
Introduction
I had a need for a combobox that would auto-complete, very much like the URL edit box in the toolbar of Netscape Navigator. It was actually surprisingly simple since the base CComboBox
is so rich in functionality.
The basic idea is that every time the text in the edit box changes, check to see if there is any text in the drop down list that is prefixed by this edit box text. Handle the CBN_EDITUPDATE
message to get the text change notifications, and use GetWindowText() to get the text. CComboBox::SelectString
will look for a string in the list which is prefixed by the given string, and select it into the edit box. I then select the portion of text that was added to the users typed text so that they can continue typing and have the additions ignored if they wish. That takes care of 90% of the work.
The only trick is in handling backspaces and deletes. When a user hits delete, the text is changed, and the auto-completion routine will try to restore that text back again. Just check in PreTranslateMessage
for a KEY_DOWN
message with a virtual key of VK_DELETE
or VK_BACK
, and temporarily disable the auto-complete mechanism for those key strokes.
Source code
#if !defined(AFX_ComboCompletion_H__115F422E_5CD5_11D1_ABBA_00A02__INCLUDED_)
#define AFX_ComboCompletion_H__115F422E_5CD5_11D1_ABBA_00A02__INCLUDED_
#if _MSC_VER >= 1000
#pragma once
#endif
class CComboCompletion : public CComboBox
{
public:
CComboCompletion();
public:
public:
public:
virtual BOOL PreTranslateMessage(MSG* pMsg);
public:
virtual ~CComboCompletion();
BOOL m_bAutoComplete;
protected:
afx_msg void OnEditUpdate();
DECLARE_MESSAGE_MAP()
};
#endif
#include "stdafx.h"
#include "ComboCompletion.h"
#ifdef _DEBUG
#define new DEBUG_NEW
#undef THIS_FILE
static char THIS_FILE[] = __FILE__;
#endif
CComboCompletion::CComboCompletion()
{
m_bAutoComplete = TRUE;
}
CComboCompletion::~CComboCompletion()
{
}
BEGIN_MESSAGE_MAP(CComboCompletion, CComboBox)
ON_CONTROL_REFLECT(CBN_EDITUPDATE, OnEditUpdate)
END_MESSAGE_MAP()
BOOL CComboCompletion::PreTranslateMessage(MSG* pMsg)
{
if (pMsg->message == WM_KEYDOWN)
{
m_bAutoComplete = TRUE;
int nVirtKey = (int) pMsg->wParam;
if (nVirtKey == VK_DELETE || nVirtKey == VK_BACK)
m_bAutoComplete = FALSE;
}
return CComboBox::PreTranslateMessage(pMsg);
}
void CComboCompletion::OnEditUpdate()
{
if (!m_bAutoComplete)
return;
CString str;
GetWindowText(str);
int nLength = str.GetLength();
DWORD dwCurSel = GetEditSel();
WORD dStart = LOWORD(dwCurSel);
WORD dEnd = HIWORD(dwCurSel);
if (SelectString(-1, str) == CB_ERR)
{
SetWindowText(str);
if (dwCurSel != CB_ERR)
SetEditSel(dStart, dEnd);
}
if (dEnd < nLength && dwCurSel != CB_ERR)
SetEditSel(dStart, dEnd);
else
SetEditSel(nLength, -1);
}