windows 程序设计 关于 窗口过程的一个有关问题

/*--------------------
   KEYVIEW2.C -- Displays Keyboard and Character Messages
                 (c) Charles Petzold, 1998
  --------------------*/

#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 ("KeyView2") ;
     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 ("This program requires Windows NT!"), 
                      szAppName, MB_ICONERROR) ;
          return 0 ;
     }
     
     hwnd = CreateWindow (szAppName, TEXT ("Keyboard Message Viewer #2"),
                          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 DWORD dwCharSet = DEFAULT_CHARSET ;            
     static int   cxClientMax, cyClientMax, cxClient, cyClient, cxChar, cyChar ;
     static int   cLinesMax, cLines ;
     static PMSG  pmsg ;
     static RECT  rectScroll ;
     static TCHAR szTop[] = TEXT ("Message        Key       Char     ")
                            TEXT ("Repeat Scan Ext ALT Prev Tran") ;
     static TCHAR szUnd[] = TEXT ("_______        ___       ____     ")
                            TEXT ("______ ____ ___ ___ ____ ____") ;

     static TCHAR * szFormat[2] = { 
          
               TEXT ("%-13s %3d %-15s%c%6u %4d %3s %3s %4s %4s"),
               TEXT ("%-13s            0x%04X%1s%c %6u %4d %3s %3s %4s %4s") } ;

     static TCHAR * szYes  = TEXT ("Yes") ;
     static TCHAR * szNo   = TEXT ("No") ;
     static TCHAR * szDown = TEXT ("Down") ;
     static TCHAR * szUp   = TEXT ("Up") ;

     static TCHAR * szMessage [] = { 
                         TEXT ("WM_KEYDOWN"),    TEXT ("WM_KEYUP"), 
                         TEXT ("WM_CHAR"),       TEXT ("WM_DEADCHAR"), 
                         TEXT ("WM_SYSKEYDOWN"), TEXT ("WM_SYSKEYUP"), 
                         TEXT ("WM_SYSCHAR"),    TEXT ("WM_SYSDEADCHAR") } ;
     HDC          hdc ;
     int          i, iType ;
     PAINTSTRUCT  ps ;
     TCHAR        szBuffer[128], szKeyName [32] ;
     TEXTMETRIC   tm ;

     switch (message)
     {
     case WM_INPUTLANGCHANGE:
          dwCharSet = wParam ;
                                   // fall through
     case WM_CREATE:
     case WM_DISPLAYCHANGE:
     
               // Get maximum size of client area

          cxClientMax = GetSystemMetrics (SM_CXMAXIMIZED) ;
          cyClientMax = GetSystemMetrics (SM_CYMAXIMIZED) ;

              // Get character size for fixed-pitch font

          hdc = GetDC (hwnd) ;

          SelectObject (hdc, CreateFont (0, 0, 0, 0, 0, 0, 0, 0,
                                   dwCharSet, 0, 0, 0, FIXED_PITCH, NULL)) ; 
               
          GetTextMetrics (hdc, &tm) ;
          cxChar = tm.tmAveCharWidth ;
          cyChar = tm.tmHeight ;

          DeleteObject (SelectObject (hdc, GetStockObject (SYSTEM_FONT))) ;
          ReleaseDC (hwnd, hdc) ;

               // Allocate memory for display lines

          if (pmsg)
               free (pmsg) ;

          cLinesMax = cyClientMax / cyChar ;
          pmsg = malloc (cLinesMax * sizeof (MSG)) ;
          cLines = 0 ;
                                   // fall through
     case WM_SIZE:
          if (message == WM_SIZE)
          {
               cxClient = LOWORD (lParam) ;
               cyClient = HIWORD (lParam) ;
          }
               // Calculate scrolling rectangle

          rectScroll.left   = 0 ;
          rectScroll.right  = cxClient ;
          rectScroll.top    = cyChar ;
          rectScroll.bottom = cyChar * (cyClient / cyChar) ;

          InvalidateRect (hwnd, NULL, TRUE) ;

          if (message == WM_INPUTLANGCHANGE)
               return TRUE ;        //  [color=#FF0000]我觉得这个 if 语句 不写也可以，为什么非要加个。。。[/color]

          return 0 ;
          
     case WM_KEYDOWN:
     case WM_KEYUP:
     case WM_CHAR:
     case WM_DEADCHAR:
     case WM_SYSKEYDOWN:
     case WM_SYSKEYUP:
     case WM_SYSCHAR:
     case WM_SYSDEADCHAR: 

               // Rearrange storage array

          for (i = cLinesMax - 1 ; i > 0 ; i--)
          {
               pmsg[i] = pmsg[i - 1] ;
          }
               // Store new message

          pmsg[0].hwnd = hwnd ;
          pmsg[0].message = message ;
          pmsg[0].wParam = wParam ;
          pmsg[0].lParam = lParam ;

          cLines = min (cLines + 1, cLinesMax) ;

               // Scroll up the display

          ScrollWindow (hwnd, 0, -cyChar, &rectScroll, &rectScroll) ;

          break;        // ie, call DefWindowProc so Sys messages work。。。 [color=#FF0000]这里面把 break 换成 return 0 为什么不可以。。比如按哪些键就不能处理了[/color]
          
     case WM_PAINT:
          hdc = BeginPaint (hwnd, &ps) ;

          SelectObject (hdc, CreateFont (0, 0