From Forum Nokia Wiki

A custom contral generally overrides the CCoeControl::Draw() method to implement its own drawing stuff. However, when stepping into the code with the debugger, the result of each command is not displayed immediatly on screen because of the cache mechanism implemented by the window server.

Happily enough, you can get rid of this behaviour, and have each drawing function being rendered immediatly on screen. Add the following code in your AppUi::ConstructL():

#ifdef _DEBUG

iEikonEnv->WsSession().SetAutoFlush(ETrue);

#endif

This force each Graphic Context draw commands to show up immediately, rather than when the window server client buffer is next flushed.

This means you can step through the draw code and see the effect each line is having. However, you must ensure this line does not make it into released software as it has efficiency implications.

For more tips please visit this link Debugging Tips and Tricks