对windows程序设计书中的第七章例子的理解！有点困惑。

/*-------------------------------------------------

   CHECKER3.C -- Mouse Hit-Test Demo Program No. 3

                 (c) Charles Petzold, 1998

  -------------------------------------------------*/



#include <windows.h>



#define DIVISIONS 5



LRESULT CALLBACK WndProc   (HWND, UINT, WPARAM, LPARAM) ;

LRESULT CALLBACK ChildWndProc (HWND, UINT, WPARAM, LPARAM) ;



TCHAR szChildClass[] = TEXT ("Checker3_Child") ;



int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,

                    PSTR szCmdLine, int iCmdShow)

{

     static TCHAR szAppName[] = TEXT ("Checker3") ;

     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 ;

     }

     

     wndclass.lpfnWndProc   = ChildWndProc ;

     wndclass.cbWndExtra    = sizeof (long) ;

     wndclass.hIcon         = NULL ;

     wndclass.lpszClassName = szChildClass ;

     

     RegisterClass (&wndclass) ;

     

     hwnd = CreateWindow (szAppName, TEXT ("Checker3 Mouse Hit-Test 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 ;

}



LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)

{

     static HWND hwndChild[DIVISIONS][DIVISIONS] ;

     int         cxBlock, cyBlock, x, y ;

     

     switch (message)

     {

     case WM_CREATE :

          for (x = 0 ; x < DIVISIONS ; x++)

               for (y = 0 ; y < DIVISIONS ; y++)

                    hwndChild[x][y] = CreateWindow (szChildClass, NULL,

                              WS_CHILDWINDOW | WS_VISIBLE,

                              0, 0, 0, 0,

                              hwnd, (HMENU) (y << 8 | x),

                              (HINSTANCE) GetWindowLong (hwnd, GWL_HINSTANCE),

                              NULL) ;

          return 0 ;

               

     case WM_SIZE :

          cxBlock = LOWORD (lParam) / DIVISIONS ;

          cyBlock = HIWORD (lParam) / DIVISIONS ;

          

          for (x = 0 ; x < DIVISIONS ; x++)

                for (y = 0 ; y < DIVISIONS ; y++)

                    MoveWindow (hwndChild[x][y],

                                x * cxBlock, y * cyBlock,

                                cxBlock, cyBlock, TRUE) ;

          return 0 ;

               

     case WM_LBUTTONDOWN :

          MessageBeep (0) ;

          return 0 ;

          

     case WM_DESTROY :

          PostQuitMessage (0) ;

          return 0 ;

     }

     return DefWindowProc (hwnd, message, wParam, lParam) ;

}



LRESULT CALLBACK ChildWndProc (HWND hwnd, UINT message, 

                               WPARAM wParam, LPARAM lParam)

{

     HDC         hdc ;

     PAINTSTRUCT ps ;

     RECT        rect ;

     

     switch (message)

     {

     case WM_CREATE :

          SetWindowLong (hwnd, 0, 0) ;       // on/off flag

          return 0 ;

          

     case WM_LBUTTONDOWN :

          SetWindowLong (hwnd, 0, 1 ^ GetWindowLong (hwnd, 0)) ;

          InvalidateRect (hwnd, NULL, FALSE) ;

          return 0 ;

          

     case WM_PAINT :

          hdc = BeginPaint (hwnd, &ps) ;

          

          GetClientRect (hwnd, &rect) ;

          Rectangle (hdc, 0, 0, rect.right, rect.bottom) ;

          

          if (GetWindowLong (hwnd, 0))

          {

               MoveToEx (hdc, 0,          0, NULL) ;

               LineTo   (hdc, rect.right, rect.bottom) ;

               MoveToEx (hdc, 0,          rect.bottom, NULL) ;

               LineTo   (hdc, rect.right, 0) ;

          }

          

          EndPaint (hwnd, &ps) ;

          return 0 ;

     }

     return DefWindowProc (hwnd, message, wParam, lParam) ;

}

头一次遇到涉及到子窗口的问题，发现理解起来有困难。
RegisterClass注册了两个窗口类，关键的问题就在于子窗口跟父窗口的联系搞不明白。
父窗口的WM_CREATE中创建了25个子窗口且保留了对应的句柄，
问题1
hwndChild[x][y] = CreateWindow (szChildClass, NULL,
WS_CHILDWINDOW | WS_VISIBLE,
0, 0, 0, 0,
hwnd, (HMENU) (y << 8 | x),
(HINSTANCE) GetWindowLong (hwnd, GWL_HINSTANCE),
NULL) ;
这句的理解，子窗口的风格必须加上WS_CHILDWINDOW|WS_VISIBLE吗？另外这里创建的25个子窗口咋么没大小？0,0,0,0 有点奇怪。(HMENU) (y << 8 | x）这个也不懂什么意思，没有涉及到菜单啊？
问题2
MoveWindow (hwndChild[x][y],
x * cxBlock, y * cyBlock,
cxBlock, cyBlock, TRUE) ;
当窗口尺寸发生变化时，移动子窗口到指定位置，并且重绘每个子窗口，但是在子窗口的消息处理中WM_PAINT
case WM_PAINT :
hdc = BeginPaint (hwnd, &ps) ;

GetClientRect (hwnd, &rect) ;
Rectangle (hdc, 0, 0, rect.right, rect.bottom) ;

if (GetWindowLong (hwnd, 0))
{
MoveToEx (hdc, 0, 0, NULL) ;
LineTo (hdc, rect.right, rect.bottom) ;
MoveToEx (hdc, 0, rect.bottom, NULL) ;
LineTo (hdc, rect.right, 0) ;
}

EndPaint (hwnd, &ps) ;
return 0 ;
}
并没有做什么工作嘛，只是重新画了一下客户区矩形，请问这里又该如何理解？如果把MOVEWINDOW的TRUE改成FALSE会发生什么？
最后问题：
SetWindowLong (hwnd, 0, 1 ^ GetWindowLong (hwnd, 0)) ;
GETWINDOWLONG(HWND,0)这里的0是HWNDFIRST吧，也就是说看看窗口是否是处在父窗口的上面或者顶层，进行异或操作刚好让他相反。最后刷新客户区？我这理解的对否？