一个关于鼠标消息处理的问题！！！

Question

本人最近看了<<windows程序设计>>这本书  在第七章286页的代码中 有一个函数不知道他为什么要调用.这个函数我用#号 标注了    晴高手指教！！！！！
#include <windows.h>
        LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM) ;
        int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
        
                                                        PSTR szCmdLine, int iCmdShow)
        
{
        
           static TCHAR szAppName[] = TEXT ("BlokOut1") ;
        
           HWND                                 hwnd ;
        
           MSG                                  msg ;
        
           WNDCLASS                      wndclass ;
        
   
        
           wndclass.style                                       = CS_HREDRAW | CS_VREDRAW ;
        
           wndclass.lpfnWndProc                                 = WndProc ;
        
           wndclass.cbClsExtra                                  = 0 ;
        
           wndclass.cbWndExtra                                  = 0 ;
        
           wndclass.hInstance                                   = hInstance ;
        
           wndclass.hIcon                                       = LoadIcon (NULL, IDI_APPLICATION) ;
        
         wndclass.hCursor                                     = LoadCursor (NULL, IDC_ARROW) ;
        
           wndclass.hbrBackground                       = (HBRUSH) GetStockObject (WHITE_BRUSH) ;
        
   wndclass.lpszMenuName                        = NULL ;
        
           wndclass.lpszClassName                       = szAppName ;
        
        
        
           if (!RegisterClass (&wndclass))
        
    {
        
                  MessageBox (  NULL, TEXT ("Program requires Windows NT!"),
        
                                                                        szAppName, MB_ICONERROR) ;
        
            return 0 ;
        
           }
        
   
        
           hwnd = CreateWindow (szAppName, TEXT ("Mouse Button Demo"),
        
                   WS_OVERLAPPEDWINDOW,
        
                    CW_USEDEFAULT, CW_USEDEFAULT,
        
                    CW_USEDEFAULT, CW_USEDEFAULT,
        
                    NULL, NULL, hInstance, NULL) ;
                   ShowWindow (hwnd, iCmdShow) ;
        
           UpdateWindow (hwnd) ;
        
   
        
           while (GetMessage (&msg, NULL, 0, 0))
        
           {
        
                  TranslateMessage (&msg) ;
        
                  DispatchMessage (&msg) ;
        
           }
        
           return msg.wParam ;
        
}
        void DrawBoxOutline (HWND hwnd, POINT ptBeg, POINT ptEnd)
        
{
        
           HDC hdc ;
        
           hdc = GetDC (hwnd) ;
        
           SetROP2 (hdc, R2_NOT) ;
        
           SelectObject (hdc, GetStockObject (NULL_BRUSH)) ;
        
           Rectangle (hdc, ptBeg.x, ptBeg.y, ptEnd.x, ptEnd.y) ;
        
   
        
           ReleaseDC (hwnd, hdc) ;
        
}
        
LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam,LPARAM lParam)
        
{
        
           static BOOL   fBlocking, fValidBox ;
        
           static POINT  ptBeg, ptEnd, ptBoxBeg, ptBoxEnd ;
        
           HDC                                  hdc ;
        
           PAINTSTRUCT  ps ;
        
           switch (message)
        
           {
        
           case   WM_LBUTTONDOWN :
        
                  ptBeg.x = ptEnd.x = LOWORD (lParam) ;
        
                  ptBeg.y = ptEnd.y = HIWORD (lParam) ;
        
        
       
                  DrawBoxOutline (hwnd, ptBeg, ptEnd) ;
        
        
        
                 SetCursor (LoadCursor (NULL, IDC_CROSS)) ;
        
        
        
                  fBlocking = TRUE ;
        
                  return 0 ;
                   case   WM_MOUSEMOVE :
        
                  if (fBlocking)
        
                  {
        
                                 SetCursor (LoadCursor (NULL, IDC_CROSS)) ;
        
             
        
                  ##########    DrawBoxOutline (hwnd, ptBeg, ptEnd) ;#######
        
            
        
                                  ptEnd.x = LOWORD (lParam) ;
        
                                  ptEnd.y = HIWORD (lParam) ;
        
             
        
                                  DrawBoxOutline (hwnd, ptBeg, ptEnd) ;
        
                  }
        
                  return 0 ;
        
        
        
           case   WM_LBUTTONUP :
        
                  if (fBlocking)
        
                  {
        
                                  DrawBoxOutline (hwnd, ptBeg, ptEnd) ;
        
             
        
                                  ptBoxBeg                             = ptBeg ;
        
                                  ptBoxEnd.x                           = LOWORD (lParam) ;
        
                                  ptBoxEnd.y                           = HIWORD (lParam) ;
        
            
        
                                  SetCursor (LoadCursor (NULL, IDC_ARROW)) ;
        
                                  fBlocking                            = FALSE ;
        
                                  fValidBox                           = TRUE ;
        
             
        
                                  InvalidateRect (hwnd, NULL, TRUE) ;
        
                  }
        
                  return 0 ;
        
        
        
           case   WM_CHAR :
        
                  if (fBlocking & wParam == '\x1B')       // i.e., Escape
        
                  {
        
                                  DrawBoxOutline (hwnd, ptBeg, ptEnd) ;
        
             
        
                                  SetCursor (LoadCursor (NULL, IDC_ARROW)) ;
        
            
        
                                  fBlocking = FALSE ;
        
                  }
        
                  return 0 ;
                   case   WM_PAINT :
        
                  hdc = BeginPaint (hwnd, &ps) ;
                   if (fValidBox)
        
                  {
        
                                         SelectObject (hdc, GetStockObject (BLACK_BRUSH)) ;
        
                                         Rectangle     (      hdc, ptBoxBeg.x, ptBoxBeg.y,
        
                                                                            ptBoxEnd.x, ptBoxEnd.y) ;
        
                  }
        
        
        
                  if (fBlocking)
        
                  {
        
                                  SetROP2 (hdc, R2_NOT) ;
        
                                  SelectObject (hdc, GetStockObject (NULL_BRUSH)) ;
        
                                  Rectangle (hdc, ptBeg.x, ptBeg.y, ptEnd.x, ptEnd.y) ;
        
                  }
        
        
        
                  EndPaint (hwnd, &ps) ;
        
                  return 0 ;
                   case   WM_DESTROY :
        
                  PostQuitMessage (0) ;
        
                 return 0 ;
        
    }
        
           return DefWindowProc (hwnd, message, wParam, lParam) ;
        
}

Answer 1

注意：           SetROP2 (hdc, R2_NOT) ;你把那句话注释掉试试？

Answer 2

看名字也知道了,应该是画控件边框的函数
DrawBoxOutline (hwnd, ptBeg, ptEnd) ;#######ptBeg是定部部分,ptEnd是底部部分,其实就是当鼠标移到上面时,显示控件阴影效果

Answer 3

楼上：如果去掉第一个DrawBoxOutline函数，执行效果会变的很奇妙，不知道什么原因，可以说说里面的实质么？建议你调试一下。

Answer 4

跟你说了，关键在于设置了光栅操作码R2_NOT，那么那句话就是在上次的位置再绘制一遍边框（NOT方式），其效果就是消除上次的边框。如果注释掉，上次的边框就不能消除，因此就看到你所谓的奇妙的效果。