Smalltalk and Object Orientation: an Introduction - Free

Instance based reuse is more complicated to achieve but has the advantage that the required
functionality is also obtained.
27.11.3 Inheritance of canvases
Subcanvas reuse as described above really relates to part-of relations hips and must be redefined for
each window which wishes to use the same subcanvas even if all the windows relate to subclasses of a
common ancestor class.
ApplicationModel
ProjectApplicationModel
AccountModel
commonSpec
behaviour
accountSpec
account behaviour
accountSpec
commonSpec
Figure 27.16: Inheritance of subcanvases
One way of alleviating this duplication is to use inheritance with the reuse of parts of a graphical
user interface. For example, rather than obtaining the subcanvas specification from another class, the
subcanvas specification is inherited from a parent class. This is illustrated in Figure 27.16.
This enables not only part of the window layout to be inherited, but instance variables, accessor
methods and value holder relationships etc. This can be done by s pecifying the inherited window
specification containing the required layout, in the canvas property of a subcanvas widget.
27.12 Method of working with the UI builder
At this point it is worth noting the sequence of steps (and their order) which are generally ta ken when
developing a window using the user interface builder. We will assume that we are constructing a new
window on a new class. The only differences that you need to bear in mind if this is not the case is to
ensure that you don’t over write existing w indow specifications or method definitions (unless that is
what you intend to do).
1. Open the user interface builder using the canvas icon on the Visual Launcher and draw the desired
window.
2. Once you have drawn the window, select Install from the canvas too l. This will bring up the
installer window (illustrated in Figure 27.6) with an empty class name. You must enter a class name
before you can continue.
3. Having specified a class name the window illustrated in Figure 27.17 will be displayed. As we are
defining a new window we select the Application option in the Define As grouping which results in
the ApplicationModel class being selected as the super class of our newly defined class. If we
wish the class to be defined in a category other than UIApplications-New, then we also need
to specify the category (if this does not already exist it will be created). We can now select OK.
4. This returns us to the installer where we have the option of specifying a window specification name
other than windowSpec. Remember windowSpec will be used as the default window layout
when a window is opened. This is done by typing in a new name into the box below the prompt
saying “or enter new Selector”.
5. We are now ready to define any action methods, accessor method etc. for the window. This is done
using the Define button on the canvas tool. This causes the definer window to be displayed (as
illustrated in Figure 27.9).
6. You are now ready to define any additional initialization code required. Once this is done you can
start the window using the Open button on the canvas tool.
7. Any subsequent editing of the window follows a similar pattern. However you will not be prompted
for the superclass nor category of the class again.
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