Smalltalk and Object Orientation: an Introduction - Free

28.3.3 The “aspects” protocol
The next thing we will do is to look at the methods which have been defined for us by the User interface
builder in the “aspects” protocol. If you examine this protocol you should find two methods defined.
One called address and o ne called name. If you examine these methods you should find that their
definition matches those presented below:
address
"This method was generated by UIDefiner. Any edits made here may be lost whenever
methods are automatically defined. The initialization provided below may have been
preempted by an initialize method."
^address isNil
ifTrue: [address := String new asValue]
ifFalse: [address]
We shall stop for a moment and consider what this method actually says. The method possesses one
expression, the result of which will be returned when the method completes. The value will returned is
dependent on the result of the isNil test. If the contents of the instance variable address “is nil”
then the ifTrue: clause will execute, if not then the ifFalse: clause will execute. We shall take
the false case first. If address does not contain nil then the value of address is returned by the
ifFalse: clause. However, if the value of address is nil, then the assignment expression in the
ifTrue: clause is executed. This assignment expression creates a new instance of the class String.
It then sends the message asValue to this new string instance. This creates a value holder around the
string. This value holder with string construct is then assigned to the instanc e variable address. The
result of this expression is the value of assignment which is returned.
Thus, if the address instance variable has not yet been set (indicated by the value nil) an
appropriate value for it is generated, otherwise its value is return ed. This form of expression is referred
to as lazy initialization . That is, address is initialized appropriately when it is required, rather than
when the system is initialized. This can be useful sometimes, however it does mean that every time the
value of address is obtained an extra expression must be considered (i.e. isNil). This means that is
less efficient than initializing the value of address appropriately in an initialize method. The
accessor method for the instance variable name is defined in a similar manner.
28.3.4 The “initialize” protocol
Now let us define the “initialize” method protocol. This method protocol will only possess one method,
initialize. This method is presented below. It instantiates a Dictionary object to use with the
addressBook insta nce variable. Notice that once again we do not access the instance variable
directly. Instead we use an updater method.
initialize
"This method is called whenever an instance of Organizer is created"
| adBook |
"ApplicationModel defines its own initialize so send a message to super"
super initialize.
"Next we set up the address book instance variable"
adBook := Dictionary new.
self addressBook: adBook
As we are currently using lazy initialization with the ‘aspect’ methods name and address we have
not init ialized them here. However, as was indicated above, it would be more efficient to do so.
However, it must be remembered that they are “aspect” variables rather than plain instance variables.
This means that for the windowing operations to work correctly they must contain a value holder, which
acts as a wrapper around the actual value they represent (see the last chapter for more details).
A common mistake (and the cause of much frustration) is to initialize the aspects in an initialize
method as a String or a Number (with no value holder). This means that the “aspect” method
isNil test fails and the String or Number etc. is returned. The problem is that this is not a value
holder and thus cannot respond to input and output properly. Often this manifests itse lf when values fail
to update themselves or updates from the user fail to propagate to the instances etc. Thus appropriate
initialization should follow that used in the aspect methods, e.g.
address := String new asValue.
name := String new asValue.
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