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COM Objects and events

Posted on 2004-10-04
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Last Modified: 2010-04-05
I am trying to get my head around the way COM objects fire events, and how I can process them (the events and any data they send).

Can someone rough out some code for

a) a COM object that is callable from javascript (maybe from an html page) and "broadcasts" a string (say the current date and time) via an event

b) an event sink receiver that gets and displays that string.

I REALLY don't understand what is going on here .. is it some form of interprocess communication? what about shared memory issues?

Code might help me understand.
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Question by:Mutley2003
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4 Comments
 
LVL 17

Expert Comment

by:Wim ten Brink
ID: 12215853
Nope. Basically, it is very simple. You provide the COM object an interface to another COM object. And the COM object just calls the methods of the other interface. Something like:

type
  IFooEvent = interface
    procedure DoFire;
  end;
  IFoo = Interface
    procedure Fire;
    function GetFooEvent: IFooEvent;
    procedure SetFooEvent(Value: IFooEvent);
    property FooEvent: IFooEvent read GetFooEvent write SetFooEvent;
  end;

The COM DLL/EXE would create a type around IFoo. The Foo method just checks if a value is assigned to the FooEvent property and if it has a value, it just calls IFooEvent.DoFire.
Your application that calls the COM object must therefore contain a class that implements IFooEvents so it can send this class to the COM object. Basically, what you just get are two COM objects that communicate with one another through function calls.

With DCOM/COM+, there are also stubs and proxies involved, resulting in the communication between the two COM objects to be marshalled, send over the network and then unmarshalled again. This is just the Windows support for DCOM/COM+ and you don't have to worry about it.

If you use the Delphi COM wrapper wizard, this wizard will create the additional class for the events in your code for you.

However, it gets more complicated if multiple clients want to receive events from the COM server. In that case, the server needs to maintain a list of client COM objects, and call them all on events. This is done through the event sink. And it gets more complicated than this. But basically,COM events just mean the client has a COM object too...
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Author Comment

by:Mutley2003
ID: 12257075
ok, I have one COM object talking to another (as in, calling the methods of another). Let me see if I have this correct
* the COM server fires the events
* the COM client(s) receives them.

But I am missing a bit of the puzzle right here. Is the standard windows messaging being employed .. some PostMessage or SendMessage or somesuch? How does the client know that the event has happened?


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Wim ten Brink earned 500 total points
ID: 12262101
Nope. If it's an in-process COM object, the message system is just a matter of calling the procedures. The COM object is located in the same process space as the client application.
An out-of-process COM object does require some inter-proces communication but this is handled by Windows itself. If client and server are located on the same system, Windows will just use the pipe mechanism that it has. If client and server are on different systems, Windows will use TCP/IP for the communication and it has some ports reserved for this purpose. (Which you have to keep open if you have a firewall, else it won't work.)

This messaging system is completely handled by Windows. This is arranged by the type library which just tells Windows how it should marshall and unmarshall all the data that needs to be sent between the two sides.

You might want to read http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dncomg/html/msdn_comppr.asp and other MSDN articles about COM.
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