Overcome Window Flicker While Dragging

While I use the CImageList control for dragging, I found it flickers in the owner window while draging and redraw the client region. I tried Google to find a solution, but there were no result, so I try to solve it myself.

Here's my solution, which I'm very glad share with you!

First, you should register a new window class and create a new top most tool window, and move it's position far off screen(I take 99000), so it's not visible. The code would look like this:

BOOL CImageDragWrapper::Initialize()
{
   if(m_hDragWnd)
   {
      return TRUE;
   }

   WNDCLASSEX wcex;
   wcex.cbSize = sizeof(WNDCLASSEX); 

   if(!::GetClassInfoEx(AfxGetInstanceHandle(), POPWINDOWCLASSNAM, &wcex))
   {   
      wcex.style           = CS_HREDRAW | CS_VREDRAW;
      //specifies default window procedure 
      wcex.lpfnWndProc     = (WNDPROC)DefWindowProc;
      wcex.cbClsExtra      = 0;
      wcex.cbWndExtra      = 0;
      wcex.hInstance       = AfxGetInstanceHandle();
      wcex.hIcon           = LoadIcon(0, IDI_INFORMATION);
      wcex.hCursor         = LoadCursor(NULL, IDC_ARROW);
      wcex.hbrBackground   = (HBRUSH)(COLOR_WINDOW+1);
      wcex.lpszMenuName    = 0;
      wcex.lpszClassName   = POPWINDOWCLASSNAM;
      wcex.hIconSm         = LoadIcon(0, IDI_INFORMATION);   
      RegisterClassEx(&wcex);
   }

   m_hDragWnd = CreateWindowEx(WS_EX_TOPMOST | WS_EX_TOOLWINDOW, POPWINDOWCLASSNAM,
                        _T(""),WS_POPUP | WS_DISABLED |  WS_CLIPSIBLINGS,99000, 
                        0, 6, 6, 0,   0,
                        AfxGetInstanceHandle(),   0   );

   if(m_hDragWnd)
   {
      SetLayeredWindowAttributes(RGB(255,255,255), 100,   /*0x00000001|*/0x0000002);

      ShowWindow(m_hDragWnd, SW_SHOW);
      UpdateWindow(m_hDragWnd);
   }
   return m_hDragWnd != 0;
}

Second, move the window to properly position and paint it's client. The code would look like this:

BOOL CImageDragWrapper::DragBegin(HWND hDragWndOwner,int cxWnd, int cyWnd, 
                int cxHotSpotOffset, int cyHotSpotOffset,
                const POINT& ptCurrentMousePos, HBITMAP hBack, 
				int cxBackOffset, int cyBackOffset)
{
   if(m_hDragWnd)
   {   
      m_hDragWndOwner = hDragWndOwner;
      m_cxHotSpotOffset = cxHotSpotOffset;
      m_cyHotSpotOffset = cyHotSpotOffset;

      POINT ptDest = ptCurrentMousePos;
      ClientToScreen(m_hDragWndOwner, &ptDest);

      ::MoveWindow(m_hDragWnd,
                ptDest.x - m_cxHotSpotOffset, 
                ptDest.y - m_cyHotSpotOffset, 
                cxWnd,
                cyWnd, TRUE);

      //Draw background image for dragging support window
      if(hBack)
      {
         HDC hDC = ::GetDC(m_hDragWnd);
         HDC hMemDC = ::CreateCompatibleDC(hDC);
            
         HBITMAP hbmpOldMem = (HBITMAP)::SelectObject(hMemDC, hBack);
         
         ::BitBlt(hDC, 0, 
               0, 
               cxWnd, 
               cyWnd, 
               hMemDC, 
               cxBackOffset, 
               cyBackOffset, 
               SRCCOPY);         

         ::SelectObject(hMemDC, hbmpOldMem);
         ::DeleteDC(hMemDC);

         ::ReleaseDC(m_hDragWnd, hDC);
      }
   }
   return m_hDragWnd != 0;
}

Third, changes the position and dimensions for dragging support window by calling MoveWindow

Forth, move dragging to support the window's postion far to screen. The code would look like this:

BOOL CImageDragWrapper::DragEnd()
{
   if(m_hDragWnd && m_hDragWndOwner)
   {
      m_hDragWndOwner = 0;
      ::MoveWindow(m_hDragWnd,
                   99000, 
                   0, 
                   6,
                   6, TRUE);
   }
   return m_hDragWnd != 0;
}

Using the Code:

First, in the InitInstance method of CWinApp's subclass add code like this:

   CImageDragWrapper::Initialize();
   CTestDragDialogDlg dlg;
   m_pMainWnd = &dlg;
   int nResponse = dlg.DoModal();
   if (nResponse == IDOK)
   {
      // TODO: Place code here to handle when the dialog is
      //  dismissed with OK
   }
   else if (nResponse == IDCANCEL)
   {
      // TODO: Place code here to handle when the dialog is
      //  dismissed with Cancel
   }

   CImageDragWrapper::Destroy();

Then in dragging the owner window's OnMouseMove, OnLButtonUp method calls DragBegin, DragMove, and DragEnd method of CImageDragWrapper, It's very simple.

Note: you can compare CImageDragWrapper with CImageList by using my sample code!