Smalltalk and Object Orientation: an Introduction - Free

29.2 The custom view widget
29.2.1 What is a custom view widget?
Any widget displayed in a window created using the window building facilities of the canvas, the
palette and the canvas tools uses a view to display itself. This view determines what will be displayed,
what the associated view does and how it links with your application. However, not everything you will
wish to do is covered by the widgets provided by the palette. For example, if you wish to display a
network of nodes, then there is no built in widget to do this - you must write the code which will display
such a network using a custom view.
Figure 29.2: The View Holder Palette icon
A custom view is held within a view holder which can be placed onto a window usi ng the user interface
builder. This is done using the view holder icon which is available on the user interface builder’s
palette. It allows the user to place a view holder widget onto the canvas being designed. Such a holder
can take advantage of some of the widget properties defined by the user interface builder such as sliders
etc. However, the developer must supply the majority of the code that connects the graphic contents of
the view holder to the application as well as the code which actually generates the graphics.
29.2.2 How do you create a custom view?
Figure 29.3: Defining a custom view
To create a custom view a user must first place a view holder onto the canvas representing the wind ow
being constructed. For example, in Figure 29.3, we have placed a view holder across the majority of the
window and added a horizontal and vertical slider. This view is then linked to the window via an
instance variable specified in the view’s properties window.
The result of this is that when the user interface window building software encounters this window’s
definition (held in a class side window specification method) it will construct a window with four
buttons and a scrollable view. However that is all that it will do. It will not attempt to perform any
operations to determine what the interior of the view should look like.
29.2.3 Managing a custom view
Associated with this view holder will be a model, a controller and a view instance. As you will note
from these classes custom views are implemented and controlled using the traditional MVC framework
described earlier in this part of the book. Remember:
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