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VB 2005 Class Event Handlers

Is there anyway to raise an event from a class and require that any program
using that class (not just inheritance) have an event handler for specific
events in the class? In my case, some events don't need to be handled, but
some must be handled by the program that uses the class.

I'm looking for something along the lines of

Class MyClass
Event OptionalEvent(p arms)
Event RequiredEvent(p arms)

end Class

Mike Ober.


Dec 13 '05 #1
7 1714
No, you cannot require that a program handle the event of a particular
component or control.

"Michael D. Ober" <obermd.@.alum. mit.edu.nospam> wrote in message
news:%2******** ********@TK2MSF TNGP10.phx.gbl. ..
Is there anyway to raise an event from a class and require that any
program
using that class (not just inheritance) have an event handler for specific
events in the class? In my case, some events don't need to be handled,
but
some must be handled by the program that uses the class.

I'm looking for something along the lines of

Class MyClass
Event OptionalEvent(p arms)
Event RequiredEvent(p arms)

end Class

Mike Ober.

Dec 13 '05 #2
Can't you do something using implemention inheritance? Not my strongest
skill, so I not sure how you "require" that a class implement another.

Public Interface IRequiredEvent
Event RequiredEvent(B yVal e As EventArgs)
End Interface

Public Class Class1
Implements IRequiredEvent
Public Event OptionEvent(ByV al e As System.EventArg s)
Public Event RequiredEvent(B yVal e As System.EventArg s) Implements
IRequiredEvent. RequiredEvent
End Class

Public Class Class2
Implements IRequiredEvent
Public Event RequiredEvent(B yVal e As System.EventArg s) Implements
IRequiredEvent. RequiredEvent
End Class

Greg
"Marina" <so*****@nospam .com> wrote in message
news:eT******** ******@TK2MSFTN GP09.phx.gbl...
No, you cannot require that a program handle the event of a particular
component or control.

"Michael D. Ober" <obermd.@.alum. mit.edu.nospam> wrote in message
news:%2******** ********@TK2MSF TNGP10.phx.gbl. ..
Is there anyway to raise an event from a class and require that any
program
using that class (not just inheritance) have an event handler for
specific
events in the class? In my case, some events don't need to be handled,
but
some must be handled by the program that uses the class.

I'm looking for something along the lines of

Class MyClass
Event OptionalEvent(p arms)
Event RequiredEvent(p arms)

end Class

Mike Ober.


Dec 13 '05 #3
Michael,
As Marina suggests you cannot require a program handle a particular event,
per se.

You could however throw an exception when you go to raise the event, if
there are no event handlers attached to it.

In VB 2005 I would rely on Custom Events to raise the exception.

http://www.panopticoncentral.net/arc...8/06/1545.aspx

Something like:

Public Class MyClass

Private m_requiredEvent Handlers As EventHandler

Public Custom Event RequiredEvent As EventHandler
AddHandler(ByVa l value As EventHandler)
m_requiredEvent Handlers =
DirectCast([Delegate].Combine(m_requ iredEventHandle rs, value), EventHandler)
End AddHandler

RemoveHandler(B yVal value As EventHandler)
m_requiredEvent Handlers =
DirectCast([Delegate].Remove(m_requi redEventHandler s, value), EventHandler)
End RemoveHandler

RaiseEvent(ByVa l sender As Object, ByVal e As System.EventArg s)
If m_requiredEvent Handlers Is Nothing Then
Throw New InvalidOperatio nException("No event handlers
attached")
End If
m_requiredEvent Handlers.Invoke (sender, e)
End RaiseEvent
End Event

Protected Overridable Sub OnRequiredEvent (ByVal e As EventArgs)
RaiseEvent RequiredEvent(M e, e)
End Sub
End Class

In VB 2002 & 2003 you can use the hidden field RequiredEventEv ent to see if
there are any handlers or not.

Public Class MyClass

Public Event RequiredEvent As EventHandler

Protected Overridable Sub OnRequiredEvent (ByVal e As EventArgs)
If RequiredEventEv ent Is Nothing Then
Throw New InvalidOperatio nException("No event handlers
attached")
End If
RaiseEvent RequiredEvent(M e, e)
End Sub

End Class

NOTE: In either case I normally follow the .NET event standard & define &
use the OnRequiredEvent sub to raise the event itself.

