Smalltalk and Object Orientation: an Introduction - Free

While this may seem to be a complicated (and rat her convoluted) way to change the value of part of
the model, it has many advantages in flexibility and modularity. No longer does the model have to
inform the interface (view) about any updates; the ValueHolder does it automatically wh enever the
value it holds changes. Indeed, it is no longer necessary for the model to concern itself with any aspect
of the view (or controller) at all. The model now really is separated from the view and controller. This
makes the task of developing highly graphical systems much easier.
27.9.2 Working with value holders
If the user interface builder described above is used to construct a window and the definer has been used
to generate the instance variables and accessor methods for the windows aspects, then valu e holders for
these instance variables will automatically be created. However, it is still possible to do this manually
(and may be used with non interface instance variables).
A new value holder can be created in two ways:
1. By creating an instance of the class ValueHolder. For example,
balance := ValueHolder with: 230.
2. By converting an object to a value holder and object. For example,
balance := 230 asValue.
Whichever approach is used, the contents of the value holder is accessed and up dated in the same
way. Accessing the valueholder’s contents is performed using the value message and updating it is
performed using the value: message. For example:
currentBalance := balance value.
balance value: (currentBalance + deposit).
If a new value is assigned to a value holder the value: message must be used, otherwise the value
holder will be deleted and the link to a window (if present) and to any dependents will be lost.
27.10 The onChangeSend:to: message
In an earlier chapter we discussed the use of the dependency mechanism as a means of informing
interested parties about a change in the value of something. Value holders take this concept one step
further and makes it more explicit. Whenever the value of a value holder is modified using the value:
message a check is made to see whether there are any interests recorded on that value holder. If there,
then the associated action is performed.
The interest in a value holder is registered using the onChangeSend: aSymbol to:
anObject message. This message states that when the value holders value changes (via value:) the
symbol is sent to the object as a message. A common use of this message is to ensure that some action
is performed whenever the user changes a value currently being displayed. For example:
balance onChangeSend: #checkInTheRed to: currentAccount
states that when the value holder held in balance is modified, the message checkInTheRed should
be sent to the object held in currentAccount. Note that unlike the version of the dependency
mechanism discussed earlier, this approach explicitly states what action should be performed on what
object when a particular value changes.
If at a later date the dependency between the value holder and the message send to the specified
object should be retracted, the (less meaningfully named) retractInterestsFor: anObject
message can be used.
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