Smalltalk and Object Orientation: an Introduction - Free

27.3.4 Widget layout control
The widget layout control butto ns are presented across the top of the canvas tool window. The first six
buttons control widget alignment, the next four control widget distribution and the last two control
widget size. Each of the operations is relative to the first widget selected. For example, the first button
will align all the currently selected widgets such that their tops are all in line with the very first widget
selected.
27.3.5 Properties, aspects and actions
The properties button on the canvas tool brings up the properties tool. This tool displays the information
appropriate for the selected widget. If no widget is selected then the window details are selected.
Figure 27.5: The Properties tool
The properties tool allows the user to specify a wide range of details about a widget. For example, it
is possible to specify any associated colors, position, etc. The precise details of what can be defined
depends on the particular widget.
Figure 27.5 illustrates the properties tool for an action button. In Figure 27.5 the developer has
specified that the selected button should have a label ‘ Add’ (this is a textual label but equally the label
could be a graphic image) and the action which is the symbol #doAdd. The action is the name of the
message which will get sent to the object displaying the window when the button is pressed. The
developer therefore defines this method to specify the add buttons’ behavior.
In a similar manner other widget’s details are defined using the properties tools. For example, for an
input/output field the tool would request the field’s name, and whether it should be output only or input
and output (buttons and input/output fields are probably the most common widgets that you will use).
The name of the input field is referred to as its aspect. Essentially it is the name of an instance variable
which will be used to hold the field’s data. Other common widgets are the list widget which displays the
contents of a list and table widgets which display data held in tabular form. For further details on the
properties tool see the VisualWorks manuals.
27.4 Window specification and creation
Once the developer has designed the window to their satisfaction you can instal l it onto a class
(essentially this means that you can compile it into Smalltalk). This is done using the ‘Install’ button on
the canvas tool. This button triggers the installer (illustrated in Figure 27.6). The develop er must then
specify the name of the class to install the window onto. If the class does not already exist it will be
created by the system. In such a situation the developer must select what category to place the class into
and what the superclass of the class will be.
In almost all circumstances the superclass of the new class will be ApplicationModel, or one
of its subclasses. ApplicationModel is an abstract class that provides the functionality required to
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