Drawing -in traditional architecture

The application framework callsCCoeControl::Draw when a control area needs -to be updated on the display. Controls may implement CCoeControl::Draw or leave the drawing -to their child controls. For more information on control hierarchies, see The run-time control hierarchy. The platform calls CCoeControl::Draw for the parent control -first, and then recursively for each control.

Controls should override CCoeControl::Draw to draw their content. -The override should do nothing else and should be as fast as possible. For -example, it is bad design to create fonts dynamically, and read any bitmaps -or resources while drawing. A good rule of thumb is that there should not -be trap handlers in the method override; any time-consuming functionality -that can be done beforehand should be cached.

In most cases controls are drawn on the display using the screen device -graphics context, accessed with CCoeControl::SystemGc(). The graphics context provides a wide set of GDI (Graphics -Device Interface - common Symbian platform graphics API) drawing primitives -that can be used for drawing virtually anything on screen.

An example of a basic override of CCoeControl::Draw for a control that is -a top-level window in an application is as follows:

Double buffering -

For controls that perform intensive drawing operations, the drawing -should be cached: a process also known as double-buffering. Here the drawing -is done to a memory context first and then in the CCoeControl::Draw method only the context's -bitmap is passed to screen. In the Symbian platform, the easiest way to implement -a double buffer is to create a CFbsBitmap and -then bind a graphics context to that - this makes it possible to use the -same GDI interface for drawing as the display context. The drawing is done -to a memory bitmap buffer. Double-buffering is a common paradigm used in games, -but can also be utilized in any application when performance of drawing controls -is important.

The following is a short example of how a double buffer is created and -used:

-iGcBmp = new (ELeave) CWsBitmap(iEikonEnv->WsSession()); -User::LeaveIfError(iGcBmp->Create(aClientRect.Size(), iEikonEnv->ScreenDevice()->DisplayMode())); -iGcDevice = CFbsBitmapDevice::NewL(iGcBmp); -User::LeaveIfError(iGcDevice->CreateBitmapContext(iGc)); - -

iGcBmp is a pointer to CWsBitmap, -the bitmap memory buffer, that is created with the same width and height as -the top-level window and with the same color bit depth as the display. iGcDevice is -a pointer to the CBitmapDevice device -class and the context iGc holds the CBitmapContext instance. iGc is then used instead of CScreenGc, obtained from the CCoeControl::SystemGc() method, when the -control draws itself. The double-buffer drawing should be done outside of -the CCoeControl::Draw method and can be called -directly when needed. Only at the end of the off-screen drawing is the memory -buffer flushed to the screen by calling CCoeControl::DrawDeferred()

-void CMyDrawingExample::Draw(const TRect& /*aRect*/) const - { - SystemGc().BitBlt(TPoint(0, 0), iGcBmp); - } -