Smalltalk and Object Orientation: an Introduction - Free

29.4.1 Initialization
3. model:
initialize
(SmallDraw)
1. new
DrawingView
(DrawingView)
4. addDependent:
2. new
DrawingModel
(DrawingModel)
Figure 29.6: Object interaction during initialization
When the SmallDraw application is opened (i.e. when the message open is se nt to the SmallDraw
class) an instance of SmallDraw is created and sent the message initialize (as illustrated in
Figure 29.6). This results in the creation of two further instances one a drawingView and the other a
drawingModel. The drawingController is automatically instantiated when the view is
displayed (note the type of controller to be used is specified by the drawingView in the instance
variable defaultController).
Once the two instances have been created the mes sage model: is sent to the drawingView. This
in turn causes the drawingView to become one of the drawing models dependents. Note that this last
message send is defined in the DependentPart class, which is one of the superclasses of
DrawingView.
At this poi nt the structure required by the custom view is in place. It is now possible for the
application model SmallDraw and the user to interact with the drawingView and
drawingController.
29.4.2 Changing the type of graphic object to be displayed
setBoxStyle
(SmallDraw)
1. style:
#box
(DrawingModel)
Figure 29.7: Changing the graphic object to be added
As we have used the user interface builder to add buttons to the main window which will be used to
determine which type of graphic object to add to the d rawing, we require some way of passing that
information onto the objects involved in managing and maintaining the custom view. That means we
must inform the model of the type of object which will be added.
The actual interaction illustrated in Figure 29.7 involves the message setBoxStyle being sent to
the SmallDraw application when the Box style button is selected in the main window. This message is
sent in response to an event being raised (we do not cover how this work s in this book: it is left as an
exercise for the reader).
When the message setBoxStyle is received by the smallDraw object, a further message is
then sent to the drawingModel (which smallDraw has a link to via the instance variable
drawingModel) setting the current style to the symbol #box. The style value can currently be one of
#box, #circle or #point.
29.4.3 Adding a new graphic object
(OrderedCollection)
4. add: (BoxWidget)
redButton
Activity
(DrawingController)
1. addWidgetAt: 2. new
(DrawingModel)
BoxWidget
3. origin:
8. displayOn:forDisplayBox:
5. changed:with:
(BoxWidget)
(border)
6. update:with:
(DrawingView)
7. displayOn: (GraphicsContext)
9. displayRectangle:
(GraphicsContext)
Figure 29.8: Adding a box graphic object
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