Implementing an autocompleting Combobox

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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 // _MSC_VER >= 1000

// ComboCompletion.h : header file
//
// Copyright (c) Chris Maunder 1997.
// Please feel free to use and distribute.


/////////////////////////////////////////////////////////////////////////////
// CComboCompletion window

class CComboCompletion : public CComboBox
{
// Construction
public:
    CComboCompletion();

// Attributes
public:

// Operations
public:

// Overrides
    // ClassWizard generated virtual function overrides
    //{{AFX_VIRTUAL(CComboCompletion)
    public:
    virtual BOOL PreTranslateMessage(MSG* pMsg);
    //}}AFX_VIRTUAL

// Implementation
public:
    virtual ~CComboCompletion();

    BOOL m_bAutoComplete;

    // Generated message map functions
protected:
    //{{AFX_MSG(CComboCompletion)
    afx_msg void OnEditUpdate();
    //}}AFX_MSG

    DECLARE_MESSAGE_MAP()
};

/////////////////////////////////////////////////////////////////////////////

//{{AFX_INSERT_LOCATION}}
// Microsoft Developer Studio will insert additional declarations immediately 
// before the previous line.

#endif 



// ComboCompletion.cpp : implementation file
//
// Copyright (c) Chris Maunder 1997.
// Please feel free to use and distribute.

#include "stdafx.h"
#include "ComboCompletion.h"

#ifdef _DEBUG
#define new DEBUG_NEW
#undef THIS_FILE
static char THIS_FILE[] = __FILE__;
#endif

/////////////////////////////////////////////////////////////////////////////
// CComboCompletion

CComboCompletion::CComboCompletion()
{
    m_bAutoComplete = TRUE;
}

CComboCompletion::~CComboCompletion()
{
}


BEGIN_MESSAGE_MAP(CComboCompletion, CComboBox)
    //{{AFX_MSG_MAP(CComboCompletion)
    ON_CONTROL_REFLECT(CBN_EDITUPDATE, OnEditUpdate)
    //}}AFX_MSG_MAP
END_MESSAGE_MAP()

/////////////////////////////////////////////////////////////////////////////
// CComboCompletion message handlers

BOOL CComboCompletion::PreTranslateMessage(MSG* pMsg)
{
    // Need to check for backspace/delete. These will modify the text in
    // the edit box, causing the auto complete to just add back the text
    // the user has just tried to delete. 

    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 we are not to auto update the text, get outta here
  if (!m_bAutoComplete) 
      return;

  // Get the text in the edit box
  CString str;
  GetWindowText(str);
  int nLength = str.GetLength();
  
  // Currently selected range
  DWORD dwCurSel = GetEditSel();
  WORD dStart = LOWORD(dwCurSel);
  WORD dEnd   = HIWORD(dwCurSel);

  // Search for, and select in, and string in the combo box that is prefixed
  // by the text in the edit box
  if (SelectString(-1, str) == CB_ERR)
  {
      SetWindowText(str);            // No text selected, so restore what 
                                     // was there before
      if (dwCurSel != CB_ERR)
        SetEditSel(dStart, dEnd);    //restore cursor postion
  }

  // Set the text selection as the additional text that we have added
  if (dEnd < nLength && dwCurSel != CB_ERR)
      SetEditSel(dStart, dEnd);
  else
      SetEditSel(nLength, -1);
}