Smalltalk and Object Orientation: an Introduction - Free

The aspect methods can then be simplified to just return the contents of the associated instance variable.
28.3.5 The “actions” protocol
Another set of methods defined by the User interface builder are the “actions” methods. These are in the
“action” protocol. We have already seen examples of these in the VisualOrganizer class. As you
saw there, by default, these methods are defined to return the value of self. We therefore need to define
them.
We shall first look at the doExit method as this is the simplest method. This method will close the
window. To do this we again use the inherited closeRequest method. We therefore merely need to
send the message closeRequest to self.
doExit
"This method closes the associated window"
self closeRequest.
Next we will look at the doInput method. This method takes input from both the name input field
and the address input field and stores them using the newAddress:for: method which we will
define later. Notice that once again we use an intermediate updater method to actually access any
instance variables (this is good Smalltalk style). The doInput method is presented below.
doInput
"The method is executed when the input button is pressed"
| aName anAddress |
aName := name value.
anAddress := address value.
self newAddress: anAddress for: aName.
Notice that we do not just access the name and address aspects (instance variables) and save them
into the temporary variables aName and anAddress. Instead, we send the message value to each of
the aspects first. This is because, if we just took the contents of name and address we would obtain a
ValueHolder object. What we really want is the contents of the value holder. To get this we must
send the message value to the value holder. Thus name value , will return the value held by the
value holder in the instance variable name.
Remember when you type this method in and accept it, the system will notify you that the method
newAddress:For: is undefined. We will define it later!
Next we will examine the doQuery action method. This method accesses the name input field and
updates the address input/output field. Again we must use the message value to access the contents
of the name value holder. We then use the addressFor: accessor method (which we will define
below) to obtain the address assoc iated with the name. If the address retrieved is not nil then we put
the address retrieved into the address input/output field. If it is nil we display an okay dialog with an
appropriate warning message.
doQuery
"The method is executed when the input button is pressed"
| aName anAddress |
aName := name value.
anAddress := self addressFor: aName.
anAddress isNil
ifTrue: [Dialog warn:'There is no address for ' , name value.]
ifFalse: [address value: anAddress asText.]
We will ju st examine the last statement in the method for a moment. This time we used the message
value: rather than the message value with the value holder in address. This is because we were
updating the contents of the value holder rather than accessing it. Thus the address value holder is set to
the value of the parameter sent with the message value:. This parameter was actually the string
returned by the addressFor: method. However, to be displayed it must be converted into a Text
object (which unde rstands how to display strings). This is achieved by sending the string in
anAddress the message asText. This is necessary because the text editor field assumes that it will
display a text (which has special properties such as font and style) rather than j ust a plain string. When
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