Re: COM Events and IDispatch - DWebBrowserEvents2 and HTMLWindowEvents2

From:
"Alexander Nickolov" <agnickolov@mvps.org>
Newsgroups:
microsoft.public.vc.atl
Date:
Tue, 9 Jan 2007 10:17:29 -0800
Message-ID:
<Ow8JtpBNHHA.3552@TK2MSFTNGP03.phx.gbl>
For starters you don't need a public COM object for
implementing a sink. This is typically done on an
internal C++ class. That's what confused me. Besides,
I don't see how can you get the sink object implemented
in a base class of the C++ class implementing your
second ATL COM object at all - it's not accessible
from the object you create, you need the raw C++
class for that. In short - get rid of your second COM
object and implement the sink on an internal C++ class.

--
=====================================
Alexander Nickolov
Microsoft MVP [VC], MCSD
email: agnickolov@mvps.org
MVP VC FAQ: http://vcfaq.mvps.org
=====================================

"Lady_A" <alexandra.lev@gmail.com> wrote in message
news:1168309334.852573.182350@51g2000cwl.googlegroups.com...

OK, now I am confused.
This is what I have :
A typelib with 2 coclasses, the first one is the one that was there and
the second one is the one I added. The class I added implements only
one interface, that event interface and inherits from
IDispEventImpl<event interface>.
Right now the first class does not have the second as its member, it
just creates it upon need and registers a sink using the created object
pointer.
As far as I can understand the containing object is the class I added.
You are saying that due to its derivation from DispEventImpl that
class, with only one dispinterface, can not do reference counting
correctly ?

Clearly I am missing something - are you saying that IDispEventImpl is
not to be used at all ? All the samples I saw on the web point to this
approach, as far as hooking into HTMLWindowEvents is concerned...
If its not trouble perhaps you can refer me to some relevant material
that explains those aspects and suggests another way of implementing
sinks.

Thank you.

Alexander Nickolov wrote:

No, you are not avoiding the reference counting problem -
you are running headfirst into it. IDispEvent[Simple]Impl
is a roundabout way into a circular reference count.
Implementing your sink on the ATL COM object is a sure
way of getting it.

--
=====================================
Alexander Nickolov
Microsoft MVP [VC], MCSD
email: agnickolov@mvps.org
MVP VC FAQ: http://vcfaq.mvps.org
=====================================

"Lady_A" <alexandra.lev@gmail.com> wrote in message
news:1168302933.548209.160360@42g2000cwt.googlegroups.com...

Thanks a lot!
I do not want to change the existing working code to use IDispEventImpl
instead.
Therefore I am adding another class that will derive only from one
IDispEventImpl and use this to hook into HTMLWindowEvents2, avoiding
the reference counting problem with deriving from 2 IDispEventImpl.

Alexander Nickolov wrote:

But beware the shallow split identity performed by
IDsipEvent[Simple]Impl - it only implements QI but
delegates AddRef/Release to the containing object
thus you have shared reference count. It's highly
recommended to not derive an ATL COM object
class from IDispEvent[Simple]Impl in order to
avoid circular reference counts locking an object
in memory indefinitely.

--
=====================================
Alexander Nickolov
Microsoft MVP [VC], MCSD
email: agnickolov@mvps.org
MVP VC FAQ: http://vcfaq.mvps.org
=====================================

"Igor Tandetnik" <itandetnik@mvps.org> wrote in message
news:OHPTdZTMHHA.2028@TK2MSFTNGP03.phx.gbl...

"Lady_A" <alexandra.lev@gmail.com> wrote in message
news:1168042166.981694.72520@v33g2000cwv.googlegroups.com

I have an IE plugin that is hooked into DWebBrowserEvents2 in the
following way :
[
...
 dual,
...
]
interface IMyInterface : IDispatch {
part of DWebBrowserEvents2 methods with their dispids
};


Why do it this way? Why not use IDispEvent[Simple]Impl, as the
nature
intended? See ATLEventHandling sample.

It works fine and now I need to add hooking into
HTMLWindowEvents2::onunload
First of all I am not clear on how can my class can expose in the
COM_MAP an interface DWebBrowserEvents that it does not implement
according to the declaration (it has all the funcs, but there is no
":
public DWebBrowserEvents2")


DWebBrowserEvents2, being a dispinterface, is basically just a
synonym
for
IDispatch, which you do implement.

Second of all, how do I add the HTMLWindowEvents2 in this case ?


In general, you don't. Implement a separate class for each separate
event
interface you want to sink events from. Or, do as I suggested and
use
IDispEventImpl; your class can derive from IDispEventImpl more than
once,
each such derivation supporting one sink interface. Internally,
IDispEventImpl plays a clever trick called "split COM identity",
which
allows one C++ class to pretend to be several COM objects at once.
--
With best wishes,
   Igor Tandetnik

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