think-cell technical report TC2003/01 A GUI-based Interaction ...

5.1 Program Architecture IMPLEMENTATION
driven by the document window.
5.1.2 Hierarchy and Interfaces of UI Classes
Some selected class diagrams are depicted in figure 38. The root class CUIObject
holds a reference to its document window and offers an interface to process all
kinds of low level user interface messages. The subclasses realize different UI wid-
gets by implementing the processing of those messages differently. In particular,
the subclasses of CHotZone offer specific functions that provide a high-level in-
terpretation of the low-level UI input, based on notions like hit test, mouse capture
and the like. For example, in the case of the CHotButton class, a sequence
of the OnLButtonDown()/OnLButtonUp() mouse messages triggers a call to the
OnClick() method, if at the times of both events the mouse pointer is found within
the UI object’s responsive area.
The application specific methods like OnClick() are abstract, meaning that the
respective class itself cannot be instantiated. To use the functionality provided by
the UI classes, each UI widget is programmed as its own subclass, implementing its
specific behavior in those abstract methods. Other UI components are too specific to
be derived from CHotZone or its subclasses. Examples are CFeatureOutline and
CUIGridLine. These classes implement the interpretation of low-level messages
themselves, deriving directly from class CUIObject (Fig. 38(a)).
The inheritance graph of CVisualElement (Fig. 38(b)) partly parallels the
hierarchy of subclasses of CUIObject. Most UI objects have some visual represen-
tation that is made up of one or more visual elements. The methods and attributes
of visual elements serve to create an appropriate texture mapping, transforming
coordinates from a precompiled texture bitmap to document-oriented coordinates
and finally to on-screen coordinates, taking zooming and scrolling into account. The
subclasses of CVisualElement are tailored to their specific use with the objective
to minimize the interface to their respective subclass of CUIObject.
As can be seen in figure 37, features are contained by a smart element and
smart elements are contained by a container. Now, figure 38(c)) shows that smart
elements also are features and that containers also are smart elements. The class
CSmartElement derives from CFeature, because the latter provides function-
ality to show user interface components, and smart elements need to have a user
interface, too. On the other hand, the relationship between CSmartElement and
CContainerSE enables hierarchical layouts with gridlines at different levels.
5.1.3 Windows Message Processing
User input processing in my prototype is based on Microsoft Windows messages.
In Windows, messages are responsible for inter-process communication. Messages
that originate in the operating system are queued in thread-specific message queues
and are processed whenever a thread makes a system call to PeekMessage() or
GetMessage(). The user thread then typically makes a call to DispatchMessage(),
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