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Inplace edit control

Thursday Aug 6th 1998 by Mario Contestabile

Inplace edit control

Have you ever wanted to allow users of your MFC application to rename strings contained in various controls? If so, you probably included a "Rename" button next to your control. Take a listbox for example. When "Rename" is pressed you retrieve the current selection from the listbox, display it in a dialog for the user to modify, and then modify the string in the listbox to the new string. Not a very aesthetic or efficient method.

A better solution is to dynamically create an edit control on top of the area where the displayed string is, preferably following a double-click of the string in the listbox. You could re-use this edit class throughout your application, thus removing the need for "Rename" buttons. This is common technique.

One method our SmartEdit can use to notify its parent that the user is done with it (<Enter> was pressed for example) is to send its parent a custom message. But how does the parent (in our example, a dialog with a list control) know what new string the user typed? We could pass "this" to our SmartEdit, which in turn could call a dialog function passing it the new string. Although convenient, this method would not make our SmartEdit very re-usable, since it would have to know a) the type of CDialog our parent is and b) every different parent dialog would need to implement this same function to accept the new string.

The method I chose was to post a message to the parent, essentially telling it "Hay, the user possibly modified this string you sent me. Here it is, do as you please." But right before posting this message to its parent, SmartEdit places the text on the clipboard. This way, SmartEdit doesn't care what kind of window its parent is, and the parent can handle it as it seems fit.

||||| Figure 1 "myDialog.cpp LBN_DBLCLK handler"

void myDialog::OnDblclkStringInListBox()
{
	const INT nIndex = m_Ctl.GetCurSel();
	if(nIndex == LB_ERR) return;
	CString string;
	m_Ctl.GetText(nIndex, string);
	RECT rect;
	INT result = m_Ctl.GetItemRect(nIndex, &rect);
	if(result==LB_ERR) return;
	SmartEdit* pEdit = new SmartEdit;
	rect.bottom += 4;
	pEdit->Create(WS_CHILD | WS_VISIBLE | WS_BORDER | ES_AUTOHSCROLL,
		rect, &m_Ctl, (UINT)-1);
	pEdit->SetWindowText(string);
	pEdit->SetFocus();
	pEdit->LimitText(MYMAX_LEN); // user defined maximum length of string
}

Figure 1 shows the ON_LBN_DBLCLK handler from the myDialog class. This is the handler called when the user double-clicks a string in the listbox. We first determine which, if any, of the listbox strings is currently selected. Note GetCurSel() can only be used with a single-selection listbox. The actual string is then retrieved from the listbox. The dimension of the item's rectangle is determined since this will be the size and position of our SmartEdit. The SmartEdit is created on the heap, and its Create() function called which creates the Windows edit control and attaches it to the CEdit object.

||||| Figure 2 "SmartEdit.cpp implementation"

SmartEdit::SmartEdit()  : bEscapeKey(FALSE)
{
}

BEGIN_MESSAGE_MAP(SmartEdit, CEdit)
//{{AFX_MSG_MAP(SmartEdit)
ON_WM_KILLFOCUS()
//}}AFX_MSG_MAP
END_MESSAGE_MAP()


void SmartEdit::OnKillFocus(CWnd*)
{
	PostMessage(WM_CLOSE, 0, 0);
	if(!bEscapeKey){
		CString str;
		GetWindowText(str);
		COleDataSource *pds = new COleDataSource;
		PTCHAR cp = (PTCHAR)GlobalAlloc(GMEM_FIXED, (str.GetLength() *
			sizeof(TCHAR)) + sizeof(TCHAR));
		_tcscpy(cp, str);
		pds->CacheGlobalData(CF_TEXT, cp);
		pds->SetClipboard();
		GetOwner()->PostMessage(EDITCLASSMSG);
		TRACE1("SmartEdit::OnKillFocus  Posting message to my owner, I have put
			[%s] on the clipboard for him\n", cp);
	}
}

// "override" base class member function
void SmartEdit::PostNcDestroy()
{
	delete this;
}

// "augment" base class member function
BOOL SmartEdit::PreTranslateMessage(MSG* pMsg)
{
	if(pMsg->wParam == VK_RETURN){
		PostMessage(WM_CLOSE, 0, 0);
		return TRUE;
	}else if(pMsg->wParam == VK_ESCAPE){
		PostMessage(WM_CLOSE, 0, 0);
		return bEscapeKey = TRUE;
	}
	
	return CEdit::PreTranslateMessage(pMsg);
}

Figure 2 shows SmartEdit's implementation. It is straightforward, but perhaps the PostNcDestroy function needs further explanation. Remember this object was created on the heap with "new" in myDialog. That means someone must "delete" it. Since PostNcDestroy() is called after the window has been destroyed, it is a perfect time to delete the object. Modeless dialogs use this method of self-destruction.

The most important action occurs in its OnKillFocus() handler. When the SmartEdit loses focus by a means other than the key, it retrieves the new string with GetWindowText(). The string is then placed on the clipboard using a COleDataSource. COleDataSource is a source actor in OLE data transfer. Notice I used PTCHAR, and _tcscpy() to allow the code to be transparantly compiled in both ANSI and UNICODE applications. After the string is placed on the clipboard, it sends EDITCLASSMSG (simply #defined as WM_APP + 100) to its parent.

||||| Figure 3 "myDialog.cpp EDITCLASSMSG handling code"

BOOL myDialog::PreTranslateMessage(MSG* pMsg)
{
	if(EDITCLASSMSG == pMsg->message){
		COleDataObject data;
		
		hClipboard()){           
			if(data.IsDataAvailable(CF_TEXT)){ 
				HGLOBAL hg;
				if(hg = data.GetGlobalData(CF_TEXT)){
					CString str = (LPCTSTR)GlobalLock(hg);
					TRACE1("myDialog::PreTrans(EDITCLASSMSG)  This is on the clipboard [%s]\n", str);
					GlobalUnlock(hg);
					NewString(str); // Appropriate action
				}
			}
			data.Release();
		}
		return TRUE;
	}
	
	return CDialog::PreTranslateMessage(pMsg);
}

Fogure 3 shows what myDialog does when handling a EDITCLASSMSG message. It knows there's a string on the clipboard, so uses COleDataObject to retrieve it. COleDataObject is the destination actor in OLE data transfer. The string from the clipboard is simply passed to a helper function, New String() in this case, which will decide a course of action. To replace the string that was 'underneath' the string double-clicked in the listbox, you would keep the item index obtained in OnDblclkStringInListBox(). For example, "const INT nIndex = m_Ctl.GetCurSel();" could be replaced by "m_nIndex = m_Ctl.GetCurSel();" after which NewString() would use myDialog's member variable 'm_nIdex' as the index for the new string.

We now have a fully functional SmartEdit capable of notifying its parent (using OLE) that it has accepted a string. One problem still remains. Can you spot it? Since the SmartEdit object 'kills' itself using 'delete' in PostNcDestroy(), it *must* be created on the heap using 'new'. If we don't enforce this, someone will inevitably use our SmartEdit in a DDX_Control(). This will cause serious havoc when the "delete this;" line is executed. So what can we do?

The answer is simple. make the ~SmartEdit destructor protected. This still allows the class to be used in inheritance, but prevents its usage on the stack.

Posted: March, 8, 98

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