ArcGIS Engine Developer Guide

VISUAL C++
Connection point mechanism for hooking source
to sink objects
The connection is now complete; methods
will be called on any listening sinks by the
source. The sink will typically hold onto
an interface pointer to the source, so when
a sink has finished listening it can be
released from the source object by calling
IConnectionPoint::Unadvise. This is implemented
with AtlUnadvise.
IDispatch events versus pure COM events
An outbound interface can be a pure dispatch interface. This means instead of
the source calling directly onto a method in a sink, the call is made via the
IDispatch::Invoke mechanism. The IDispatch mechanism has a performance overhead
to package parameters compared to a pure vtable COM call. However, there
are some situations where this must be used. ActiveX controls must implement
their default outbound interface as a pure IDispatch interface; for example,
IMapControlEvents2 is a pure dispatch interface. Also, Microsoft Visual Basic 6 can
only be a source of pure IDispatch events. The connection point
mechanism is the same as for pure COM mechanisms, the main
difference being in how the events are fired.
ATL provides some macros to assist with listening to IDispatch
events; this is discussed on MSDN under ‘Event Handling and
ATL’. There are two templates available, IDispEventImpl and
IDispEventSimpleImpl, that are discussed in the following sections.
Visual Studio C++ Class Wizard. Adding event
handler to an ActiveX control on a dialog box.
Using IDispEventImpl to listen to events
The ATL template IDispEventImpl will use a type library to
“crack” the IDispatch calls and process the arguments into C++
method calls. The Visual Studio Class wizard can provide this
mechanism automatically when adding an ActiveX control to a
dialog box. Right-click the Control and click Events. In the Class
wizard, choose the resource ID of the control, choose the event,
then click Add Handler.
The following code illustrates the event handling code added by the wizard, with
some modifications to ensure advise and unadvise are performed.
#pragma once
There is a bug in the wizard: it does not add
the advise and unadvise code to the dialog box.
To fix this issue, add a message handler for
OnDestroy. Then in the OnInitDialog handler,
call AtlAdviseSinkMap with a TRUE second
parameter to begin listening to events. Place a
corresponding call to AtlAdviseSinkMap (with
FALSE as the second parameter) in the
OnDestroy handler. This is discussed further in
the MSDN article “BUG: ActiveX Control Events
Are Not Fired in ATL Dialog (Q190530)”.
#include "resource.h" // Main symbols
#include
////////////////////////////////////////////////////////////////////////////
// CMyDialog
class CMyDialog :
public CAxDialogImpl,
public IDispEventImpl
{
130 • ArcGIS Engine Developer Guide