解析 MFC 中的 FromHandle

MFC 对 Windows API 进行了封装，在很多方面都会提供便利。用 FromHandle 返回零时对象的指针，就可以调用各种类的方法。临时对象会在 OnIdle 中销毁。这里对 FromHandle 的实现原理从源码上进行解析。

//
// 1
//
CWnd* PASCAL CWnd::FromHandle(HWND hWnd)
{
    CHandleMap* pMap = afxMapHWND(TRUE); //create map if not exist
    ASSERT(pMap != NULL);
    CWnd* pWnd = (CWnd*)pMap->FromHandle(hWnd);
#ifndef _AFX_NO_OCC_SUPPORT
    pWnd->AttachControlSite(pMap);
#endif
    ASSERT(pWnd == NULL || pWnd->m_hWnd == hWnd);
    return pWnd;
}

这是 CWnd 的 FromHandle 方法，大致的意思为从 CHandleMap 中获取临时 CWnd 对象的指针。

//
// 2
//
CHandleMap* PASCAL afxMapHWND(BOOL bCreate)
{
    AFX_MODULE_THREAD_STATE* pState = AfxGetModuleThreadState();
    if (pState->m_pmapHWND == NULL && bCreate)
    {
        BOOL bEnable = AfxEnableMemoryTracking(FALSE);
#ifndef _AFX_PORTABLE
        _PNH pnhOldHandler = AfxSetNewHandler(&AfxCriticalNewHandler);
#endif

        pState->m_pmapHWND = new CHandleMap(RUNTIME_CLASS(CTempWnd),
            offsetof(CWnd, m_hWnd));
#ifndef _AFX_PORTABLE
        AfxSetNewHandler(pnhOldHandler);
#endif
        AfxEnableMemoryTracking(bEnable);
    }
    return pState->m_pmapHWND;
}

再看

#define offsetof(s,m)   (size_t)&(((s *)0)->m)

继续

//
// 3
//
CObject* CHandleMap::FromHandle(HANDLE h)
{
    ASSERT(m_pClass != NULL);
    ASSERT(m_nHandles == 1 || m_nHandles == 2);

    if (h == NULL)
        return NULL;

    CObject* pObject = LookupPermanent(h);
    if (pObject != NULL)
        return pObject;   // return permanent one
    else if ((pObject = LookupTemporary(h)) != NULL)
    {
        HANDLE* ph = (HANDLE*)((BYTE*)pObject + m_nOffset);
        ASSERT(ph[0] == h || ph[0] == NULL);
        ph[0] = h;
        if (m_nHandles == 2)
        {
            ASSERT(ph[1] == h || ph[1] == NULL);
            ph[1] = h;
        }
        return pObject;   // return current temporary one
    }

    // This handle wasn't created by us, so we must create a temporary
    // C++ object to wrap it.  We don't want the user to see this memory
    // allocation, so we turn tracing off.

    BOOL bEnable = AfxEnableMemoryTracking(FALSE);
#ifndef _AFX_PORTABLE
    _PNH pnhOldHandler = AfxSetNewHandler(&AfxCriticalNewHandler);
#endif

    CObject* pTemp = NULL;
    TRY
    {
        pTemp = m_pClass->CreateObject();
        if (pTemp == NULL)
            AfxThrowMemoryException();

        m_temporaryMap.SetAt((LPVOID)h, pTemp);
    }
    CATCH_ALL(e)
    {
#ifndef _AFX_PORTABLE
        AfxSetNewHandler(pnhOldHandler);
#endif
        AfxEnableMemoryTracking(bEnable);
        THROW_LAST();
    }
    END_CATCH_ALL

#ifndef _AFX_PORTABLE
    AfxSetNewHandler(pnhOldHandler);
#endif
    AfxEnableMemoryTracking(bEnable);

    // now set the handle in the object
    HANDLE* ph = (HANDLE*)((BYTE*)pTemp + m_nOffset);  // after CObject
    ph[0] = h;
    if (m_nHandles == 2)
        ph[1] = h;

    return pTemp;
}