Smalltalk and Object Orientation: an Introduction - Free

In the updating protocol we need to define a method which will catch the result of the change
message set when a new widget is added to the model. This is the update:with: message. In our
case we merely wish to draw the newly added object in the view:
update: anAspect with: anObject
anAspect == #object ifTrue: [anObject displayOn: self graphicsContext].
We now need to define the displayOn: method for the drawing object within a displaying
protocol. This is sent to the drawing view when what is being displayed needs updating (e.g. when part
of the window is uncovered). This method sends the displayOn: message to all the elements
currently held in the nodes list of the drawingModel:
displayOn: aGraphicsContext
self model nodes do: [:node | node displayOn: aGraphicsContext]
Finally we need to extend the class side method in the instance creation protocol so that it sends the
message initialize to the newly created instance:
new
^super new initialize
29.5.4 The DrawingController class
The DrawingController class is a subclass of the ControllerWithMenu class. This is
because we wish to catch the left mouse button pressed event. This is simplified for the
ControllerWithMenu cla ss as a method called redButtonActivity is called whenever the
left mouse button is pressed.
ControllerWithMenu subclass: #DrawingController
instanceVariableNames: ''
classVariableNames: ''
poolDictionaries: ''
category: 'SmallDraw'
The class comment for the DrawingController is left as an exercise for the reader. Only two
methods are defined for this class: controlLoop and redButtonActivity. Both methods are
defined in the control defaults protocol. The controlLoop method is called whenever the curso r
enters the area of the window displayed by the DrawingView. This controlLoop method merely
changes the type of cursor displayed (to that of a cross) when the user moves the cursor onto the
drawing area. It then sends the message super controlLoop. This a llows the control operations
defined higher up the class hierarchy to be invoked:
controlLoop
"Change the cursor to a cross-hair for drawing."
Cursor crossHair showWhile: [super controlLoop].
Finally the redButtonActivity method defines what should happen when the left mouse
button is pressed. In this case a new graphic object is added to the drawing at the point indicated by the
current cursor position:
redButtonActivity
"Place widget at the current cursor position when left button pressed"
self model addWidgetAt: self sensor cursorPoint.
29.5.5 The DrawingWidget class
The DrawingWidget class is an abstract superclass which captures all the elements which are
common to graphic objects which will be displayed within a drawing. The DrawingWidget class (a
subclass of Object) defines three instance variables origin (the cursor position when the mouse is
clicked), the extent (or size of the object) and the border:
Object subclass: #DrawingWidget
instanceVariableNames: 'origin extent border '
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