ON_MESSAGE和ON_REGISTERED_MESSAGE

on_message用于处理一切消息，但是on_registered_message只用处理在系统已经注册过的消息。
The RegisterWindowMessage function is used to define a new window message that is guaranteed to be unique throughout the system. The macro ON_REGISTERED_MESSAGE is used to handle these messages. This macro accepts a name of a UINT NEAR variable that contains the registered windows message ID.

class CMyWnd : public CMyParentWndClass
{
public:
    CMyWnd();

    //{{AFX_MSG(CMyWnd)
    afx_msg LRESULT OnFind(WPARAM wParam, LPARAM lParam);
    //}}AFX_MSG

    DECLARE_MESSAGE_MAP()
};

static UINT NEAR WM_FIND = RegisterWindowMessage("COMMDLG_FIND");

BEGIN_MESSAGE_MAP(CMyWnd, CMyParentWndClass)
    //{{AFX_MSG_MAP(CMyWnd)
    ON_REGISTERED_MESSAGE(WM_FIND, OnFind)
    //}}AFX_MSG_MAP
END_MESSAGE_MAP()