Expose an event handler to VBScript users of my COM object

Question

Suppose I have a COM object which users can access via a call such as:

Set s = CreateObject("Server")

What I'd like to be able to do is allow the user to specify an event handler for the object, like so:

Function ServerEvent

   MsgBox "Event handled"

End Function

s.OnDoSomething = ServerEvent

Is this possible and, if so, how do I expose this in my type library in C++ (specifically BCB 2007)?

Answer 1

I'm a little hazy on the details, but maybe the link below might help:

http://msdn.microsoft.com/en-us/library/ms974564.aspx

It looks like your server object needs to implement IProvideClassInfo and then you call ConnectObject in your VBScript code. See also:

http://blogs.msdn.com/ericlippert/archive/2005/02/15/373330.aspx

Answer 2

This is how I did it just recently. Add an interface that implements IDispatch and a coclass for that interface to your IDL:

[
    object,
    uuid(6EDA5438-0915-4183-841D-D3F0AEDFA466),
    nonextensible,
    oleautomation,
    pointer_default(unique)
]
interface IServerEvents : IDispatch
{
    [id(1)]
    HRESULT OnServerEvent();
}

//...

[
    uuid(FA8F24B3-1751-4D44-8258-D649B6529494),
]
coclass ServerEvents
{
    [default] interface IServerEvents;
    [default, source] dispinterface IServerEvents;
};

This is the declaration of the CServerEvents class:

class ATL_NO_VTABLE CServerEvents :
    public CComObjectRootEx<CComSingleThreadModel>,
    public CComCoClass<CServerEvents, &CLSID_ServerEvents>,
    public IDispatchImpl<IServerEvents, &IID_IServerEvents , &LIBID_YourLibrary, -1, -1>,
    public IConnectionPointContainerImpl<CServerEvents>,
    public IConnectionPointImpl<CServerEvents,&__uuidof(IServerEvents)>
{
public:
    CServerEvents()
    {
    }

    // ...

BEGIN_COM_MAP(CServerEvents)
    COM_INTERFACE_ENTRY(IServerEvents)
    COM_INTERFACE_ENTRY(IDispatch)
    COM_INTERFACE_ENTRY(IConnectionPointContainer)
END_COM_MAP()

BEGIN_CONNECTION_POINT_MAP(CServerEvents)
    CONNECTION_POINT_ENTRY(__uuidof(IServerEvents))
END_CONNECTION_POINT_MAP()

    // ..

    // IServerEvents
    STDMETHOD(OnServerEvent)();

private:
    CRITICAL_SECTION m_csLock;        
};

The key here is the implementation of the IConnectionPointImpl and IConnectionPointContainerImpl interfaces and the connection point map. The definition of the OnServerEvent method looks like this:

STDMETHODIMP CServerEvents::OnServerEvent()
{
    ::EnterCriticalSection( &m_csLock );

    IUnknown* pUnknown;

    for ( unsigned i = 0; ( pUnknown = m_vec.GetAt( i ) ) != NULL; ++i )
    {    
        CComPtr<IDispatch> spDisp;
        pUnknown->QueryInterface( &spDisp );

        if ( spDisp )
        {
            spDisp.Invoke0( CComBSTR( L"OnServerEvent" ) );
        }
    }

    ::LeaveCriticalSection( &m_csLock );

    return S_OK;
}

You need to provide a way for your client to specify their handler for your events. You can do this with a dedicated method like "SetHandler" or something, but I prefer to make the handler an argument to the method that is called asynchronously. This way, the user only has to call one method:

STDMETHOD(DoSomethingAsynchronous)( IServerEvents *pCallback );

Store the pointer to the IServerEvents, and then when you want to fire your event, just call the method:

m_pCallback->OnServerEvent();

As for the VB code, the syntax for dealing with events is a little different than what you suggested:

Private m_server As Server
Private WithEvents m_serverEvents As ServerEvents

Private Sub MainMethod()
    Set s = CreateObject("Server")
    Set m_serverEvents = New ServerEvents

    Call m_searchService.DoSomethingAsynchronous(m_serverEvents)
End Sub

Private Sub m_serverEvents_OnServerEvent()
    MsgBox "Event handled"
End Sub

I hope this helps.

Answer 3

Thanks for the comments. I ended up following the technique described here.