Chapter 1

Figure 11.1
As it happens, this screen includes two windows – the app view and the button bar. The app
view is a single control, while the button bar comprises several controls such as
� a container for the whole button bar, which is a compound control;
� component controls, including the four buttons.
This application already allows us to make some generalizations about controls:
� For an application, a control is the basic unit of GUI interaction: a control can do any
sensible combination of drawing, pointer handling, and key handling.
� A window is the basic unit of interaction for the system: controls always use all or part
of a window.
� Controls can be compound: that is, they can contain component controls. A compound
control is sometimes known as a container.
In this chapter, we'll see the role that controls (and windows) play in drawing. We'll continue
to use the hellogui and Battleships applications as examples. In the next chapter, we'll
look more closely at keyand pointer-based interaction.
11.1.2 Walking through Draw()
In Symbian OS, all drawing is done through a graphics context (GC). In this section, we'll
take a closer look at the example function CHelloGuiAppView::Draw() to see how the
GC is used.
Getting the graphics context
The CHelloGuiAppView::Draw() function begins by getting hold of a GC using
SystemGc(), a function in CCoeControl:
CWindowGc& gc = SystemGc();
All graphics context classes are derived from CGraphicsContext. Each derived class –
such as CWindowGc here – is used for drawing on a particular graphics device (in our
example, a window) and implements all the functionality specified by the base class, plus
(and optionally) some extra functionality appropriate for the device in question. Therefore,
we can clear the screen through the graphics context:
gc.Clear();