Force an event listener

[eedarley]

Is there a way to force the consumer of my object to handle events
that I've fired off? I don't care how they handle the event, just that
they do. I think in c# there is a way but not sure about vb.net.
Please advise.

 

[Guest]

(e-mail address removed) wrote in @o61g2000hsh.googlegroups.com:

Is there a way to force the consumer of my object to handle events
that I've fired off? I don't care how they handle the event, just that
they do. I think in c# there is a way but not sure about vb.net.
Please advise.

AddHandler allows you to assign event handlers to functions.

 

[eedarley]

(e-mail address removed) wrote in @o61g2000hsh.googlegroups.com:


AddHandler allows you to assign event handlers to functions.

Thanks for the response, But what I'm after is not how to handle an
event but ensuring that the consumer is in fact handling my events. So
in my object, before firing off the event, I want to check and see if
in fact the consumer has in fact added an event handler.

Regards

 

[Guest]

(e-mail address removed) wrote in @o61g2000hsh.googlegroups.com:

Thanks for the response, But what I'm after is not how to handle an
event but ensuring that the consumer is in fact handling my events. So
in my object, before firing off the event, I want to check and see if
in fact the consumer has in fact added an event handler.

The best way to do this is with a plug-in system.

Create an interface and insure any classes that need to listen to your
events implement the interface.

Your loader class will bind the events to the proper event handlers.

This will ensure:

1. All required handlers are implemented
2. Your loader class attaches all the events to the proper handlers

 

[rowe_newsgroups]

(e-mail address removed) wrote in @o61g2000hsh.googlegroups.com:


The best way to do this is with a plug-in system.

Create an interface and insure any classes that need to listen to your
events implement the interface.

Your loader class will bind the events to the proper event handlers.

This will ensure:

1. All required handlers are implemented
2. Your loader class attaches all the events to the proper handlers

The problem with that is that there isn't a way to force the consumer
to implement the interface (is there?), and also this would mean the
handlers would have to be public.

Would it be possible to pass a delegate in the class's constructor and
map it that way?

Thanks,

Seth Rowe

 

[eedarley]

Seth I think your approach is more friendly. This will require the
consumer to pass itself. From that I can check to see if they are
handling the event. Thanks for everyones help.

 

[John]

Take a look at custom events--you can use them to take control of
the process. In your code for the RaiseEvent method you could
check if there are any subscribers before actually firing the
event. For example:

Private _Test As EventHandler

Public Custom Event Test As EventHandler
AddHandler(ByVal value As EventHandler)
_Test = DirectCast([Delegate].Combine(_Test, value), EventHandler)
End AddHandler

RemoveHandler(ByVal value As EventHandler)
_Test = DirectCast([Delegate].Remove(_Test, value), EventHandler)
End RemoveHandler

RaiseEvent(ByVal sender As Object, ByVal e As System.EventArgs)
If _Test IsNot Nothing Then
_Test.Invoke(sender, e)
End If
End RaiseEvent
End Event

J

 

[eedarley]

Here is an example of what I interpreted to be Seth's approach and
which I am going to use.


Public Class Car
Public Delegate Sub CarAboutToExplode(ByVal
sender As Car, ByVal e As EventArgs)

Public Sub New(byval evnt as CarAboutToExplode)
'... do some stuff
'... oops forgot to put oil in engine
evnt.Invoke(me, eventargs.Empty)
end Sub

Public Sub ShutDown()
'shut down engine
End Sub
end Class



Public Class ConsumerOfCar

Public Sub SomeFunction()
dim del as new Car.CarAboutToExplode(addressof OnCarAboutToExplode)

dim car as new Car(del)


end Sub

Private Sub OnCarAboutToExplode(byval sender as car, bybal e as
eventargs)

sender.ShutDown()

End Sub


End Class

 

[eedarley]

John, I like your approach as well. Thank you for the example. I may
consider that approach as well. It's very clear.

Best Regards.

 

[Phill W.]

Is there a way to force the consumer of my object to handle events
that I've fired off? I don't care how they handle the event, just that
they do. I think in c# there is a way but not sure about vb.net.
Please advise.

I'd suggest a custom Event Arguments class to which you add a Handled
property. Pass this along with your Event and, if the Handled property
comes back False ...

Public Class CustomEventArgs
Inherits EventArgs

Friend Sub New()
End Sub

Public Property Handled() as Boolean
Get
Return m_bHandled
End Get
Set( Value as Boolean )
m_bHandled = Value
End Set
End Property

Private m_bHandled = False

End Class

Public Event SomeEvent( _
ByVal sender As Object _
, ByVal e As CustomEventArgs _
)

Protected Sub OnSomeEvent()
Dim e as New CustomEventArgs

RaiseEvent SomeEvent( Me, e )

If e.Handled = False Then
Throw New YouDidntHandleMyEventException( ...
End If

End Sub

HTH,
Phill W.

 

[Patrice]

This is not needed as this is done for you ? (not sure what is the exact
problem).

If you meant you want to know if a particuler event has been handled, in
most cases this is done using a "Handled" boolean in the event arguments.
See for example :
http://msdn2.microsoft.com/en-us/library/system.windows.forms.keypresseventargs.handled.aspx

You can then take the appropriate default action if this boolean is still
false (the consumer could also have registered something but not being
insterested in this particular event).