VC++学习:分析MFC中的映射

　条件查找映射

  MFC中大量使用了BEGIN_XXX_MAP这样的宏，以及映射进行查找优化，例如消息映射，OLE命令映射，以及接口等等。每个映射包含一个指向基类的映射的指针。这样，当一个类需要根据一定的条件查找一个对象时，它会查找本类对象，如果没有找到，那么会查找基类，直到根基类。这类查找包含Windows消息，命令，事件和OLE命令的分发，和对象实现的接口的查询等等。

  下面是函数BOOL CWnd::OnWndMsg(UINT message, WPARAM wParam, LPARAM lParam, LRESULT* pResult)的部分代码，演示了如何根据消息的ID查找处理函数。

  const AFX_MSGMAP* pMessageMap; pMessageMap = GetMessageMap();
  UINT iHash; iHash = (LOWORD((DWORD_PTR)pMessageMap) ^ message) & (iHashMax-1);
  AfxLockGlobals(CRIT_WINMSGCACHE);
  AFX_MSG_CACHE* pMsgCache; pMsgCache = &_afxMsgCache[iHash];
  const AFX_MSGMAP_ENTRY* lpEntry;
  if (message == pMsgCache->nMsg && pMessageMap == pMsgCache->pMessageMap)
  {
  // cache hit
  lpEntry = pMsgCache->lpEntry;
  AfxUnlockGlobals(CRIT_WINMSGCACHE);
  if (lpEntry == NULL)
  return FALSE;

  // cache hit, and it needs to be handled
  if (message < 0xC000)
  goto LDispatch;
  else
  goto LDispatchRegistered;
  }
  else
  {
  // not in cache, look for it
  pMsgCache->nMsg = message;
  pMsgCache->pMessageMap = pMessageMap;

  #ifdef _AFXDLL
  for (/* pMessageMap already init'ed */; pMessageMap->pfnGetBaseMap != NULL;
  pMessageMap = (*pMessageMap->pfnGetBaseMap)())
  #else
  for (/* pMessageMap already init'ed */; pMessageMap != NULL;
  pMessageMap = pMessageMap->pBaseMap)
  #endif
  {
  // Note: catch not so common but fatal mistake!!
  // BEGIN_MESSAGE_MAP(CMyWnd, CMyWnd)
  #ifdef _AFXDLL
  ASSERT(pMessageMap != (*pMessageMap->pfnGetBaseMap)());
  #else
  ASSERT(pMessageMap != pMessageMap->pBaseMap);
  #endif

  if (message < 0xC000)
  {
  // constant window message
  if ((lpEntry = AfxFindMessageEntry(pMessageMap->lpEntries,
  message, 0, 0)) != NULL)
  {
  pMsgCache->lpEntry = lpEntry;
  AfxUnlockGlobals(CRIT_WINMSGCACHE);
  goto LDispatch;
  }
  }
  else
  {
  // registered windows message
  lpEntry = pMessageMap->lpEntries;
  while ((lpEntry = AfxFindMessageEntry(lpEntry, 0xC000, 0, 0)) != NULL)
  {
  UINT* pnID = (UINT*)(lpEntry->nSig);
  ASSERT(*pnID >= 0xC000 || *pnID == 0);
  // must be successfully registered
  if (*pnID == message)
  {
  pMsgCache->lpEntry = lpEntry;
  AfxUnlockGlobals(CRIT_WINMSGCACHE);
  goto LDispatchRegistered;
  }
  lpEntry++; // keep looking past this one
  }
  }
  }
pMsgCache->lpEntry = NULL;
  AfxUnlockGlobals(CRIT_WINMSGCACHE);
  return FALSE;
  }

  LDispatch: