Chapter 1

Words used in MVC
I should finish this section with a word on nomenclature. A 'control' in Symbian OS is not
usually a 'controller' in the MVC sense, which is precisely why the word 'controller' is not
used. A Symbian OS control does, however, usually contain pure MVC view functionality: its
Draw() function draws a model without changing it.
In Symbian OS literature, the word 'view' is used for a control or some drawing/interaction
code to highlight the fact that the 'view' is entirely separate from the 'model'. A good example
is ETEXT and FORM, Symbian OS rich text components:
� ETEXT is a model without views
� FORM provides views but has no model.
We use 'app view' in this sense, while the model is often contained in the document class. In
Battleships, I use 'fleet view' and 'player status view' as the names of my controls, because
the fleet and player status 'model' data is kept in separate classes.
Often, in Symbian OS literature, the word 'model' is used for application data that can be
saved to file.
11.3.2 The Draw() Contract
Symbian OS controls use the Draw() function to implement MVC view functionality.
CCoeControl::Draw() is defined in coecntrl.h as
IMPORT_C virtual void Draw(const TRect& aRect) const;
A derived class will override this virtual function to draw – or redraw – its model. In the rare
cases in which this function is not overridden, there's a default implementation that leaves
the control blank.
Important
Because CCoeControl::Draw() is strictly an MVC view function, it
should not update the model. It is therefore const, and nonleaving.
Your Draw() implementation must not leave.
This is another reason CGraphicsContext functions return void: if they could fail,
Draw() could fail also.
Redraw handling
System-initiated redraw handling starts in the window server, which detects when you need
to redraw part of a window. In fact, it maintains an invalid region on the window, and sends
an event to the application that owns the window, asking it to redraw the invalid region.
CONE works out the control that intersects the invalid region and converts the event into a
call to Draw() for all affected controls. A system-initiated redraw must redraw the model
exactly as the previous draw.
Application-initiated redraw handling starts (by definition) in the application. If you update a
model and need to redraw a control, you can simply call its DrawNow() function.
DrawNow() is a nonvirtual function in CCoeControl that:
� tells the window server that the control is about to start redrawing,
� calls Draw(),
� tells the window server that the control has finished redrawing.