PRB: ClassWizard Doesn't Support Property Change Notifications

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, versions 4.0, 5.0

SYMPTOMS

ClassWizard makes adding event handlers in an OLE control container much easier by providing the required macros, function declarations, and the function body. However, ClassWizard currently doesn't provide support to add handlers for property change requests and property change notifications. You must manually add the macros, function declarations, and the function body required for defining an event sink map entry in order to handle property change notifications. This article explains the steps necessary for handling one such property change notification.

RESOLUTION

The following steps illustrate how to handle property change notifications for the "Caption" stock property of the Circ3 sample control in a dialog based control container application:

1. Place the Circ3 control in the main dialog template of the container application.

2. Add an event sink map manually to the control container as follows:

   a. In the .h file of your Main Dialog class, add a

         DECLARE_EVENTSINK_MAP() macro inside the AFX_MSG section.
   

   b. In the .cpp file of your Main Dialog Class, add the following:

         BEGIN_EVENTSINK_MAP(COleCntrDlg, CDialog)
           //{{AFX_EVENTSINK_MAP(COleCntrDlg)
           //}}AFX_EVENTSINK_MAP
         END_EVENTSINK_MAP()
   
         In the this event sink map, COleCntrDlg is the name of your main
         dialog class.
   
   

3. Add the following two functions to the protected section in the .h file of your Main Dialog Class:

   BOOL CaptionRequest(BOOL *pChange); BOOL CaptionChanged(void);

4. Add the following ON_PROPNOTIFY macro outside of the AFX_EVENTSINK_MAP comments section in the event sink map:

   ON_PROPNOTIFY(COleCntrDlg, IDC_CIRC3CTRL1, DISPID_CAPTION,

                     CaptionRequest, CaptionChanged )
   
      In this macro, IDC_CIRC3CTRL1 is the ID of the Circ3 control. Please
      refer to the ON_PROPNOTIFY documentation for the syntax of the macro and
      its parameters. The documentation incorrectly states that the changed
      notification function should take a UINT parameter. However, the
      property changed notification function takes VOID as its only parameter.
   
   

5. Add the following implementations for CaptionRequest and CaptionChanged in the .cpp file of your Main Dialog Class:

      // Property Change Notifications
      BOOL COleCntrDlg::CaptionRequest(BOOL *pChange)
      {
          // Perform customized processing here
          AfxMessageBox("IPropertyNotifySink::OnRequestEdit called");
   
          // Return TRUE to allow the property to change, else return
          // FALSE
          *pChange = TRUE;
          return TRUE;    // Notification handled
      }
   
      BOOL COleCntrDlg::CaptionChanged(void)
      {
          // Perform customized processing here
          AfxMessageBox("IPropertyNotifySink::OnChanged called");
   
          return TRUE;    // Notification handled
      }
   
   

MORE INFORMATION

An OLE control container would typically implement IPropertyNotifySink in order to receive notifications about property changes from an OLE control. The control container creates a sink with this interface and connects it to the control through the connection point mechanism.

The OLE control itself is required to call the methods of IPropertyNotifySink only for those properties marked with the [bindable] and [requestedit] attributes in the object's type information. When the control is about to change a [requestedit] property, it must call IPropertyNotifySink::OnRequestEdit before changing the property and must also honor the action by the sink on return from this call. Also, when the control changes a [bindable] property, it is required to call IPropertyNotifySink::OnChanged. An OLE control will send these notifications only for property changes occurring after it is fully constructed and initialized.

REFERENCES

Online documentation for ON_PROPNOTIFY and ON_PROPNOTIFY_RANGE. Online documentation for Property Change Notification.