深入理解 Windows 視窗程序機制

1. Win32 API 程式設計

使用 Win32 API 程式設計時，視窗程序很明確，就是程式設計師自訂的視窗程序，但 Win32 提供了 DefWindowProc() API 函數，預設處理交給它即可。

int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    LPSTR lpCmdLine, int nCmdShow)
{
    WNDCLASSEX wcex;
    wcex.lpszClassName = "MyClass";
    wcex.lpfnWndProc = (WNDPROC)MyWndProc;
    ...
    RegisterClassEx(&wcex);
    HWND hWnd = CreateWindow("MyClass", szTitle, WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);
    if (!hWnd) return FALSE;
    ShowWindow(hWnd, nCmdShow);
    UpdateWindow(hWnd);
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
}

LRESULT CALLBACK MyWndProc(HWND hWnd, UINT message,
    WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
        ...
        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

2. MFC 視窗程序機制

MFC 的視窗程序較複雜，先看靜態的部分（MFC 預先註冊的類別），簡單說：MFC 幫你處理好視窗程序的一切。

2.1 最抽象層：MFC 將視窗程序封裝起來，程式設計師只需「programming by difference」，對哪個訊息感興趣，就建立該訊息的回應函數（當然還有虛函數覆寫...）。

void CMyClass::OnLButtonDown(UINT nFlags, CPoint pt)
{
    ...
}

2.2 再深入一層：CWnd::WindowProc() 是目前的視窗程序，它是一個 template method。被你「programming by difference」的訊息，會由它交給 CWnd::OnWndMsg() 處理，預設的交給 CWnd::DefWindowProc() 處理。注意 CWnd::OnWndMsg() 和 CWnd::DefWindowProc() 都是虛函數。CWnd::DefWindowProc() 中呼叫了 ::DefWindowProc()（Win32 API 的）。

class CWnd : public CCmdTarget
{
    ...
protected:
    virtual LRESULT WindowProc(UINT message, WPARAM wParam, LPARAM lParam);
    virtual BOOL OnWndMsg(UINT message, WPARAM wParam, LPARAM lParam, LRESULT* pResult);
    ...
};

// template method
LRESULT CWnd::WindowProc(UINT message, WPARAM wParam, LPARAM lParam)
{
    LRESULT lResult = 0;
    if (!OnWndMsg(message, wParam, lParam, &lResult))
        lResult = DefWindowProc(message, wParam, lParam);
    return lResult;
}

// primitive method
LRESULT CWnd::DefWindowProc(UINT nMsg, WPARAM wParam, LPARAM lParam)
{
    if (m_pfnSuper != NULL)
        return ::CallWindowProc(m_pfnSuper, m_hWnd, nMsg, wParam, lParam);
    ...
}

2.3 最底層：MFC 預先註冊的類別

Pre-Registers Window Classes 沒什麼神秘的，因為 Window Classes 只是個 struct。當你想用某個 Pre-Registers Window Classes 時，無非是傳個 parameter，某段程式判斷後給 wc 結構賦值，呼叫 AfxRegisterClass(&wc) 完成。關鍵在於：使用的仍是 Win32 API 的 ::DefWindowProc()。

BOOL CWnd::CreateEx(DWORD dwExStyle, LPCTSTR lpszClassName, ...)
{
    CREATESTRUCT cs;
    cs.lpszClass = lpszClassName;
    ...
    PreCreateWindow(cs); // ######## pass a cs with lpszClass null
    ...
}

BOOL CWnd::PreCreateWindow(CREATESTRUCT& cs) // ######## pass a cs with lpszClass NULL
{
    if (cs.lpszClass == NULL) // ######## pass a cs with lpszClass NULL
    {
        VERIFY(AfxDeferRegisterClass(AFX_WND_REG)); // ######## pass a para AFX_WND_REG
        ASSERT(cs.style & WS_CHILD);
        cs.lpszClass = _afxWnd;
    }
    return TRUE;
}

#define AfxDeferRegisterClass(fClass) AfxEndDeferRegisterClass(fClass)
BOOL AFXAPI AfxEndDeferRegisterClass(LONG fToRegister) // ######## pass a para AFX_WND_REG
{
    ...
    WNDCLASS wndcls;
    memset(&wndcls, 0, sizeof(WNDCLASS));
    wndcls.lpfnWndProc = DefWindowProc; // ########## here, Win32 API ::DefWindowProc()
    wndcls.hInstance = AfxGetInstanceHandle();
    wndcls.hCursor = afxData.hcurArrow;
    ...
    if (fToRegister & AFX_WND_REG) // ######## pass a para AFX_WND_REG
    {
        wndcls.style = CS_DBLCLKS | CS_HREDRAW | CS_VREDRAW;
        wndcls.lpszClassName = _afxWnd; // ######## pass a para _afxWnd
        AfxRegisterClass(&wndcls);
        ...
    }
    ...
}
const TCHAR _afxWnd[] = AFX_WND;
#define AFX_WND AFX_WNDCLASS("Wnd")
#define AFX_WNDCLASS(s) _T("Afx") _T(s) _T("42") _STATIC_SUFFIX _UNICODE_SUFFIX _DEBUG_SUFFIX

2.4 總結

class CWnd : public CCmdTarget
{
    ...
protected:
    virtual LRESULT WindowProc(UINT message, WPARAM wParam, LPARAM lParam);
    virtual BOOL OnWndMsg(UINT message, WPARAM wParam, LPARAM lParam, LRESULT* pResult);
    virtual LRESULT DefWindowProc(UINT message, WPARAM wParam, LPARAM lParam);
    virtual BOOL OnCommand(WPARAM wParam, LPARAM lParam);
    virtual BOOL OnNotify(WPARAM wParam, LPARAM lParam, LRESULT* pResult);
    ...
}

LRESULT CALLBACK AfxWndProc(HWND hWnd, UINT nMsg, WPARAM wParam, LPARAM lParam)
{
    CWnd* pWnd = CWnd::FromHandlePermanent(hWnd);
    return AfxCallWndProc(pWnd, hWnd, nMsg, wParam, lParam);
}

LRESULT AFXAPI AfxCallWndProc(CWnd* pWnd, HWND hWnd, UINT nMsg,
    WPARAM wParam = 0, LPARAM lParam = 0)
{
    LRESULT lResult;
    lResult = pWnd->WindowProc(nMsg, wParam, lParam);
    return lResult;
}

LRESULT CWnd::WindowProc(UINT message, WPARAM wParam, LPARAM lParam)
{
    LRESULT lResult = 0;
    if (!OnWndMsg(message, wParam, lParam, &lResult))
        lResult = DefWindowProc(message, wParam, lParam);
    return lResult;
}

BOOL CWnd::OnWndMsg(UINT message, WPARAM wParam, LPARAM lParam, LRESULT* pResult)
{
    if (message == WM_COMMAND)
        OnCommand(wParam, lParam);
    else if (message == WM_NOTIFY)
        OnNotify(wParam, lParam, &lResult);
    else
        ... // msg map related
}

LRESULT CWnd::DefWindowProc(UINT nMsg, WPARAM wParam, LPARAM lParam)
{
    if (m_pfnSuper != NULL)
        return ::CallWindowProc(m_pfnSuper, m_hWnd, nMsg, wParam, lParam);
    ...
}