Chapter 1

In theory, then, you don't need to code any new functions in order to do an applicationinitiated
redraw. You can simply call DrawNow() so that your Draw() function is called in
turn.
Where to draw
It's possible that only a part of your control will need to be drawn (or redrawn). To
understand this, you need to distinguish between the four regions shown in Figure 11.6:
Figure 11.6
Your control is part of a window. The window server knows about the window and knows the
regions of the window that are invalid – that is, the parts that need to be redrawn. Your
Draw() function must draw the entire invalid region, but it must not draw outside the
boundary of the control.
The window server will clip drawing to the invalid region – which is clearly bounded, in turn,
by the boundary of the window itself.
Important
But if your control doesn't occupy the entire window, you are
responsible for ensuring that your redraw doesn't spill beyond the
boundaries of the control.
Often, this turns out to be not too onerous a responsibility: many controls such as buttons
and the various sections of the Battleships application screen, draw rectangles, lines, and
text are guaranteed to be inside the control's boundary in any case.
In the few cases in which this doesn't happen, you can issue a SetClippingRect() call to
the graphics context that ensures that future drawing is clipped to the control's rectangle.
Here's an example, from the drawing example developed in Chapter 15:
aGc.SetClippingRect(aDeviceRect);
aGc.SetPenColor(KRgbDarkGray);
aGc.DrawRect(surround);
This is necessary because surround could have been bigger than aDeviceRect, which is
the region of the control that this code is allowed to draw into. You can cancel this later, if
you wish, with CancelClippingRect(), but since CGraphicsContext::Reset() does
this anyway and Reset() is called prior to each control's Draw(), you don't need to do this
explicitly from a control.