Smalltalk and Object Orientation: an Introduction - Free

5.3 Inspectors
Another type of tool available in the Smalltalk environment are Inspectors. Inspectors permit the
internal state of an individual object to be viewed. I n particular they allow the user to view and change
instance variables directly. In effect inspectors are to instances as browsers are to class definitions.
Unlike other programming language development tools, you can use an inspector to change the state of
an instance (for example, to set up an object ready for testing a piece of code) as well as examine its
contents.
The inspector window is divided into two as shown in Figure 5.2. The left hand side s hows the
instance variables of the object, while the right hand side shows the contents of those variables. For
example, in the figure, the prefs instance variable has been selected (in the left hand side) while the
dictionary contained by that instance va riable is displayed (in the right hand side). If the contents is a
composite object (that is an object which contains other objects e.g. such as the dictionary shown), then
you can open another inspector which will present the contents of that object. To d o this you select the
inspect option off the right (middle on the three button mouse) button menu in the left hand side.
The right hand view of the window not only allows you to examine and change the contents of
instance variable, it also allows you to evaluate expressions (just as you can in a Workspace) except that
the evaluation happens within the context of the instance. This means that instance and class variable s
can be referenced in the same way as they can within any method definition.
Note that two inspectors are built into the bottom of the debugger window and can be extremely
useful when attempting to decide why something unexpected has happened.
The use of inspectors provides a powerful debugging and testin g tool. All objects respond to the
message inspect; the basic inspector method is implemented in the class Object’s instance
protocol. Most objects respond by opening an inspector window, labeled with the class of the receiver.
Figure 5.2: An example inspector
5.4 Notifiers
You should also get familiar with the system notifier, such as those you will encounter when you type in
an incorrect Smalltal k expression. These notifiers are there to help you. For example, in Figure 5.3, I
have mis-typed show, the system recognizes this and informs me of the fact. If I select the correct it
option, it will try to find what it thinks I was trying to type. In this case it will give a select list
containing the show: message, which I can select. This will replace my misspelling and the code will
successfully execute.
Figure 5.3: A system notifier
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