Exposing Controls Based on System Controls

In most cases, Active Accessibility exposes information about superclassed or subclassed controls. Superclassing and subclassing allow an application developer to create a custom control with the basic functionality of a system control and include enhancements provided by the application. A superclassed control has a different window class name than the system control on which it is based. A subclassed control has the same window class name. For more information about superclassing and subclassing, see the Platform SDK documentation.

Because Active Accessibility exposes information about system-provided controls, Active Accessibility exposes the modified control unless a superclassed or subclassed control is significantly different from the base control. To determine if the modified control is accessible, application developers should use utilities such as Inspect Objects and Accessible Event Watcher to compare the behavior of the modified control with the base control.

If after using these utilities you determine that the modified control is not accessible, you must treat the control as any other custom control. The control must trigger events, and the application's window procedure must respond to the WM_GETOBJECT message by supplying an IAccessible interface that client applications use to obtain information about the control.

CreateStdAccessibleProxy and CreateStdAccessibleObject

If all or most of the IAccessible properties for the modified control are the same as the base control, use CreateStdAccessibleProxy or CreateStdAccessibleObject to simplify the implementation of the control's IAccessible interface.

Both of these functions retrieve an IAccessible interface pointer for the specified system control. The difference in these functions is that CreateStdAccessibleObject uses the window class name obtained from its hwnd parameter whereas CreateStdAccessibleProxy uses the window class name specified in its szClassName parameter. Therefore, if you decide to use these functions, use CreateStdAccessibleProxy to expose information about superclassed controls, and either function with subclassed controls.

After obtaining an IAccessible interface pointer to the system control, use the pointer in your implementation of the IAccessible interface for the modified control. If a property or method for the modified control is the same as the base control, use the IAccessible pointer to return the information provided by the base control. If a property for the modified control is different from the base control, override the base control's property.

In the following example, CAccCustomButton is the application-defined class derived from IAccessible. The member variable m_pAccDefaultButton is a pointer to an IAccessible interface that was retrieved from CreateStdAccessibleObject during the initialization procedure for the control. In this example, the Role property for the custom control is the same as the Role property of the system control, so the Role property of the base control is returned. However, the Description property is different from that of the base control, so this property is overridden.

HRESULT CAccCustomButton::Initialize( HWND hWnd, HINSTANCE hInst )
{
.
.
.
    hr = CreateStdAccessibleObject( m_hWnd, 
                                    OBJID_CLIENT, 
                                    IID_IAccessible, 
                                    (void **) &m__pAccDefaultButton );
.
.
.
}

STDMETHODIMP CAccCustomButton::get_accRole( VARIANT varID )
{
    return m_pAccDefaultButton->get_accRole(varID);
}


STDMETHODIMP CAccCustomButton::get_accDescription( VARIANT varChild,
                                                   BSTR* pszDesc )
{
    TCHAR   szString[256];
    OLECHAR wszString[256];

    LoadString( m_hInst, ID_DESCRIPTION, szString, 256 );
    MultiByteToWideChar( CP_ACP, 0, szString, -1, wszString, 256 );
   *pszDesc = SysAllocString( wszString );
   if ( !pszDesc )
       return S_OK;
   else
       return E_OUTOFMEMORY;
}

For more information about the IAccessible properties and methods of system controls, see Appendix A: Supported User Interface Elements Reference.