--
Hope this helps
Jay [MVP - Outlook]
..NET Application Architect, Enthusiast, & Evangelist
T.S. Bradley - http://www.tsbradley.net
"Michael D. Ober" <obermd.@.alum. mit.edu.nospam> wrote in message
news:%2******** ********@TK2MSF TNGP10.phx.gbl. ..
| Is there anyway to raise an event from a class and require that any
program
| using that class (not just inheritance) have an event handler for specific
| events in the class? In my case, some events don't need to be handled,
but
| some must be handled by the program that uses the class.
|
| I'm looking for something along the lines of
|
| Class MyClass
| Event OptionalEvent(p arms)
| Event RequiredEvent(p arms)
|
| end Class
|
| Mike Ober.
|
|
|
|
Dec 13 '05 #4
Define an interface that all users of your class must implement
Ensure that they do so by requiring a reference to it to be passed to the
constructor

Public Interface IUsersMustImple mentThisInterfa ce
Sub Method1()
Sub Method2()
End Interface

Public MyCoolClass
Public Sub New(ByVal user As IUsersMustImple mentThisInterfa ce)
' Keep a reference to the user
End Sub
End Class

/claes

"Michael D. Ober" <obermd.@.alum. mit.edu.nospam> wrote in message
news:%2******** ********@TK2MSF TNGP10.phx.gbl. ..
Is there anyway to raise an event from a class and require that any program using that class (not just inheritance) have an event handler for specific
events in the class? In my case, some events don't need to be handled, but some must be handled by the program that uses the class.

I'm looking for something along the lines of

Class MyClass
Event OptionalEvent(p arms)
Event RequiredEvent(p arms)

end Class

Mike Ober.

Dec 14 '05 #5
OK, given that I can't force an event handler to be used, what is the syntax
to do the following:

dim MyClass as new MyObject(requir edexternalhandl er)

where

Public Function RequiredExterna lHandler(byval unhandledvalue as string) as
string
end function

Basically I have a class that is using application specific login (uses the
integrated windows login most of the time), but when it fails, it needs to
call a platform specific login hander. The platforms are
ConsoleApplicat ion, Windows Forms Application, and ASPNet Application.

Thanks,
Mike.
"Greg Burns" <bl*******@news groups.nospam> wrote in message
news:eG******** *****@TK2MSFTNG P14.phx.gbl...
Can't you do something using implemention inheritance? Not my strongest
skill, so I not sure how you "require" that a class implement another.

Public Interface IRequiredEvent
Event RequiredEvent(B yVal e As EventArgs)
End Interface

Public Class Class1
Implements IRequiredEvent
Public Event OptionEvent(ByV al e As System.EventArg s)
Public Event RequiredEvent(B yVal e As System.EventArg s) Implements
IRequiredEvent. RequiredEvent
End Class

Public Class Class2
Implements IRequiredEvent
Public Event RequiredEvent(B yVal e As System.EventArg s) Implements
IRequiredEvent. RequiredEvent
End Class

Greg
"Marina" <so*****@nospam .com> wrote in message
news:eT******** ******@TK2MSFTN GP09.phx.gbl...
No, you cannot require that a program handle the event of a particular
component or control.

"Michael D. Ober" <obermd.@.alum. mit.edu.nospam> wrote in message
news:%2******** ********@TK2MSF TNGP10.phx.gbl. ..
Is there anyway to raise an event from a class and require that any
program
using that class (not just inheritance) have an event handler for
specific
events in the class? In my case, some events don't need to be handled,
but
some must be handled by the program that uses the class.

I'm looking for something along the lines of

Class MyClass
Event OptionalEvent(p arms)
Event RequiredEvent(p arms)

end Class

Mike Ober.




Dec 14 '05 #6
Michael,
| OK, given that I can't force an event handler to be used, what is the
syntax
| to do the following:

Dim anObject As New Something(Addre ssOf RequiredExterna lHandler)

Where:

Class Something
Public Delegate Sub EventHandler(By Val parms As Object)

Event OptionalEvent As EventHandler
Event RequiredEvent As EventHandler

Public Sub New(ByVal handler As EventHandler)
AddHandler RequiredEvent, handler
End Sub

End Class

Public Sub RequiredExterna lHandler(ByVal parms As Object)

End Sub

Note Event handlers cannot be functions. You can change the "Delegate Sub
EventHandler" to include the correct parameters as needed.

I normally use System.EventHan dler rather then define my own, unless I need
a specific type derived from EventArgs.

Using:
Event OptionalEvent As EventHandler
Event RequiredEvent As EventHandler
Instead of:
| > Public Event OptionEvent(ByV al e As System.EventArg s)
| > Public Event RequiredEvent(B yVal e As System.EventArg s)

Is generally better as the first uses a predefined delegate (EventHandler)
where as the second will create a hidden delegate for each event that you
define, seeing as the events have the signuture you are getting a number of
identical duplicate types defined in your assembly...

