Smalltalk and Object Orientation: an Introduction - Free

The action of adding a new graphic object to the drawing displayed by the custom view in the
SmallDraw application is initiated by the user pressing the left mouse button. Historically the mouse
buttons were known as red, yellow and blue for the left, middle and righ t mouse buttons. This was
because the original system on which Smalltalk was developed had multi -color mouse buttons (colored
red, yellow and blue). Although Smalltalk is now available on systems with one, two and three mouse
buttons (all of which tend to be shades of the same color e.g. white, gray or cream) there are still places
in the system where reference is made to the color of the mouse button - this is one of them.
When the left mouse button is pressed a redButtonActivity message is sent to the
drawingController (as illustrated in Figure 29.8). This allows the drawingController to
decide what action to perform in response to the left mouse button being pressed. For example, an
application may specify that a menu should be displayed or that any object under the mouse is selected
etc. In the case of the SmallDraw application the action is to place a graphic object (of the appropriate
style) at the current cursor location. To do this, the
drawingController sends an
addWidgetAt: message to the drawingModel.
Next the drawingModel creates a new instance of the appropriate type of object. In the diagram
illustrated in Figure 29.8 the current style is that of #box. Therefore a new instance of BoxWidget is
created. This boxWidget instance is then sent a message origin: which sets its position within the
view. The boxWidget is then added to the orderedCollection used to hold all the graphic
objects. Next the drawingModel sends itself the changed:with: message so that all of its
dependents can be informed that a change has taken place.
This results in an update:with: message being sent to the drawingView via the dependency
mechanism. The drawingView then sends a displayOn: message to the o bjects to be displayed.
The boxWidget then uses a border object to display itself on the graphicsContext.
This section has outlined the most difficult part of constructing a custom view for the novice -
working out what is sent where, by what and when. The thing to remember is that you are slotting your
application specific code into a generic framework. However at this stage it is often much easier to look
at what someone else has done and to use it as the basis of what you require. While this approach can be
extremely useful as a learning exercise it is still important to become familiar with this framework. One
way to do this is to place self halt messages into strategic methods (such as update:with: and
displayOn: in the drawingView object) and then to u se the debugger to explore the objects and
messages which have been sent.
29.4.4 Refreshing
The final set of interactions which you should be aware of relate to refreshing the window following
some operation which invalidates part or all of the screen display (su ch as the movement of a partially
overlapping window). In such circumstances the message checkForEvents is sent to the
applicationWindow, the application controller (which acts as the controller for the overall
application). (Note that in Figure 29.9 the large dashed line indicates missing message sends).
The applicationWindow then sends a displayDamageEvent: aRectangle message to itself.
This message basically means that an event is raised which indicates that there ma y be some damage to
the displayed area (specified by a rectangle). If the applicationWindow is open, then the message
displayOn: will be sent to the drawingView.
The displayOn: method in drawingView (as opposed to that defined in the “wi dget” classes)
obtains a collection of objects to be displayed from the drawingModel using the nodes message
and sends each object a displayOn: message as part of a do: message expression. This results in
each of the graphic objects being redraw on the screen.
(BoxWidget)
5. displayOn:
checkForEvents
(ApplicationWindow)
2. displayOn:
(DrawingView)
4. do: aBlock 6. displayOn:
(OrderedCollection)
(CircleWidget)
3. nodes
7. displayOn:
1. displayDamageEvent:
(DrawingModel)
(PointWidget)
Figure 29.9: Refreshing a custom view
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