questions on COM - COM Discussion Boards

7-Jan-05 0:24

I have been trying to receive event from a COM object in C++ with no success, I think I have no found the correct way to go about it (I have never used COM before), here's what I've done:

I have created a new project as an 'ATL Project', I then used the 'Implement Interface' function to add all parts of the COM object I need which generated a ton of code but in the code was the event I am trying to receive.

Firstly, am I going about this the right way now? And if I am, how do I use this new object in my application instead of the existing object?

I'm sorry if this is a stupid question but I have read about COM and searched the net etc but I just can't figure it out, I'm having real trouble understanding COM, any help is greatly appreciated.

Thanks in advance,
Dan.

Lim Bio Liong7-Jan-05 6:07

Lim Bio Liong

7-Jan-05 6:07

Hello __makaveli__,

Check out this article :

"Understanding COM Event Handling"

http://www.codeproject.com/com/TEventHandler.asp[^]

__makaveli__9-Jan-05 22:05

9-Jan-05 22:05

Thanks for the reply but that's not really helping me, I understand the theory on using COM (I think) and I have downloaded and played around with the example but I still can't see how I handle an event from an existing object.

Now I have a new project with the correct methods and events, how do I use it in a C project?

Martin Koorts9-Jan-05 22:44

9-Jan-05 22:44

Hi
To process COM events, you first have to implement an interface - this is what you've achieved by running the wizard. You'll have to create an instance of this class somewhere is your app. Secondly you'll have to pass this instance's interface to the COM object you want to listen to - typically this is done through IConnectPointContainer, etc. and associated IConnectionPoint::Advise/Unadvise methods (an observer pattern). Call QueryInterface(__uuidof(IConnectPointContainer), ...) on the object you want to listen to to see if it supports connection points.

If you're hosting an ActiveX control in an MFC application, this is easiest done with MFC's EVENT_MAP mechanism. ATL has a similar setup with SINK_MAP. Both of these MAPs hide the details of making calls to Advise and Unadvise, and the preceding discovery of IConnectPoint interfaces on the observable.

Connection Points is just one standard mechanism of binding the observer to the observable, and may or may not be supported by the object you want to listen to. Another way to achieve the same (and functionally similar) is a straight callback - the object you want to listen to has a method for registering the observer (eg. StartListening(IMyObjectsEvents *pObserver), and StopListening(...)). But since this is a more bespoke interaction, one that is less often supported. Also, since Connection Points is a generic implementation, it incurs an overhead in number of round-trips necessary to establish the 'connections', but I doubt that it's of any concern in your case.

HTH
Martin

__makaveli__9-Jan-05 23:32

9-Jan-05 23:32

Thanks, I think I have now made some progress, I have now regsitered the object successfully and created the new instance within my C project without any errors, but now I get a "Not Implemented" error when I try to use any of the commands from the original object.

In my C project, I have simply replaced the ID of the original object (CrystalDesignRunTime2.Application.Net) with the ID of the new object, what else do I need to do to make the functions work?

(I have started again on this to simply make the object work before I try implementing any event handling, any help is greatly appreciated on this, it's driving me mad!)

Martin Koorts10-Jan-05 1:03

10-Jan-05 1:03

Hang on there; It's not required to register the event wrapper, and probably isn't going to help at all.
Let's work through this one step at a time.

Forget about the events for now.
How are you hosting this Crystal object in your application - is it an MFC app with the Crystal control dragged onto a dialog?

Martin

__makaveli__10-Jan-05 1:09

10-Jan-05 1:09

The Crystal object is hosted in a C DLL (It's a plugin for another application), it is created using this code:

m_pApp.CreateInstance("CrystalDesignRunTime2.Application.Net");

(m_pApp being an IApplicationPtr)

I then open the report with this code:

m_pRep = data->m_pApp->OpenReport(reportName, varOpenType);

(m_pRep begin an IReportPtr)

I then use the Export() function from the IReportPtr to export the report as necessary.

That all works fine, how can I create an event wrapper to work as above and capture the event?

(Thanks for your help on this!)

Martin Koorts10-Jan-05 3:33

10-Jan-05 3:33

Hi
The following bit of code demonstrates what is possible with ATL and catching events from the MS Calendar ActiveX control;

<<<<<<<<< START OF CODE >>>>>>>>>>
// main.cpp
#define _WIN32_WINNT 0x0500 // if you want to use ::CoInitializeEx
#include <atlbase.h>
#include <atlcom.h>

#import "C:\Program Files\Microsoft Office\OFFICE11\mscal.ocx" \
rename_namespace("cal") \
raw_interfaces_only

const int CALENDAR_SOURCE_ID = 0;

class CMyCalendarEvents;

typedef IDispEventSimpleImpl<calendar_source_id, cmycalendarevents,="" &__uuidof(cal::dcalendarevents)="">
CalendarEventsImpl;

_ATL_FUNC_INFO OnNewMonthInfo = { CC_STDCALL, VT_EMPTY, 0, 0 };

class CMyCalendarEvents
: public CalendarEventsImpl
{
public:
BEGIN_SINK_MAP(CMyCalendarEvents)
SINK_ENTRY_INFO(CALENDAR_SOURCE_ID, __uuidof(cal::DCalendarEvents), 1, OnNewMonth, &OnNewMonthInfo)
END_SINK_MAP()

void __stdcall OnNewMonth()
{
_RPT0(_CRT_WARN, "## Month Changed!\n");
}
};

void main()
{
if (FAILED(::CoInitialize(0)))
return;

try
{
CMyCalendarEvents ce;

