrawingController
Figure 29.4: Inheritance in SmallDraw
Note that the dashed line between the Object class and the View class indicates that there are a
number of classes between these two in the actual class hierarchy. It is useful to examine these classes
yourself to see what facilities they add.
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