C / C++ / MFC

On WM_LBUTTONDOWN...
 
::SetCapture(hwnd);
 
MSG Msg;
memset(&Msg,0,sizeof(MSG));
while (Msg.message != WM_LBUTTONUP &&
            (Msg.message != WM_MOUSEMOVE ||
               (Msg.wParam & MK_LBUTTON)))
{
   if (::PeekMessage(&Msg,0,0,0,PM_REMOVE))
   {
      ::TranslateMessage(&Msg);
      ::DispatchMessage(&Msg);
   }
  
   // Increment displayed field
   // Call ::InvalidateRect(hWnd, NULL, bErase) on displayed field control
   // Call ::UpdateWindow(hWnd) on displayed field control
}
 
::ReleaseCapture();

// (pseudocode)

On WM_LBUTTONDOWN
   Capture the mouse (::SetCapture())
   Create a timer for the desired interval of increments (::SetTimer())
On WM_TIMER   
   Increment displayed field
   Call ::InvalidateRect(hWnd, NULL, bErase) on displayed field control
   Call ::UpdateWindow(hWnd) on displayed field control
On WM_LBUTTONUP
   Kill the timer (::KillTimer())
   Release the mouse (::ReleaseCapture())