about offscreen bitmap

#include "bitmapmethods.h"
#include <game.mbg>

_LIT(KGameMbm, "\\system\\apps\\game\\game.mbm");

CGameMainContainer::CGameMainContainer()
: fade(ETrue), aBlackMap(0), aWhiteMap(255)
{
}

void CGameMainContainer::ConstructL(const TRect& /*aRect*/)
{
CreateWindowL();
TRect screenRect = CEikonEnv::Static()->EikAppUi()->ApplicationRect();
LoadArtL();
iPeriodicTimer = CPeriodic::NewL(CActive::EPriorityStandard);
StartTimerL();
SetRect(screenRect);
ActivateL();
}

void CGameMainContainer::StartTimerL()
{
if(!iPeriodicTimer->IsActive())
iPeriodicTimer->Start(0, 500, TCallBack(CGameMainContainer::Tick, this));
}

void CGameMainContainer::StopTimer()
{
if(iPeriodicTimer)
iPeriodicTimer->Cancel();
}

void CGameMainContainer::LoadArtL()
{
    iBitmap = NBitmapMethods::CreateBitmapL(KGameMbm, EMbmGameTitle);
// Create the off screen bitmap and device / gc
iOffScreenBitmap = NBitmapMethods::CreateBitmapL(Rect().Size(),KColourDepth);
iOffScreenBitmapDevice = NBitmapMethods::CreateBitmapDeviceL(*iOffScreenBitmap);
iOffScreenBitmapGc = NBitmapMethods::CreateGraphicsContextL(*iOffScreenBitmapDevice);
}

void CGameMainContainer::Draw(const TRect& aRect) const
{
UpdateDisplay();
}

void CGameMainContainer::UpdateDisplay() const
{
CWindowGc& gc = SystemGc();
//gc.Clear(aRect);

if(fade)
{
//gc.SetFadingParameters(aBlackMap, aWhiteMap);
//gc.SetFaded(ETrue);
//gc.BitBlt(TPoint(0,0), iBitmap);
iOffScreenBitmapGc->SetFadingParameters(aBlackMap, aWhiteMap);
iOffScreenBitmapGc->SetFaded(ETrue);
iOffScreenBitmapGc->BitBlt(TPoint(0,0), iBitmap);
gc.BitBlt(TPoint(0,0),iOffScreenBitmap);
}
//draw main screen
else
{
//gc.SetFaded(EFalse);
//gc.BitBlt(TPoint(0,0), iBitmap);
iOffScreenBitmapGc->SetFaded(EFalse);
iOffScreenBitmapGc->BitBlt(TPoint(0,0), iBitmap);
gc.BitBlt(Rect().iTl,iOffScreenBitmap);
}
}

TInt CGameMainContainer::Tick(TAny* aObject)
{
//cast and call a non-static function
((CGameMainContainer*)aObject)->DoFade();
return TRUE;
}

void CGameMainContainer::DoFade()
{
CWindowGc& gc = SystemGc();
if(iPeriodicTimer->IsActive())
{
if(aBlackMap < aWhiteMap)
{
aBlackMap += 5;

gc.Activate(*DrawableWindow());
UpdateDisplay();
gc.Deactivate();
//DrawDeferred();
}
else
{
fade = EFalse;
StopTimer();

gc.Activate(*DrawableWindow());
UpdateDisplay();
gc.Deactivate();
//DrawDeferred();
}
}
}