這兩個函數是MFC的基類CWnd兩個很重要的虛函數,萬惡的MFC總是讓人很煩惱。
{
public:
virtual BOOL PreCreateWindow(CREATESTRUCT& cs);
virtual void PreSubclassWindow();
BOOL SubclassWindow(HWND hWnd);
};
父類中的虛擬函數是可以被子類重新實現的,以實現多態。一般基類會有一個默認的實現。
msdn對這兩個函數的定義。
PreCreateWindow:Called by the framework before the creation of the Windows window
attached to this CWnd object.
(譯:在窗口被創建並attach到this指針所指的CWnd對象之前,被framework調用)
PreSubclassWindow:
This member function is called by the framework to allow other necessary
subclassing to occur before the window is subclassed.
(譯:在window被subclassed之前被framework調用,用來允許其它必要的subclassing發生)
雖然我已有譯文,但還是讓我對CWnd的attach和窗口的subclass作簡單的解釋吧!要理解attach,我們必須要知道一個C++的CWnd對象和窗口句柄(window)的區別:窗口句柄是windows的系統級的概念,代表着一個具體的窗口的句柄,可以通過操作這個句柄來修改窗口的屬性已經行爲。對應sdk編程
的HWND。
而CWnd就是MFC用類對window所進行C++封裝。attach,就是在CWnd對象和窗口句柄建立一個關聯。這樣就可以用CWnd對象的接口來修改實際窗口的
屬性和行爲。
好了,PreCreateWindow由framework在窗口創建前被調用,函數名也說明了這一點,Pre應該是previous的縮寫,PreSubclassWindow由framework在subclass窗口前調用。 這段話說了等於沒說,你可能還是不知道,什麼時候該改寫哪個函數。羅羅嗦嗦的作者,還是用代碼說話吧!源碼之前,了無祕密(候捷語)。我們就看看MFC中的這三個函數都是這樣實現的吧!LPCTSTR lpszWindowName, DWORD dwStyle,
int x, int y, int nWidth, int nHeight,
HWND hWndParent, HMENU nIDorHMenu, LPVOID lpParam)
{
// allow modification of several common create parameters
CREATESTRUCT cs;
cs.dwExStyle = dwExStyle;
cs.lpszClass = lpszClassName;
cs.lpszName = lpszWindowName;
cs.style = dwStyle;
cs.x = x;
cs.y = y;
cs.cx = nWidth;
cs.cy = nHeight;
cs.hwndParent = hWndParent;
cs.hMenu = nIDorHMenu;
cs.hInstance = AfxGetInstanceHandle();
cs.lpCreateParams = lpParam;
if (!PreCreateWindow(cs))
{
PostNcDestroy();
return FALSE;
}
AfxHookWindowCreate(this);
HWND hWnd = ::CreateWindowEx(cs.dwExStyle, cs.lpszClass,
cs.lpszName, cs.style, cs.x, cs.y, cs.cx, cs.cy,
cs.hwndParent, cs.hMenu, cs.hInstance, cs.lpCreateParams);
return TRUE;
}
// for child windows
BOOL CWnd::PreCreateWindow(CREATESTRUCT& cs)
{
if (cs.lpszClass == NULL)
{
// make sure the default window class is registered
VERIFY(AfxDeferRegisterClass(AFX_WND_REG));
// no WNDCLASS provided - use child window default
ASSERT(cs.style & WS_CHILD);
cs.lpszClass = _afxWnd;
}
return TRUE;
}
BOOL CWnd::SubclassWindow(HWND hWnd)
{
if (!Attach(hWnd))
return FALSE;
// allow any other subclassing to occur
PreSubclassWindow();
// now hook into the AFX WndProc
WNDPROC* lplpfn = GetSuperWndProcAddr();
WNDPROC oldWndProc = (WNDPROC)::SetWindowLong(hWnd, GWL_WNDPROC,
(DWORD)AfxGetAfxWndProc());
ASSERT(oldWndProc != (WNDPROC)AfxGetAfxWndProc());
if (*lplpfn == NULL)
*lplpfn = oldWndProc; // the first control of that type created
#ifdef _DEBUG
else if (*lplpfn != oldWndProc)
{
::SetWindowLong(hWnd, GWL_WNDPROC, (DWORD)oldWndProc);
}
#endif
return TRUE;
}
void CWnd::PreSubclassWindow()
{
// no default processing
}
在CWnd的實現中,除了CWnd::SubclassWindow會調用PreSubclassWindow外,還有一處。上面所列函數CreateEx的代碼,其中調用了一個AfxHookWindowCreate函數,見下面代碼:
BOOL CWnd::CreateEx()
{
// allow modification of several common create parameters
if (!PreCreateWindow(cs))
{
PostNcDestroy();
return FALSE;
}
AfxHookWindowCreate(this);
HWND hWnd = ::CreateWindowEx(cs.dwExStyle, cs.lpszClass,
cs.lpszName, cs.style, cs.x, cs.y, cs.cx, cs.cy,
cs.hwndParent, cs.hMenu, cs.hInstance, cs.lpCreateParams);
return TRUE;
}
// From VS Install PathVC98MFCSRCWINCORE.CPP
void AFXAPI AfxHookWindowCreate(CWnd* pWnd)
{
if (pThreadState->m_hHookOldCbtFilter == NULL)
{
pThreadState->m_hHookOldCbtFilter = ::SetWindowsHookEx(WH_CBT,
_AfxCbtFilterHook, NULL, ::GetCurrentThreadId());
if (pThreadState->m_hHookOldCbtFilter == NULL)
AfxThrowMemoryException();
}
}
這樣設定完成後,回到CWnd::CreateEx函數中,執行::CreateWindowEx進行窗口創建,窗口一產生,就會調用上面設定的Hook函數_AfxCbtFilterHook。而正是在_AfxCbtFilterHook中對函數PreSubclassWindow進行了第二次調用。見如下代碼:
/**//////////////////////////////////////////////////////////////////////////////
// Window creation hooks
LRESULT CALLBACK
_AfxCbtFilterHook(int code, WPARAM wParam, LPARAM lParam)
{
// connect the HWND to pWndInit
pWndInit->Attach(hWnd);
// allow other subclassing to occur first
pWndInit->PreSubclassWindow();
{
// subclass the window with standard AfxWndProc
oldWndProc = (WNDPROC)SetWindowLong(hWnd, GWL_WNDPROC, (DWORD)afxWndProc);
ASSERT(oldWndProc != NULL);
*pOldWndProc = oldWndProc;
}
}
也在調用函數SetWindowLong進行窗口subclass前調用了PreSubclassWindow.