Smalltalk and Object Orientation: an Introduction - Free

Edit button, then the user interface builder is Launcher for the given specification. If the Start button is
selected then an instance of the application class is created and opened on the specified specification.
Figure 27.13: The Resource Finder
The Resource Finder is a useful way of finding the specifications provided for various classes,
menus, starting applications and editing existing applications.
27.9 ValueHolders
In VisualWorks the biggest departure from the original ParcPlac e version of Smalltalk, which was
known as Smalltalk -80 and then ObjectWorks, is the use of ValueHolders. ValueHolders represent a
modification to the way information is exchanged within the MVC architecture. This modification is
intended to further buffer the user interface from the application model. The idea is that the model
should present only that - a data structure and its associated methods. However in the MVC
architecture, information about the interface often crept into the model. For example, the model would
have to send the change message to itself at the right times and make the appropriate information
available to the view (when the view received the resulting update: message). The use of
ValueHolders is an attempt to remove this concern from the model.
aView
aController
aModel
aValueHolder
anObject
Figure 27.14: The modified MVC architecture in VisualWorks
27.9.1 How they work
The ValueHolder acts as an intermediate object whic h buffers the application model from the actual
objects it maintains. The model now refers to ValueHolders, which refer to the actual objects being
modeled (this is illustrated in Figure 27.14). Thus the model does not make direct reference to other
objects, instead it refers to a ValueHolder. When the model wants to access a value it requests that the
ValueHolder provides it. Similarly, if it wants to update a value it sends a message telling the
ValueHolder to update its value.
When the object referred to by a ValueHolder changes, it is the responsibility of the ValueHolder to
inform the associated view that its value has changed and that the view should display this new value (in
the original MVC it was the responsibility of the model). Similarly, if the controller receives input from
the user which directly changes the value of the ValueHolder, the controller merely informs the value
holder that its value should change.
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