CCheckSK’�An Extended Check Box Class

Environment: VC6 SP4

Introduction

This class is derived from the MFC CButton class. It supports the following features:


  • Showing of On/Off LEDs to indicate the state of the check box
  • Showing an icon to indicate the state of the check box. The icon can be specified by using a resource ID, filename, or HICON
  • Left/Center/Right aligned texts
  • Text on the left or right of the LED
  • Enabled/disabled check boxes
  • Tool tips

This article demonstrates how to extend MFC to subclass common controls and to apply owner drawing to give them any desired look.

Using the Code

To use the class, follow these steps:


  1. Add the files CCheckSK.h and CCheckSK.cpp to your project.
  2. Create check boxes on the dialog.
  3. Open the class wizard and create control variables for the check boxes. Choose “Control” for the “Category” and “CButton” for the “Variable Type.”
  4. Include CCheckSK.h in the .h file for your dialog class (in the demo, the file is checkDlg.h).
  5. If you had named the check box variable m_chk, in the header file for the dialog, there will be a line CButton m_chk;. Replace the CButton with CCheckSK.
  6. At the end of OnInitDialog of your dialog class, add calls to the appropriate methods in CCheckSK.

  7. BOOL CCheckDlg::OnInitDialog()
    {

    m_chk1.SetCheck(TRUE);
    m_chk1.SetLedColor(RGB(255, 0, 0), RGB(128, 0, 0));
    m_chk1.SetToolTip(“Click on this to change the state”);

    m_chk5.SetIcon(IDI_ON, IDI_OFF);

    }

  8. If you want to change the look of the check boxes later, you can call any of these methods from your code at run time.

How the Code Works

We start by deriving the CCheckSk class from the CButton MFC.


class CCheckSK : public CButton
{
….
}

To make the control owner drawn, the BS_OWNERDRAW style needs to be added to the window style. For this, subclassing is used. To do this, the PreSubclassWindow method is overridden as follows:


void CCheckSK::PreSubclassWindow()
{
UINT nBS = GetButtonStyle();

// the button should not be owner draw
ASSERT((nBS & SS_TYPEMASK) != BS_OWNERDRAW);

// This class supports only check boxes
ASSERT(nBS & BS_CHECKBOX);

// Switch to owner-draw
ModifyStyle(SS_TYPEMASK, BS_OWNERDRAW, SWP_FRAMECHANGED);

m_nStyle = GetWindowLong(GetSafeHwnd(), GWL_STYLE);

CButton::PreSubclassWindow();
}

After the owner-draw style has been set, the framework will call the DrawItem method each time it requires the control to be redrawn. So, this method has to be implemented and all code to draw the LED or display the icon has to be added to it. The implementation of this method is a bit long and the following snippet just gives the flow.


void CCheckSK::DrawItem(LPDRAWITEMSTRUCT lpDIS)
{
// this class works only for push buttons
ASSERT (lpDIS->CtlType == ODT_BUTTON);

// get the device context pointer to draw into
CDC* pDC = CDC::FromHandle(lpDIS->hDC);

// get button state, selected? has focus? disabled?

// draw bounding rectangle with color based on whether
// the mouse is over the check box

// if check box has focus, draw the focus rectangle

// calculate LED’s and text’s rectangle

// If icon is given, draw the icon;
// otherwise, draw the LED using GDI functions

// Calculate the rectangle for the text based on the
// horizontal alignment style of the check box

// if the button is disabled, draw etched text or draw
// plain text

// Release all resources

}

Methods

The following methods are present in the CDialogSK class:


  1. DWORD SetIcon(int nIconOn, int nIconOff);

    Sets the icon based on resource ID


  2. DWORD SetIcon(HICON hIconOn, HICON hIconOff);

    Sets the icon based on the handle to a icon


  3. DWORD SetIcon(LPCTSTR lpszFileNameIn, LPCTSTR lpszFileNameOut);

    Sets the icon based on the name of an icon file


  4. BOOL SetLedColor(COLORREF colLedOn, COLORREF colLedOff);

    If any icons are not used, this method is used to set the color of the on/off LED


  5. BOOL SetLedSize (int nSize);

    The size of the LED is changed with this method


  6. void SetToolTip (LPCTSTR lpszText);

    The tool tip for the check box can be changed by using this method


Downloads


Download demo project – 15 Kb


Download source – 6 Kb

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