--
Hope this helps
Jay [MVP - Outlook]
..NET Application Architect, Enthusiast, & Evangelist
T.S. Bradley - http://www.tsbradley.net
"Michael D. Ober" <obermd.@.alum. mit.edu.nospam> wrote in message
news:u0******** *****@tk2msftng p13.phx.gbl...
| OK, given that I can't force an event handler to be used, what is the
syntax
| to do the following:
|
| dim MyClass as new MyObject(requir edexternalhandl er)
|
| where
|
| Public Function RequiredExterna lHandler(byval unhandledvalue as string) as
| string
| end function
|
| Basically I have a class that is using application specific login (uses
the
| integrated windows login most of the time), but when it fails, it needs to
| call a platform specific login hander. The platforms are
| ConsoleApplicat ion, Windows Forms Application, and ASPNet Application.
|
| Thanks,
| Mike.
|
|
| "Greg Burns" <bl*******@news groups.nospam> wrote in message
| news:eG******** *****@TK2MSFTNG P14.phx.gbl...
| > Can't you do something using implemention inheritance? Not my strongest
| > skill, so I not sure how you "require" that a class implement another.
| >
| > Public Interface IRequiredEvent
| > Event RequiredEvent(B yVal e As EventArgs)
| > End Interface
| >
| > Public Class Class1
| > Implements IRequiredEvent
| > Public Event OptionEvent(ByV al e As System.EventArg s)
| > Public Event RequiredEvent(B yVal e As System.EventArg s) Implements
| > IRequiredEvent. RequiredEvent
| > End Class
| >
| > Public Class Class2
| > Implements IRequiredEvent
| > Public Event RequiredEvent(B yVal e As System.EventArg s) Implements
| > IRequiredEvent. RequiredEvent
| > End Class
| >
| > Greg
| >
| >
| > "Marina" <so*****@nospam .com> wrote in message
| > news:eT******** ******@TK2MSFTN GP09.phx.gbl...
| > > No, you cannot require that a program handle the event of a particular
| > > component or control.
| > >
| > > "Michael D. Ober" <obermd.@.alum. mit.edu.nospam> wrote in message
| > > news:%2******** ********@TK2MSF TNGP10.phx.gbl. ..
| > >> Is there anyway to raise an event from a class and require that any
| > >> program
| > >> using that class (not just inheritance) have an event handler for
| > >> specific
| > >> events in the class? In my case, some events don't need to be
handled,
| > >> but
| > >> some must be handled by the program that uses the class.
| > >>
| > >> I'm looking for something along the lines of
| > >>
| > >> Class MyClass
| > >> Event OptionalEvent(p arms)
| > >> Event RequiredEvent(p arms)
| > >>
| > >> end Class
| > >>
| > >> Mike Ober.
| > >>
| > >>
| > >>
| > >>
| > >
| > >
| >
| >
| >
|
|
|
Dec 14 '05 #7

Michael D. Ober wrote:
Is there anyway to raise an event from a class and require that any program
using that class (not just inheritance) have an event handler for specific
events in the class? In my case, some events don't need to be handled, but
some must be handled by the program that uses the class.

I'm looking for something along the lines of

Class MyClass
Event OptionalEvent(p arms)
Event RequiredEvent(p arms)

end Class


That's not really what events are for, so any solution is going to be
kludgey at best. When you find yourself trying to work against the
framework, it's time to reconsider.

What it looks like you want is that any caller of a method (methods
have callers, not classes per se) has a way for you to call it back.
There are loosely speaking two ways to enforce a behavioural contract,
subclassing and interface implementation. The latter is the more
flexible so we'll go with that.

First make a list of the methods you are going to require your callers
to implement; put these all in an interface:

Interface ICanBeCalledBac k
Sub ProcessStarted( )
Sub ProcessFinished ()
End Interface

Now, in each method that is going to call its caller back, accept a
parameter of this interface type:

Class TheClass 'avoid MyClass even in examples as it is a keyword :)

Sub TheProcess(call er As ICanBeCalledBac k, ' ... the other parameters
caller.ProcessS tarted
'the process
caller.ProcessF inished
End Sub

Finally, in an object that is going to call this method, implement the
interface:

Class ACaller
Implements ICanBeCalledBac k

Sub Started() Implements ICanBeCalledBac k.ProcessStarte d
'whatever
End Sub

Sub Finished() Implements ICanBeCalledBac k.ProcessFinish ed
'whatever
End Sub

Sub MakeTheCall
Dim tc As New TheClass

tc.TheProcess(M e)
End Sub
End Class

Now the compiler will enforce for you that any caller of TheProcess
must pass as the 'caller' an objet that *definitely* implements the
callback procedures defined in the interface.

You can do the optional callback stuff similarly - define an interface
for the optional callbacks, then in TheProcess do

If TypeOf caller Is ICanBeCalledBac kOptionally Then
DirectCast(call er,
ICanBeCalledBac kOptionally).Op tionalCallbackP rocedure
End If

Please note that Claes has already given you this answer in this thread
but I thought a little more motivation might help.

--
Larry Lard
Replies to group please

Dec 15 '05 #8

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