Smalltalk and Object Orientation: an Introduction - Free

classVariableNames: ''
poolDictionaries: ''
category: 'SmallDraw'
The initialize method for this class (defined in the
values for each of the instance variables. For example:
initialize protocol) sets the default
initialize
origin := 0 @ 0.
extent := 30 @ 30.
border := Border width: 1.
You can experiment with these values by changing the extent or the border (do so and see what the
effects are).
Next we will define the methods in the accessing protocol. These methods retrieve and set the
border, the origin and the extent:
border
^border
border: aBorder
border := border.
origin
^origin
origin: aPoint
origin := aPoint.
extent
^extent
extent: aPoint
extent := aPoint
The displaying protocol has two methods in it: displayOn: and bounds. The displayOn:
method is a subclass responsibility. This is because each subclass needs to specify what it should look
like independently. This is done in the displayOn: method.
displayOn: aGraphicsContext
self subclassResponsibility
The bounds method calculates the bounds of the object using the origin and the extent. The
return object is an instance of rectangle.
bounds
^origin extent: extent.
Finally, as before we need to extend the basic instance creation method new by se nding the message
initialize to the newly created instance. This is left as an exercise for the reader.
29.5.6 The PointWidget class
The PointWidget is a subclass of DrawingWidget that specifies how a point should be drawn in
a drawing. It defines a single instance variable diameter. This indicates the size of the point to draw:
DrawingWidget subclass: #PointWidget
instanceVariableNames: 'diameter '
classVariableNames: ''
poolDictionaries: ''
category: 'SmallDraw'
The initialize method specifies the default size of the diameter. As the DrawingWidget
defines some initializations as well, we must send the message super initialize.
initialize
super initialize.
diameter := 4.
254