MFC OnEnChange handler function - infinite loop

Hi,

(I'm using VS++2005)

I put edit box control (with ID - ID_edit_box) on my dialog, and associate (with handler wizard) two varibles for it: control (c_editbox) and value (v_editbox) variable. Also I associate handler function OnEnChangeedit_box with that edit box control. Suppose that we may enter just one digit in edit box, and that digit can be 0 or 1. If we enter some other value - what I want is that content of that edit box is automaticaly cleared, so user can't see that he type anything (in other words user can not enter anything except 0/1 in edit box). I do that check in onEnChangeedit_box function. Here is the code:

void CSDRDlg::OnEnChangeedit_box()
{
CWnd* pWnd;
CString edit_box_temp;

pWnd = GetDlgItem(ID_edit_box);
pWnd->GetWindowText(edit_box_temp);

if ((edit_box_temp == "0" || edit_box_temp == "1")
{...do something - i.e. setfocus on some other edit box }
else
{
pWnd->SetWindowText(""); // clear the content of edit box
//... any other statement below will not be executed because the
//above line cause again call of this function
}
}
I debug and discover that line: "pWnd->SetWindowText("");" cause infinite loop because we change control content in this function which truggers again her call.

But I change above code like this:
void CSDRDlg::OnEnChangeedit_box()
{
UpdateData(TRUE);
if ((v_editbox == "0" || v_editbox== "1")
{...do something - i.e. setfocus on some other edit box }
else
{
v_editbox = "";
UpdateData(FALSE);
}
}

and that works what I want but can someone explain to me why when we call
v_editbox = "";
UpdateData(FALSE);
that dont cause infinite loop.

Best regards

The answer can be found in UpdateData source code:

Code:

---

BOOL CWnd::UpdateData(BOOL bSaveAndValidate)
{
        ASSERT(::IsWindow(m_hWnd)); // calling UpdateData before DoModal?

        CDataExchange dx(this, bSaveAndValidate);

        // prevent control notifications from being dispatched during UpdateData
        _AFX_THREAD_STATE* pThreadState = AfxGetThreadState();
        HWND hWndOldLockout = pThreadState->m_hLockoutNotifyWindow;
        ASSERT(hWndOldLockout != m_hWnd);  // must not recurse
        pThreadState->m_hLockoutNotifyWindow = m_hWnd;

        BOOL bOK = FALSE;      // assume failure
        TRY
        {
                DoDataExchange(&dx);
                bOK = TRUE;        // it worked
        }
        CATCH(CUserException, e)
        {
                // validation failed - user already alerted, fall through
                ASSERT(!bOK);                                                                                       
                // Note: DELETE_EXCEPTION_(e) not required
        }
        AND_CATCH_ALL(e)
        {
                // validation failed due to OOM or other resource failure
                e->ReportError(MB_ICONEXCLAMATION, AFX_IDP_INTERNAL_FAILURE);
                ASSERT(!bOK);
                DELETE_EXCEPTION(e);
        }
        END_CATCH_ALL

        pThreadState->m_hLockoutNotifyWindow = hWndOldLockout;
        return bOK;
}

---

According to the notes, line
pThreadState->m_hLockoutNotifyWindow = m_hWnd;
prevents notifications. Since they use undocumented functions, we cannot do the same in our own code, and need to use some flag which must be checked in notification handler:

Code:

---

void CSDRDlg::OnEnChangeedit_box()
{
    if ( m_bNotificationDisabled )
        return;


    m_bNotificationDisabled = TRUE;
    pWnd->SetWindowText("");
    m_bNotificationDisabled = FALSE;    // return to defaut
}

---

Another way is clearing editbox asynchronously. You can post user-defined message to the dialog (PostMessage, not SendMessage), and clear editbox in the message handler.

Another solution would be to check the length of the string:

Code:

---

pWnd = GetDlgItem(ID_edit_box);
pWnd->GetWindowText(edit_box_temp);

if (strlen(edit_box_temp)>0)
{
    if ((edit_box_temp == "0" || edit_box_temp == "1")
    {
        ...do something - i.e. setfocus on some other edit box
    }
    else
    {
        pWnd->SetWindowText(""); // clear the content of edit box
        //... any other statement below will not be executed because the
        //above line cause again call of this function
    }
}

---

Hope that helps.

Quote:

---

Originally Posted by melinda123

Suppose that we may enter just one digit in edit box, and that digit can be 0 or 1. If we enter some other value - what I want is that content of that edit box is automaticaly cleared, so user can't see that he type anything (in other words user can not enter anything except 0/1 in edit box).

---

If this is really the case, and not just to explain the question, would it not be much clearer to the user to dispay a check box, instead of an edit box?