CComPtr<cal::icalendar> ptrCalendar;
HRESULT hr = ptrCalendar.CoCreateInstance(OLESTR("MSCAL.Calendar"));
_com_util::CheckError(hr);

// start listening
hr = ce.DispEventAdvise(ptrCalendar, &__uuidof(cal::DCalendarEvents));
_com_util::CheckError(hr);

hr = ptrCalendar->Today(); // OnNewMonth fires ??
_com_util::CheckError(hr);

hr = ptrCalendar->Today(); // OnNewMonth fires ??
_com_util::CheckError(hr);

// stop listening
hr = ce.DispEventUnadvise(ptrCalendar, &__uuidof(cal::DCalendarEvents));
_com_util::CheckError(hr);

hr = ptrCalendar->Today(); // OnNewMonth fires ??
_com_util::CheckError(hr);
}
catch (...)
{
// do nothing
}

::CoUninitialize();
}
<<<<<<<<< END OF CODE >>>>>>>>>>

In your case, the event sink would look different, for example, the IReportEvent::NoData handler would be (I think);

<<<<<<<<< START OF CODE >>>>>>>>>>
const int REPORT_SOURCE_ID = 0;

class CMyReportEvents;

typedef IDispEventSimpleImpl<report_source_id, cmyreportevents,="" &__uuidof(cr::ireportevent)="">
ReportEventsImpl;

_ATL_FUNC_INFO OnNoDataInfo = { CC_STDCALL, VT_EMPTY, 1, { VT_PTR | VT_BOOL } };

class CMyReportEvents
: public ReportEventsImpl
{
public:
BEGIN_SINK_MAP(CMyReportEvents)
SINK_ENTRY_INFO(REPORT_SOURCE_ID, __uuidof(cr::IReportEvent), 1, OnNoData, &OnNoDataInfo)
END_SINK_MAP()

void __stdcall OnNoData(VARIANT_BOOL *pCancel)
{
_ASSERT(false);
}
};
<<<<<<<<< END OF CODE >>>>>>>>>>

HTH
Martin

__makaveli__10-Jan-05 4:54

10-Jan-05 4:54

Thanks very much for the code!

I'm a little confused, do I use this code in the C project I have (The DLL I am working on) or in the ATL project? If I use it in the ATL project, how do I use this class in my DLL.

(Sorry if I sound really stupid but I am having a really hard time understanding how I should use COM/ATL objects)

Thanks in advance!

Martin Koorts10-Jan-05 5:58

10-Jan-05 5:58

Chill

It's not really feasible to have 2 DLLs - you can have both, but to my mind it just confuses things too much.
The DLL that uses the COM object (eg. calling OpenReport, etc) should be the same one that provides the events object that implements the IReportEvent interface. The theory is; if you see the following in a type library;

coclass Report {
[default] interface IReport;
interface IDReport;
[default, source] dispinterface IReportEvent;
};

it means that the Report class promises to implement IReport and IDReport, but *not* IReportEvent because that's the one you need to implement and give to the Report instance (you've created) by using the Advise/Unadvise methods mentioned earlier - this is indicated with the 'source' IDL attribute.

So, the code I gave you resembles something to be put in your work DLL and replaces the need for an ATL wrapper DLL that is generated from the type library. You can use the wizard-generated ATL DLL code as a hint to what the SINK_MAP entries should look like ....a handy reference.

As an aside, if you haven't already, use OleView to inspect Crystal's CRAXDDRT20.DLL (or similar) to see what the type library looks like.

Also, get 'ATL Internals' by Brent Rector, Chris Sells - it explains all this stuff much better than I can.

Wink | ;-)

Martin

__makaveli__10-Jan-05 22:16

10-Jan-05 22:16

I think I'm getting somewhere at last, just two questions I can't seem to figure out, when I try to use DispEventAdvise() function of CMyReportEvents, I get the message that it "must have class/struct/union type" which is obviously because that function does not exist in the class, how do I add the function to the class? Also, I get the message that there is no default constructor available for CMyReportEvents, is there any specific code I need in the constructor or can I use an empty constructor?

Thanks in advance!

__makaveli__10-Jan-05 22:39

10-Jan-05 22:39

I have worked out where my problem is, I need to declare the ReportEventsImpl properly, I have changed the code to this:

#import "C:\Program Files\Common Files\Crystal Decisions\1.0\Bin\craxddrt20.dll" rename_namespace("cr")

using namespace cr;

const int REPORT_SOURCE_ID = 0;

class CMyReportEvents;

typedef IDispEventSimpleImpl<report_source_id, cmyreportevents,="" &cr::ireportevent=""> ReportEventsImpl;

_ATL_FUNC_INFO OnNoDataInfo = { CC_STDCALL, VT_EMPTY, 1, { VT_PTR | VT_BOOL } };

class CMyReportEvents : public ReportEventsImpl
{
public:
BEGIN_SINK_MAP(CMyReportEvents)
SINK_ENTRY_INFO(REPORT_SOURCE_ID, __uuidof(IReportEvent), 1, OnNoData, &OnNoDataInfo)
END_SINK_MAP()

bool CMyReportEvents()
{
}

void __stdcall OnNoData(VARIANT_BOOL *pCancel)
{
_ASSERT(false);
}
};

I get the error that using "IReportEvent" in the typedef and that "Use of template requires argument list", could you point me in the direction of what I use instead of IReportEvent.

Thanks!

Martin Koorts10-Jan-05 23:06