How to frequently update data in edit box from data in hooked edit box?

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C++

I'm working on a hooking application. I am able to hook on XPs system calculator. I can make it function using a separate application (similar in UI, since I manually copied the entire sys calc UI). My problem right now, is I can't update my own edit box in real time (Or once I click my app, system calc's edit box updates but not my edit box). My idea is to use ON_MESSAGE but I don't know how to make it loop like what a WindowProc could do (Where I placed my button clicks, ::PostMessage(hwnd, WM_LBUTTONDOWN, 0, 0); ::PostMessage(hwnd, WM_LBUTTONUP, 0, 0);). Or do you have any idea on how I can loop within my process so that all I have to do is get and display data from sys calc to my app? I'm a beginner.... :) So please, somehow, provide some codes. In additon, I tried to place my UpdateEditBox() code within the WindowProc() together with PostMessage()s but it will cause a Stack Overflow.

Some parts of my code:
=================================================================

BEGIN_MESSAGE_MAP(CCalcHookExDlg, CDialog)<br />
       //{{AFX_MSG_MAP(CCalcHookExDlg)<br />
          ON_MESSAGE(UWM_HOOKLINK, OnHookMessage) // user-defined Msg Map<br />

....

//}}AFX_MSG_MAP<br />
END_MESSAGE_MAP()

=================================================================

LRESULT CCalcHookExDlg::OnHookMessage(WPARAM wParam, LPARAM lParam)<br />
{<br />
    UpDateEditBox((HWND)lParam);<br />
    return 0;<br />
}

=================================================================

void CCalcHookExDlg::UpDateEditBox(HWND hwnd)<br />
{<br />
    char str[35];<br />
    // momentarily,I substituted the specific handler; sys calc edit box<br />
    // I'll be using hwnd later once I get to  solve the problem<br />
    ::SendMessage(this->hwndHookedCalcDisp, WM_GETTEXT, sizeof(str),    (long)str);<br />
    CString s;<br />
    s.Format("%s", str);<br />
    this->cwndMyCalcDisp->SetWindowText(s);<br />
    s.ReleaseBuffer();<br />
}

=================================================================

LRESULT CCalcHookExDlg::WindowProc(UINT message, WPARAM wParam, LPARAM lParam) <br />
{   <br />
	switch(message)<br />
	{<br />
        case WM_CLOSE:<br />
            {<br />
                PostQuitMessage(0);<br />
                this->DestroyWindow();<br />
                break;<br />
            }<br />
        case WM_COMMAND:<br />
	    {<br />
                CWnd* cWnd = GetFocus();<br />
                char className[20];<br />
                GetClassName(cWnd->GetSafeHwnd(), (LPTSTR)className,20);<br />
                if(strcmp(className, "Button") == 0)<br />
                {<br />
                // this gets a stored sys calc handler, w/c is editbox<br />
                HWND hwnd = hr->RetrieveHookedAppHandler(cWnd->GetDlgCtrlID());<br />
                ::PostMessage(hwnd, WM_LBUTTONDOWN, 0, 0);<br />
                ::PostMessage(hwnd, WM_LBUTTONUP, 0, 0);<br />
                }<br />
                m_dummy.SetFocus();<br />
		break;               <br />
	    }   <br />
	}<br />
	return CDialog::WindowProc(message, wParam, lParam);<br />
}<br />

=================================================================
I hope, the code will help you figure out what I mean. :)