843: Highlights

If you want to build an application, then use the VisualWorks GUI builder to create a subclass of the ApplicationModel model class to serve as the hub.
The VisualWorks GUI builder generally installs value holders in instance variables of the application model. The value-holder installation is done by an application-model getter that creates a value holder the first time that it is called. The getter exhibits the following general pattern, in which the selector and instance variable have the same name:
```
selector 
  ^ instance variable isNil 
      ifTrue: [instance variable := initial value asValue] 
      ifFalse: [instance variable] 
```
In typical applications, controllers send value-setting messages to value holders, which then send change messages to themselves, initiating the propagation of update messages to viewers, which look back into the value holders in the course of redrawing themselves.
In complex applications, the application model, with value holders and aspect adapters, separates domain-model classes from viewer and controller classes. This separation is the essence of the model–viewer–controller paradigm.